And daft question about boost [Archive] - The Great Planes and warbirds Community

GregP

11th May 2009, 05:53

A great question. The entinre question of boost has several aspects.

First is the boost itself:

1) You have to know if the boost is aboslute pressure or gauge pressure. 1 atmosphere is 29.92 inches of Mercury on a standard day (sea level, 59°F or 15°C). However, if you check a tire with no air in it, the reading will be zero on a gauge since both the inside and outside of the tire have 1 atmosphere of pressure. If the tire has two atmospheres in it, the outside has 1 aotmosphere on it, the difference is 1 atmosphere, or 29.92 inches.

2) In the USA, we used inches of Mercury absolute (as opposed to gauge pressure). By way of example, 1 atmosphere is 29.92 inches of Mercury on a standard day (sea level, 59°F or 15°C). Anything over 29.92 is starting to be a charged intake and probably is due to a supercharger or turbocharger ... there being no OTHER way to charge the intake except maybe Nitrous Oxide boost.

3) In the UK, they used gauge pressure, also in inches of Mercury. So, 9 pounds of boost in the UK is 9.00 + 29.92 = 38.92 inches of manifold pressure in the USA.

4) In Germany, they used technical atmospheres or "ata" units. 1 ata is about 1 atmosphere. So ... 1.1 ata is about 29.92 + 2.992 = 32.8 inches of manifold pressure in teh USA or 2.99 ~3 pounds of boost in the UK.

5) Each individual engine reacts differently to charged intake pressure. Some engine thrive on it, and some are less than wonderfully affected. A great deal depends on the charge system. Most WW2 engines had an internal supercharger. Some also had an external turbo-supercharger, or turbocharger in today's parlance.

Many Allisons had a 9.5-inch impeller because the Allison was intended from the outset to be turbocharged. The war department decided ot use the available turbocargers for the bombers to be used in Europe, and deleted the turbocharger from the Allison-powered fighters. That left the Allison a bit underboosted and a bit anemic above about 12,000 - 15,000 feet. If it HAD the turbocharger, it would have been a greater engine in history.

If the Allison had used a 10.5 inch impeller, it would have been better, but not a high altitide engine since the internal supercharger is sugle-stage and single-speed.

For reference, the radials ALSO had internal superchargers as did the Merlin, the Griffin, and the DB-300 / 601 / 605 series.

6) Each individual engine has different reactions to the boost pressure. You can characterize the reaction of a particular Merlin series, but the characterization will not apply if the impeller design changes from series to series. A 2 stage, multi-speed Merlin will not react ANYTHING like a single stage, single speed supercharged engine.

I realize this is NOT a great explanation, but there is no general rule for how much power any particular boost will add to any engine. It dpends on the engine, the boost device, and the altitude at which the engine is running. Also, it is greatly dependent on fuel.The fuel available during WW2 in Europe was not particularly palatable to the Allisons in the P-38s. This is not due to the European fuel being "bad." Rather it is due to the fact that the carburetors, fuel mixtures and systems were developed with American fuels. In other theaters of operation, the P-38s used US fuel and flew FINE, with VERY few engine problems. Conversely, the DB engines the Germans used were DESIGNED for European fuels and ran fine on them. Ditto the Merlins and Griffons. They perhaps were not as happy on US fuel, but that is not recorded as such. Also, Merlins run fine today on the fuel we have avialble. But, so do the Allisons.

Wuzak

11th May 2009, 06:27

3) In the UK, they used gauge pressure, also in inches of Mercury. So, 9 pounds of boost in the UK is 9.00 + 29.92 = 38.92 inches of manifold pressure in the USA.

The UK used Pounds per Square Inch (PSI) gauge, often referred to as "pounds" of boost for short.

The conversion factor for psi into inches of Mercury is:

1 pound per square inch = 2.03625437 inches of mercury

(Thanks Google).

So an engine running 9psi at sea level would have 48.2 inHg Manifold Absolute Pressure.

30psi boost at sea level would be about 91 inHg, 36psi (Merlin RM.17SM) equates to about 103 inHg.

Wuzak

11th May 2009, 06:32

The way the boost is generated is important, as are factors such as intercooling and aftercooling.

In Merlins the two stage engines were more powerful at the same boost settings as the single stage engines because using two stages required less power and generated less heat than the single stage supercharger.

Generally, though, for a given engine configuration the power should roughly track the MAP. If you double the MAP then you should get approximately double the power, mainly because you have pushed twice as much air in and therefore twice as much fuel.

Kutscha

11th May 2009, 16:12

here is a boost conversion chart.

http://img.villagephotos.com/p/2005-12/1114844/Boost.jpg

Nick Sumner

11th May 2009, 19:55

I was actually thinking of two separate engines when I posed this question, the Griffon and the Pennine.

The climb ratings for the 100 series Griffons

2440hp at 25lbs low Supercharge 2750 rpm 6000 feet altitude
2300hp at 18lbs medium Supercharge 2750 rpm 15,750 feet altitude
2085hp at 9lbs high Supercharge 2750 rpm 23,000 feet altitude

Takeoff rating was 1900 hp at 25lbs 2750 rpm

All of these are with 115/145 grade fuel

I should l mention there's an assumption built in there, I have assumed the boost pressures would be the same in the 100 series Griffons as in the 80 series because I've not been able to find the actual figures for boost for the 100 series engines in Lumsden, Bingham or White.

The 100 series griffons used a two-stage three speeds supercharger and IIRC it was inter and after cooled.

How much more power would be developed at 30lbs or 35lbs?

By contrast the Pennine used a single stage two speed centrifugal supercharger and the maximum boost was 12lbs which gave 2800 hp at 5000 feet and 3500 rpm.

With the Pennine I'm guessing it's development path would have included a two-stage three speed supercharger with at least 25 - 30lb boost which might be why (as came out in our discussions about the RM.17SM) that when Fairey were working out the scheme for a Pennine Firefly, RR told them to calculate for a t/o power output of 3480 hp

Red Admiral

11th May 2009, 22:07

Scaling off the Merlin figures I get 2755hp at 30lb and 3070hp at 35lb for the Griffon.

I suspect the power for the Pennine would be more limited by cooling considerations.

Wuzak

12th May 2009, 02:31

Scaling off the Merlin figures I get 2755hp at 30lb and 3070hp at 35lb for the Griffon.

I suspect the power for the Pennine would be more limited by cooling considerations.

Yes, air cooled engines tend to need to have lower boost.

It is also difficult to compare dissimilar engines, particularly when one revs 25% more than the other.

Wuzak

12th May 2009, 03:44

The best method for comparing dissimilar engines is using the BMEP calculations.

Nick Sumner

12th May 2009, 04:02

The Pennine used forced air cooling. A fan was mounted concentrically with the prop shaft and turned at 1.03 times the engine speed. Wasn't a similar arrangement used in the BMW 801?

Nick Sumner

12th May 2009, 04:03

The best method for comparing dissimilar engines is using the BMEP calculations.

I'm not really sure how to calculate the BMEP of the Pennine.

Red Admiral

12th May 2009, 10:20

BMEP = Power * 960 / displacement * rpm for the results in Bar.

This works out as 16.7bar which is about the same as for the Centaurus at 2520hp. Looking at other air cooled engines you could probably get up to 20bar / 3350hp with some development.

Lightning

12th May 2009, 18:43

Hi Greg,

3) In the UK, they used gauge pressure, also in inches of Mercury. So, 9 pounds of boost in the UK is 9.00 + 29.92 = 38.92 inches of manifold pressure in the USA.

Standard Atmosphere = 14.7 psi

Add 9 psi = 23.7 psi

Multiply by Wuzak's conversion factor of 2.04 = 48.3 in Hg

i.e.

(14.7 + 9.0) x 2.04 = 48.3 (All values rounded for simplicity.)

This agrees with Wuzak's calculation and with Kutcha's conversion chart.

Just another way of looking at it.

Regards,
Lightning

Nick Sumner

12th May 2009, 18:52

Yes, air cooled engines tend to need to have lower boost.

That's a good point, the highest boost pressure I can find in an air cooled engine is 16.5 lb in one version of the Hercules. The Centaurus topped out at 13.5 lb and the R2800 at the equivalent of 13.3 lb. The maximum figure I can find for the BMW 801 is 1.75 ATA, which equates to about 10.2 lbs.

Lightning

12th May 2009, 18:53

Hi again, Greg,

Many Allisons had a 9.5-inch impeller because the Allison was intended from the outset to be turbocharged. The war department decided ot use the available turbocargers for the bombers to be used in Europe, and deleted the turbocharger from the Allison-powered fighters. That left the Allison a bit underboosted and a bit anemic above about 12,000 - 15,000 feet. If it HAD the turbocharger, it would have been a greater engine in history. (Italics mine.)

Don't forget my P-38. :)

Regards,
Lightning

Nick Sumner

12th May 2009, 18:54

Red Admiral

12th May 2009, 21:13

Thanks for this - is that displacement in cubic inches or litres?

Litres. I'm sure I could dig out a formula for bmep in psi and displacement in cu in.

Sid447

13th May 2009, 11:13

BMEP = Peak Torque in lbs-ft x 2473 and divided by ccm. This gives figure in p.s.i.

1] Liquid-cooled engines will always be able to run higher specific output per cid/capacity than air-cooled types.

2] Ram-air effect can also give positive MAP readings. The air intakes on F1 cars were once said to be worth around 80hp because of this.

Interesting read here:- http://www.supercoolprops.com/articles/gwhite_reno.php
(just over half-way down the page where a foto of a modified Sea Fury "September Fury" mentions the effect of ram-air).

3] Allisons were a good engine. In some respects better-built (and sweeter running) than the Merlins. But the supercharger impeller design and technology was at RR.
The big advance in hp with the Merlin XX series was the result of supercharger entry, impeller tip and blade curvature designs by (sir) Stanley Hooker.
It would be safe to say if both Allison and Merlin were of equal cid (or even if they stayed as is!) ...and run at the same rpm and boost the Merlin would be making a good bit more hp, purely because of the supercharger design being much further ahead (This all started with the RR-Buzzard race engine for the Schneider Trophy planes, so RR had got into their stride by WW2).

4] A two-stage two-speed supercharger WILL absorb more crankshaft hp than a single stage single speed, or two speed at the same rpm.

5] On intercoolers and after-coolers.....

A coolant jacket in the supercharger housing acts as the intercooler between the two stages, but it really does not accomplish much. Most of the heat from compression is rejected via the after-cooler. This is a rectangular, boxy-looking heat exchanger that sits on top of the engine towards the rear. It is a radiator core with compressed fuel/air mixture on the outside of the radiator tubes and coolant flowing though the tubes
which is all temperature stabilized by a system radiator fitted in the airstream.

Most forced induction car and truck engines use what should correctly be termed as an after-cooler.

Wuzak

14th May 2009, 02:49

1] Liquid-cooled engines will always be able to run higher specific output per cid/capacity than air-cooled types.

Correct

2] Ram-air effect can also give positive MAP readings. The air intakes on F1 cars were once said to be worth around 80hp because of this.

WW2 aero engine manufacturers were well aware of ram air effects. The Supermarine S6B used it, as I'm sure did most of its contemporaries.

3] Allisons were a good engine. In some respects better-built (and sweeter running) than the Merlins. But the supercharger impeller design and technology was at RR.
The big advance in hp with the Merlin XX series was the result of supercharger entry, impeller tip and blade curvature designs by (sir) Stanley Hooker.
It would be safe to say if both Allison and Merlin were of equal cid (or even if they stayed as is!) ...and run at the same rpm and boost the Merlin would be making a good bit more hp, purely because of the supercharger design being much further ahead (This all started with the RR-Buzzard race engine for the Schneider Trophy planes, so RR had got into their stride by WW2).

Allison V1710s were a good engine, and were very close in capacity to the Merlins. They also did have some advantages over the Merlin.

Due to the political situation in the US in the '30s (isolationist) compared with the UK (we're probably going to war in a few years) the Allison never had the funding and aggressive development of teh Merlin.

4] A two-stage two-speed supercharger WILL absorb more crankshaft hp than a single stage single speed, or two speed at the same rpm.

A two stage supercharger will absorb less power for a given boost, or produce more boost for a given power absorption. If they didn't there would be no point in having them.

5] On intercoolers and after-coolers.....

A coolant jacket in the supercharger housing acts as the intercooler between the two stages, but it really does not accomplish much. Most of the heat from compression is rejected via the after-cooler. This is a rectangular, boxy-looking heat exchanger that sits on top of the engine towards the rear. It is a radiator core with compressed fuel/air mixture on the outside of the radiator tubes and coolant flowing though the tubes
which is all temperature stabilized by a system radiator fitted in the airstream.

Most forced induction car and truck engines use what should correctly be termed as an after-cooler.

You have described the Rolls Royce methods of intercooling and aftercooling. Some othe manufacturers with twin stage supercharged engines didn't bother with the intercooler, and relied heavily on ADI for aftercooling.

Turbocharged engines can be thought of as twin stage engines too, btw.

Red Admiral

14th May 2009, 11:20

Allison V1710s were a good engine, and were very close in capacity to the Merlins. They also did have some advantages over the Merlin.

Due to the political situation in the US in the '30s (isolationist) compared with the UK (we're probably going to war in a few years) the Allison never had the funding and aggressive development of the Merlin.

I think I'd also argue that the US didn't have the same experience with developing supercharged liquid cooled inlines. They pretty much gave up in the early 1920s in favour of radials so its not much of a surprise that the next offering wasn't quite as good as contemporaries.

A two stage supercharger will absorb less power for a given boost, or produce more boost for a given power absorption. If they didn't there would be no point in having them.

It also depends on what pressure ratio or boost you need to supply. For low pressure ratios the single stage is better. In the Merlin type with superchargers mounted back to back you get losses in the 180° turn into the second supercharger impeller. In other types you get losses in the ducting between the two superchargers.

ChrisMcD

14th May 2009, 12:55

I think I'd also argue that the US didn't have the same experience with developing supercharged liquid cooled inlines. They pretty much gave up in the early 1920s in favour of radials so its not much of a surprise that the next offering wasn't quite as good as contemporaries.

Hi RA,
I would also suggest that Allison, as part of GE, were happy to rely on their parent organisation's expertise with turbochargers - after all the P 38 with turbocharged Allisons was a very nice piece of work (thanks Lightning). It was hardly their fault that the USAAF decided to give turbocharger priority to bombers.

Taking your point about radials, P&W were competing hard with Wright and that seems to have led to some very good superchargers - including some of the best of the early two stage ones - like the Twin Wasp R-1830-76 on the F4F-3 Wildcat.

Red Admiral

14th May 2009, 17:48

Speaking of turbochargers, I found the ultimate design from Bristol. It was intended to be mounted in the rear of a Buckingham nacelle and feed the Centaurus at the front. It consisted of a W.1 gas turbine and a retractable ventral intake. There were a couple of schemes but the W.1 provided high pressure air and extra thrust at the same time. It was able to raise the critical altitude of the Centaurus from ~18,000ft to 56,000ft!

Wuzak

15th May 2009, 03:19

Hi RA,
I would also suggest that Allison, as part of GE, were happy to rely on their parent organisation's expertise with turbochargers - after all the P 38 with turbocharged Allisons was a very nice piece of work (thanks Lightning). It was hardly their fault that the USAAF decided to give turbocharger priority to bombers.

Taking your point about radials, P&W were competing hard with Wright and that seems to have led to some very good superchargers - including some of the best of the early two stage ones - like the Twin Wasp R-1830-76 on the F4F-3 Wildcat.

Allison were part of GM - not GE.

Wuzak

15th May 2009, 03:23

It also depends on what pressure ratio or boost you need to supply. For low pressure ratios the single stage is better. In the Merlin type with superchargers mounted back to back you get losses in the 180° turn into the second supercharger impeller. In other types you get losses in the ducting between the two superchargers.

Yes, but each impeller will be smaller and have a smaller pressure ratio than a single impeller. It is true, however, that as the boos pressure is reduced the advantage of the two stage is reduced.

FWIW when the Merlin went initially two a two stage supercharger it maintained the same maximum boost - 18psi IIRC. Yet it gained almost 200hp....

Nick Sumner

15th May 2009, 03:31

Nick Sumner

15th May 2009, 03:41

Allison were part of GM - not GE.

were Allison GM's only aircraft engine manufacturer?

When we read of the GM/Bristol plan to build the Centaurus, would it have been Allison building them?

Wuzak

15th May 2009, 08:37

were Allison GM's only aircraft engine manufacturer?

When we read of the GM/Bristol plan to build the Centaurus, would it have been Allison building them?

Some (all?) of GM's auto divisions built aero engines under licence during WW2, I believe. Chevrolet built some Pratt & Whitney R2800s IIRC (just not as many as Ford did).

ChrisMcD

15th May 2009, 11:41

Allison were part of GM - not GE.

Sorry, my bad.

Mind you, I still think they were relying on turbochargers being available.

Red Admiral

15th May 2009, 11:48

http://i25.photobucket.com/albums/c84/AviationImages/superchargerefficiency.jpg

Comparative supercharger performance showing superiority of single stage engines at low pressure ratios. Things aren't the same between all the types above, with different impellers and intakes. With a proper comparative diagram with all things equal, the superiority of the single stage superchargers would be more noted at low pressure ratios.

Woah! Where did you find that?

I obtained a few copies of RRHT Bristol's magazine the other day which have lots of little gems in them. I'll have a go at scanning the pictures and putting some more information up.

Sid447

15th May 2009, 16:11

WW2 aero engine manufacturers were well aware of ram air effects. The Supermarine S6B used it, as I'm sure did most of its contemporaries.

Agreed,
this was in response to "GregP" mentioning in post #3 ...."Anything over 29.92 is starting to be a charged intake and probably is due to a supercharger or turbocharger ... there being no OTHER way to charge the intake......."

A two stage supercharger will absorb less power for a given boost, or produce more boost for a given power absorption. If they didn't there would be no point in having them.

I'll dis-agree with you on this.
From my perspective it isn't about how much boost a a single-stage or twin-stage supercharger is giving and where, altitude-wise.
As that is affected also by gearing, ratios and impeller size.

The actual benefits to the engine of either a single or twin are tailor-made to where you want full throttle altitude(s).

Mechanically speaking, if a crank has more hardware to turn, there is more drive-train loss at the same constant....
Therefore a heavier more complex, two stage centrifugal super-charger, because of the increased rotational friction will absorb more power to drive it than a single-stage type will.
It would also suffer more boost pressure loss going from stage to stage and then again through an inter-cooler (if fitted) which all cause restrictions to airflow.

A simple single stage, single speed type would from that point of view have the least amount of drive-train loss for a given amount of crank rpm.
And would be more efficient at it's rated altitude than an identical engine fitted with a two-stage, two-speed type that had the same FTA.

GregP

16th May 2009, 09:04

The only reason to have a two-stage or multi-speed boost device for a piston engine is to get altitude performance. yes, you CAN get some sea level power from it, too, but that can be gotten with just a bigger engine or turning more RPM or gearing it.

The multi-stage turbosupercharger engine's reason to be is for higher altitude performance. Many turbosupercharged engines made more power at their critical altitude than at takeoff.

The European theater of operations was a high-altitude theater. The Russian Front was not.

it was a toss-up in the Pacific theater, with the B-29s high because they could GET there, and the Japanese trying to climb to altitude and still be fast enough to catch the B-29s. Mostly, they weren't.

So, most of the high-altitude combat that took place, took place over Europe, with Mustangs, P-47s, Focke-Wulfs, Spifires, Mosquitos, and a few Ta-152s (not many ever), and a handful of other fighter types. The real high-altitude types were the recon planes like the Junkers Ju-86P and a few other German types, but they were too few to do much more than annoy the opposite side.

True, the Me-163s could get to altitude in record time, but their endurance as in minutes, and not even tens of minutes.

Many like to tout the Ta-152s, but too few ever saw service to be of even insignificant use to the Germans. They were GREAT airplanes ... just not available for combat except in limited, prototype batches not suited to actually fight or win an air war. They were too few to even defend their own airfields or even to have supplies of spare parts ..., much less repairs for a damaged Ta-152. So, their first mechanical glitch was usually the end of their flying career. It hapened frequently, as was the case for most new designs that saw only limite production.

So, the boosted engines could get you up to a specific high altitude where, theoretically, you could outfight the opponent,

Wuzak

18th May 2009, 02:33

Wuzak

18th May 2009, 02:43

Agreed,
this was in response to "GregP" mentioning in post #3 ...."Anything over 29.92 is starting to be a charged intake and probably is due to a supercharger or turbocharger ... there being no OTHER way to charge the intake......."

I'll dis-agree with you on this.
From my perspective it isn't about how much boost a a single-stage or twin-stage supercharger is giving and where, altitude-wise.
As that is affected also by gearing, ratios and impeller size.

The actual benefits to the engine of either a single or twin are tailor-made to where you want full throttle altitude(s).

Mechanically speaking, if a crank has more hardware to turn, there is more drive-train loss at the same constant....
Therefore a heavier more complex, two stage centrifugal super-charger, because of the increased rotational friction will absorb more power to drive it than a single-stage type will.
It would also suffer more boost pressure loss going from stage to stage and then again through an inter-cooler (if fitted) which all cause restrictions to airflow.

A simple single stage, single speed type would from that point of view have the least amount of drive-train loss for a given amount of crank rpm.
And would be more efficient at it's rated altitude than an identical engine fitted with a two-stage, two-speed type that had the same FTA.

For a given boost level and rpm the single stage supercharger impeller has to be larger than either of a two stage supercharger's impeller.

If it is the same size then it needs to run at higher rpm to get the boost.

Either way, the supercharging part is higher than for a two stage supercharger, and far more significant than mechanical power losses.

The reason why two stage engines were better for high altitude is that they could gain the necessary boost with less power lost - high altitude operations required far higher pressure ratios.

At low boost pressures the mechanical losses are more significant in the power required for the supercharger.

A similar thing is the power required to drive the car. The mechanical losses that need to be overcome are more or less directly proportional to the speed of the car. The power to overcome aerodynamic drag, however, is roughly proportional to the cube of the vehicle speed. So the car doesn't have to be going very fast for the drag to be a more significant factor than the mechanical losses.

Wuzak

18th May 2009, 06:30

Wuzak

18th May 2009, 06:39

I'll dis-agree with you on this.
From my perspective it isn't about how much boost a a single-stage or twin-stage supercharger is giving and where, altitude-wise.
As that is affected also by gearing, ratios and impeller size.

The actual benefits to the engine of either a single or twin are tailor-made to where you want full throttle altitude(s).

Mechanically speaking, if a crank has more hardware to turn, there is more drive-train loss at the same constant....
Therefore a heavier more complex, two stage centrifugal super-charger, because of the increased rotational friction will absorb more power to drive it than a single-stage type will.
It would also suffer more boost pressure loss going from stage to stage and then again through an inter-cooler (if fitted) which all cause restrictions to airflow.

A simple single stage, single speed type would from that point of view have the least amount of drive-train loss for a given amount of crank rpm.
And would be more efficient at it's rated altitude than an identical engine fitted with a two-stage, two-speed type that had the same FTA.

My earlier thoughts ignored the mechanical friction losses, as I thought that they were insignificant. I was both right and wrong. As Red's graph shows there is a point where the single stage is more efficient than the twin stage, this being at low pressure ratios. That corresponds to where the friction losses are outweighed by the work required to sompress the gas.

Lightning

19th May 2009, 18:59

Hi Wuzak,

Yes, but each impeller will be smaller and have a smaller pressure ratio than a single impeller. It is true, however, that as the boos pressure is reduced the advantage of the two stage is reduced.

FWIW when the Merlin went initially two a two stage supercharger it maintained the same maximum boost - 18psi IIRC. Yet it gained almost 200hp....

All other things being equal, what would the respective parasitic power consumptions be between a single- and a two-stage supercharger? That is, for a given boost, what power would be consumed by each? Could you give an actual example? If actual values are unavailable, the percentage difference would be informative.

I believe that at low altitudes the single-stage supercharger is superior since the second stage is not necessary.

Regards,
Lightning

Kutscha

20th May 2009, 05:27

To drive the single stage supercharger on the Merlin took ~150hp. For the 2 stage, it was ~400hp.

Wuzak

20th May 2009, 06:27

To drive the single stage supercharger on the Merlin took ~150hp. For the 2 stage, it was ~400hp.

At what boost levels?

Wuzak

20th May 2009, 07:12

There is an article about the development of the Merlin by Lovesey at the Spitfire Performance page:

http://www.spitfireperformance.com/

It is under the Articles heading.

I quote:

During the course of research and development on superchargers it became apparent to us that any further increase in the altitude performance of the Merlin engine necessitated the employment of a two-stage supercharger. It is an easy matter to increase the compression ratio of the supercharger, in other words the boost pressure obtained at altitude, but naturally in order to get more horsepower out of the engine this has to be done efficiently in order to avoid wasted power in driving the blower and excessive temperature of the charge for the compression ratio obtained. It would appear that the limit of compression ratio for the single stage supercharger is about 4:1 and for compression ratios beyond this it is necessary to go to two stages. This is shown clearly in illustration number 11 [the graph that RA posted above]

The article then discusses the thought given to utilising exhaust turbine superchargers, and the development process, then continues:

The gain in engine power at 30,000ft with the two-stage supercharger was 300hp compared with the Merlin 46, a gain of nearly 50 per cent of power. (Merlin 61, 1,020hp at 30,000ft; Merlin 46, 720hp at 30,000ft)

The reason that single stage superchargers are generally thought to be better at low altitudes is because the boost pressure is limited down low by fuels. Later in teh war with improved grades of fuel the Merlin 66 was the low altitude Merlin, and it produced as much as 2200hp on 150 grade fuel, much more than could be achieved with a single stage engine.

I think Kutscha's numbers are misleading, as they are very dependent on boost.

GregP

20th May 2009, 07:20

Wuzak

20th May 2009, 07:28

Driving the second stage of a Merlin at the boost levels seen at the Reno Air Races today takes ~800 HP. Of course, they are running 140 inches or more and making 3,000 to 3,500 HP out of a racing Merlin.

Those numbers are from Joe Yancey of Yancey's Allison, and he has seen them on the dyno. Therefore, a 3,500 HP Merlin at Reno delivers about 2,450 HP to the propeller. The rest is first and second stage supercharger losses.

I'd suggest that the 3500hp figure is the nett value, and that without the supercharger drive losses the engine would be making 4300hp.

Using a single stage supercharger to get those boost levels/MAPs would have much higher levels of supercharger losses.

Kutscha

20th May 2009, 15:52

I think Kutscha's numbers are misleading, as they are very dependent on boost.

Certainly the numbers are. I can't say for sure but would say the numbers are at maximum boost.

GregP

21st May 2009, 03:17

Hi Wuzak,

If the information were not from Joe Yancey, I might agree.

Since it is from Joe directly in conversation specifically about Reno engines, and since he has a long Reno history, and has built Allisons, Merlins, Griffons, and radials, I tend to believe him at face value.

Wuzak

22nd May 2009, 05:29

Lightning

22nd May 2009, 17:55

Hi Wuzak

I'd suggest that the 3500hp figure is the nett value, and that without the supercharger drive losses the engine would be making 4300hp.

I'm not disputing the 800hp figure, just wondering if the 3500hp he quoted is at the prop.

I was reading about a proposed (circa 1998) unlimited Reno racer design called "Intrepid" which was designed by a Mr. Hal Dantone. It was to be powered by a Merlin 9 engine having a displacement of 1640 cu. in., a two-stage supercharger, and giving a maximum power of 3500 hp.

In another article (this one on racing boats) it was stated that the Merlin, which delivered around 3500 hp in racing airplanes, was able to produce about 4000 hp in these boats because of turbo charging.

From the foregoing, I would say that the 3500 hp stated by Greg P was the power produced by the engine and not that delivered to the propeller.

Regards,
Lightning

Kutscha

22nd May 2009, 18:50

Red Admiral

23rd May 2009, 00:50

Would that be the Packard-Merlin V-1650-9

Yes. The equivalent of the 100 series Merlins.

Given the wartime power of 2640hp I think its likely that 3500hp net could be produced. I imagine most from titanium cranks allowing the engine to run a lot faster, though heavy ADI would help as well.

GregP

23rd May 2009, 05:01

There isn't a Merlin that can produce 3500 HP at the propeller.

Want 3500 HP at the prop? Get a Wright R-3350 or a very fresh, very rare R-2800 turbo-counound that doesn't exist anymore.

Want 3000HP at the prop? Get an R-4360.

Want 2650HP at the Prop? A good, well-built Merlin can deliver it for a while. So can a good, well-built Allison. If you build it right, you might even get 2850 HP at the prop with a good well-built Allison at Reno. The Merlin's second-stage supercharger simply prohibits getting much more than about 2700 HP at the prop ... but it can make make about 3500 HP internally with S/C and other losses taking the rest, even with ADI and a spray bar.

Now, if you go to 25,000 feet, the Merlin has a real advantage ... unless you turbocharge the the Allison. Still, these are FAR from wartime stock engines.

Heck, the REAL racing Mouse Merlins use Allison G-series rods! The Merlin rods can't handle the power. Ask a Reno Merlin crew chief with a real Mouse Motor.

In the last 20 years, can you tell me how often the Championship at Reno has gone to a Merlin ... if the R-3350 or R-4360 didn't break?

There were some really great piston engines in development that were simply abandoned when jets took over. It's VERY possible some of them could have produced 5000+ HP in stock form ... but the potential sales just didn't justify the development after jets blew away the 600 mph barrier.

While it MAY be possible for a propeller driven, piston powered aircraft to go 600 mph, the cost to anyone but a government would be prohibitive. Jets routinely did that in the late 1940's, so the urge to DO IT with pistons simply evaporated in the light of reality.

Maybe we could win Reno with a Tupolev Bear turboprop! 50,000 smooth HP!

Wuzak

25th May 2009, 04:17

Hi Wuzak

I was reading about a proposed (circa 1998) unlimited Reno racer design called "Intrepid" which was designed by a Mr. Hal Dantone. It was to be powered by a Merlin 9 engine having a displacement of 1640 cu. in., a two-stage supercharger, and giving a maximum power of 3500 hp.

In another article (this one on racing boats) it was stated that the Merlin, which delivered around 3500 hp in racing airplanes, was able to produce about 4000 hp in these boats because of turbo charging.

From the foregoing, I would say that the 3500 hp stated by Greg P was the power produced by the engine and not that delivered to the propeller.

Regards,
Lightning

Your example actually supports the opposing hypothesis. If the engine power quoted was at the engine and not the prop then the engine would produce 3500hp regardless of whether it has a supercharger or turbocharger. That the engine made 4000hp with a turbo suggests that 500hp in losses were removed - ie the supercharger.

Wuzak

25th May 2009, 04:32

Driving the second stage of a Merlin at the boost levels seen at the Reno Air Races today takes ~800 HP. Of course, they are running 140 inches or more and making 3,000 to 3,500 HP out of a racing Merlin.

Those numbers are from Joe Yancey of Yancey's Allison, and he has seen them on the dyno. Therefore, a 3,500 HP Merlin at Reno delivers about 2,450 HP to the propeller. The rest is first and second stage supercharger losses.

If you download the Lovesey article, Fig 24 shows that an RM.17SM did 2380hp for 50 minutes on 150 grade fuel - I believe at 30psi boost and 3300rpm.

The RM.17SM also did 2640hp at 36psi boost and 3150rpm, for 15 minutes. Both runs used PN150 fuel and ADI.

"140 inches" equates to 54psi boost, or 68.8psi absolute. That is 1 1/3 more MAP than the RM.17SM, which would mean approximately 1 1/3 the power. Which works out at about 3525hp.

I am quite sure that the RM.17SM numbers quoted are at the prop.

Wuzak

25th May 2009, 04:39

There isn't a Merlin that can produce 3500 HP at the propeller.

Want 3500 HP at the prop? Get a Wright R-3350 or a very fresh, very rare R-2800 turbo-counound that doesn't exist anymore.

Want 3000HP at the prop? Get an R-4360.

Want 2650HP at the Prop? A good, well-built Merlin can deliver it for a while. So can a good, well-built Allison. If you build it right, you might even get 2850 HP at the prop with a good well-built Allison at Reno. The Merlin's second-stage supercharger simply prohibits getting much more than about 2700 HP at the prop ... but it can make make about 3500 HP internally with S/C and other losses taking the rest, even with ADI and a spray bar.

Now, if you go to 25,000 feet, the Merlin has a real advantage ... unless you turbocharge the the Allison. Still, these are FAR from wartime stock engines.

As has been shown at high boost pressures the two stage supercharger is far more efficient than a single stage supercharger. ie it would require a lot more power to make "140 inches" of boost from a sngle stage impeller than a two stage impeller, because it would have to be a) bigger diameter, b) spinning faster, or c) both bigger and spinning faster to get the required pressure ratio.

Note that the Merlin 66 was cleared for 2200hp at sea level by the end of the war, running PN150 fuel and ADI. And that RM.17SM managed its 2600hp at low altitude also.

GregP

25th May 2009, 07:32

Wuzak

25th May 2009, 08:19

Thank you for making my point Wuzak.

We don't have any better gasoline today than 150 Octane, and we don't get anymore than about 2650 hp at the prop at Reno's altitude, second stage nothwithstanding.

Now an Allison, if built correctly, could probably get about 2950 hp at the propeller at Reno ... but would fall short at 25,000 feet without a turbo or a second-stage supercharger. Also, the Allison would run 110 - 120 inches instead of 140 inches, and would run at probably 3200 - 3300 RPM.

Did I make your point???

I would guess that the energy content of the fuel today would be similar to the PN150, but without the octane rating. That's where the copious ADI comes in.

So, if you can run 140 inches MAP in your Merlin you are basically putting in 1.3 times the air that the RM.17SM. That means you can put in 1.3 times the fuel, and make roughly 1.3 times the power.

Wuzak

27th May 2009, 05:32

Thank you for making my point Wuzak.

We don't have any better gasoline today than 150 Octane, and we don't get anymore than about 2650 hp at the prop at Reno's altitude, second stage nothwithstanding.

Now an Allison, if built correctly, could probably get about 2950 hp at the propeller at Reno ... but would fall short at 25,000 feet without a turbo or a second-stage supercharger. Also, the Allison would run 110 - 120 inches instead of 140 inches, and would run at probably 3200 - 3300 RPM.

Let's put it this way...

If fuels are no better than for WW2, why would we use an engine running 140inHg MAP/54psi boost to achieve the same power that can be had with 103inHg MAP/36psi boost (which was held for 15 minutes continuously, btw)?

Where do you get the Allison numbers from Greg?

GregP

27th May 2009, 06:34

Hi Wuzak,

From Joe Yancey of Yancey's Allison V-12 1710's.

No one has paid him to build such a "Reno" Allison variant, but he already has engineering fixes for the very few known Allison weaknesses, and development is complete.

Before you ask, Joe's engineering fixes are for those people who want to buy one and race it, not for internet dissemination. If I gave away the beans, I'd never get another fact.

Suffice to say that he demonstrated them to me to my satisfaction as an engineer, with the provision that I not give away the story. There are already enough people out there building Allisons the WRONG way, and Joe would like to cement the Allison reputation by adding his own variety of power and reliability to the equation.

One thing to note above ALL the other Allison builders ... Joe Yancey's engines come with a WARRANTY. Try getting THAT from the other guys, especially for the number of hours Joe will stand behind his work, personally.

Oh yeah, if you want to get a serious Allison of your very own, I have Joe's address and phone number. Post here. I'll see it and pass it on. Serious inquiries only, please.

Wuzak

27th May 2009, 10:12

Here's an article that may shed some light on whether the 3500hp is nett or gross:

http://machinedesign.com/article/unlimited-air-racers-the-ultimate-hot-rods-0418

Two of the fastest aircraft on the circuit are Critical Mass and Dago Red. Critical Mass is a TMK-20 Sea Fury, a British fighter carrying an air-cooled radial engine with two offset rows of nine cylinders for a total displacement of 3,350 cu in. Dago Red, a P-51 Mustang, carries a water-cooled engine, a 60°, 1,650-cu-in. V12 Rolls-Royce Merlin engine. This is stock displacement for both engines. With all the other modifications, Dago Red claims 3,600 hp, a significant increase over its original 2,200 hp, and Critical Mass ups the ante to 4,000 hp.

They burn 180-octane aviation fuel, a special air-race brew that contains manganese. During a race, Dago Red carries 110 gallons of fuel.

Both planes use pressurized fuel injection, not the electrical injection used on cars, and both run mechanical superchargers. Dago Red, for example, gets a 75-psi boost with manifold pressure between 140 and 150 in. of mercury. Its engine uses a two-stage two-speed centrifugal supercharger that is always kept in the low range — it runs at 23,000 rpm. The high manifold pressures, however, produces blow-by, and some of the air-fuel mixture is forced past the piston rings. To take care of the problem, Dago Red has an oil separator to remove oil from the crankcase breather system. The pump in this system pressurizes the tank to remove entrained air from the oil.

The Dago Red team does not use turbochargers because the engine compartment and structure can't dispose of the extra heat. Turbocharging would also mean extra cooling for the lubrication oil.

Antidetonation injection (ADI) prevents premature detonation due to the high manifold pressures. Dago Red carries 60 gallons of an ADI mixture, 50% water, and 50% methanol, that is injected into the cylinders during the race.

Lightning

27th May 2009, 21:25

Hi Wuzak,

Your example actually supports the opposing hypothesis. If the engine power quoted was at the engine and not the prop then the engine would produce 3500hp regardless of whether it has a supercharger or turbocharger. That the engine made 4000hp with a turbo suggests that 500hp in losses were removed - ie the supercharger.

Maybe we are talking about different things. Usually when the power of an airplane engine is given, it is given in "brake horsepower" which is the power at the crankshaft before losses due to driving accessories are taken into consideration. This is not the "shaft horsepower" that is delivered to the prop.

Because of large prop diameter and engine speed on the order of 3000 rpm, the speed of the prop on fighters and unlimited racers had to be reduced by half or more in order to keep tip speed below the speed of sound. (I know that you are fully aware of this, so please bear with me.) Engine-driven accessories--to include the propeller speed reduction gearing--lowered the power that was actually delivered to the prop itself.

I don't know about the airplanes under discussion, but some prop-speed reduction mechanisms can consume up to 10% of their input power. In the present case, this would amount to 350 hp thereby reducing power to the prop to 3150 hp. Subtract the power consumptions of any other engine-driven accessories, and this could easily drop below 3000 hp.

The mechanical supercharger certainly consumes considerable power, but it gives back more than it receives. It's not a matter of the engine's being able to provide 800 more horsepower if the supercharger were removed since without it, the 3500 bhp would have never been available in the first place. Also, is the supercharger considered an "accessory"? It was, after all, an integral part of the engine as delivered from the factory, and it was was always in play when the Merlin's specifications were stated.

As I said, maybe we are talking about different things.

Regards,
Lightning

Wuzak

28th May 2009, 04:22

Hi Wuzak,

Maybe we are talking about different things. Usually when the power of an airplane engine is given, it is given in "brake horsepower" which is the power at the crankshaft before losses due to driving accessories are taken into consideration. This is not the "shaft horsepower" that is delivered to the prop.

Because of large prop diameter and engine speed on the order of 3000 rpm, the speed of the prop on fighters and unlimited racers had to be reduced by half or more in order to keep tip speed below the speed of sound. (I know that you are fully aware of this, so please bear with me.) Engine-driven accessories--to include the propeller speed reduction gearing--lowered the power that was actually delivered to the prop itself.

I don't know about the airplanes under discussion, but some prop-speed reduction mechanisms can consume up to 10% of their input power. In the present case, this would amount to 350 hp thereby reducing power to the prop to 3150 hp. Subtract the power consumptions of any other engine-driven accessories, and this could easily drop below 3000 hp.

The mechanical supercharger certainly consumes considerable power, but it gives back more than it receives. It's not a matter of the engine's being able to provide 800 more horsepower if the supercharger were removed since without it, the 3500 bhp would have never been available in the first place. Also, is the supercharger considered an "accessory"? It was, after all, an integral part of the engine as delivered from the factory, and it was was always in play when the Merlin's specifications were stated.

As I said, maybe we are talking about different things.

Regards,
Lightning

I doubt that the supercharger hp is excluded from the quoted hp of an engine. The supercharger hardly counts as an accessory.

My understanding was that brake horsepower was the power measured using a brake system at the power takeoff (crank), and that some standards would test the engine with accessories (altenator/generator, water pump, etc) and others would exclude them. Wiki says that the gross hp (without accessories) was the normal brake hp.

Quoting higher hp numbers that comes with the gross figure is great for marketing, but not so useful for estimating aircarft performance.

GregP

28th May 2009, 04:56

Hi Wuzak,

A 3,600 HP Merlin delivers about 2,800 HP to the propeller.

There is no gasoline with an Octane rating of 180. Anything over 100 is not an Octane rating; it is a performance rating. As far as I know, the highest available that is real is about a 160 performance rating.

No one I have ask in the aviation industry so far has heard of 180 either.

Maybe its like horsepower ratings back in the 1960's .... Blanton-power or Blanton-Octane rating?

I'll ask Steve Hinton this weekend. He starts the Reno races every year in our T-33 jet.

Lightning

28th May 2009, 18:21

Hi Wuzak

I doubt that the supercharger hp is excluded from the quoted hp of an engine. The supercharger hardly counts as an accessory.

This was exactly my point. When all is said and done, the engine's power (brake horsepower) is quoted at the crankshaft regardless of how that power was arrived at. The supercharger is just a part of the overall system. My position falls somewhere between yours and Greg's. What I'm saying is that the stated power is at the input to the reduction gearing (the crankshaft) and is greater than that supplied to the propeller (shaft horsepower). The losses are due to the reduction gearing and any other power-draining accessories that are not taken into consideration when arriving at brake horsepower.

My understanding was that brake horsepower was the power measured using a brake system at the power takeoff (crank), and that some standards would test the engine with accessories (altenator/generator, water pump, etc) and others would exclude them. Wiki says that the gross hp (without accessories) was the normal brake hp.

It says that auto makers before 1972 used brake horsepower that was "frequently referred to as SAE gross horsepower." This is more a matter of jargon than of technical definition.

Quoting higher hp numbers that comes with the gross figure is great for marketing, but not so useful for estimating aircarft performance.

Go back and check those definitions given by Wiki, especially the one for shaft horsepower. Since this is the power where the "rubber meets the road," it is the most significant in determining the airplane's speed.

Let's let Greg check with Steve Hinton to see what he has to say on the subject. Maybe we're all a little bit right and a little bit wrong.

Regards,
Lightning

Sid447

28th May 2009, 19:24

Very interesting thread!

I'd just like to ask a simple question here.... the merlin 45M and 50M were optimised for max power at low level and would imagine these suffered the lowest drivetrain loss for net hp made.

Yes understand boost was lower at lower altitudes, it had to be with the higher content of oxygen.

As I understand it quoted hp is at the power take-off point. Which in automobile engines would be the flywheel end of the crank. I would have thought there must have been some understanding of quoted hp from different manufacturers even in those days and that hp must surely have been quoted at the power take-off point of an aero engine, the prop shaft. Thus having a standard measure.
Even though fitting a metal or wooden prop would absorb different amounts of some of that power.
Does that make any sense? :)

GregP

29th May 2009, 03:35

Wuzak

29th May 2009, 07:07

Hi Sid,

As I understand it, the WW2 V-12 engines all produced about 1,000 HP when released, give or take a bit. Then, the manufacturers started to be asked for more power (go figure ...) and started development.

When they went to 2-stage superchargers, they needed to know what was happening at expected shaft speed, so they ran the superchargers at the expected shaft speed and measured the power it took to run them. Then they measured the shaft horsepower and added the supercharger horsepower to get the big numbers.

That's how they found out the practical limit of supercarged engines was about 6,000 HP. Anything more than about 6,000 Hp and they didn't net anymore at the shaft ... it was all eaten up by the mechanical boost mechanism.

The Lycoming R-7750 was really the limit of piston development for aircraft. It produced about 5,000 HP when developed and then development was dropped in favor of jets. The expectation was about 6,000 to 6,500 HP at the shaft, but they never got better than 4,800 to 5,000 before the end of the program. One survives in the US National Air & Space Museum for all to see.

The Russians made a bigger radial, but it was not an aircraft powerplant ... it was for tractor pulling at entertainment events.

Tell the top fuel dragsters that 6000hp is the limit for supercharged engines - since their supercharged engines make 8000hp+.

Their is a point where more boost becomes pointless, as losses equal the gains. But if you can make your engine larger, or make it run faster, you can get a supercharger which makes the same boost but gives a greater mass flow rate of air and thus more power.

Boost is also limited by the fuel and its resistance to detonation.

Rolls Royce definitely tested their superchargers separately to map its performance parameters - mass flow rate/volumetric flow rate, pressure ratio, efficiency etc.

But when they tested the two stage Mk61 Merlin for service approval they tested it as a complete engine with the 114 hour type approval test, which is how engine's power was determined.

The MkI Merlin was initially rated at 930hp on 87 octane fuel.

Wuzak

29th May 2009, 07:25

Yes understand boost was lower at lower altitudes, it had to be with the higher content of oxygen.

It is to do with the absolute pressure that can be put into the cylinder. The fuel quality is a big factor in determining how much Manifold Absolute Pressure (MAP) can be used. Boost is the gauge pressure, which is added to the air absolute pressure to determine the MAP. Air pressure reduces with altitude.

As I understand it, the Oxygen content of air remains pretty much 21% for the practical altitude of most WW2 aircraft.

The reason why mountain climbers can have difficulty at altitude is not because a lower percentage of oxygen in the air, but that their is less air to breathe.

I'd just like to ask a simple question here.... the merlin 45M and 50M were optimised for max power at low level and would imagine these suffered the lowest drivetrain loss for net hp made.

I believe the 45M and 50M were modifed in service 45 and 50 Merlins, with the supercharger impellers cut down to reduce the power loss.

In Merlins the supercharger is geared directly to the crank, in most case with two speeds. The engine was throttled so that the boost was limited to maintain MAP to a level which gave good performance but didn't risk damaging the engine.

By cutting down the impeller there was less boost being made, so the height at which full throttle could be used (which meant less restrictions and less power loss from the supercharger system) was lower. Most of the development of the Merlin was aimed at raising the full throttle height.

As I understand it quoted hp is at the power take-off point. Which in automobile engines would be the flywheel end of the crank. I would have thought there must have been some understanding of quoted hp from different manufacturers even in those days and that hp must surely have been quoted at the power take-off point of an aero engine, the prop shaft. Thus having a standard measure.
Even though fitting a metal or wooden prop would absorb different amounts of some of that power.
Does that make any sense? :)

Engine manufacturers often quoted a number of different hp levels:
Maximum climb power
Continuous climb power
Maximum continuous power
Max cruise power
Economical cruise power
Military power
War emergency power
Takeoff power

And the altitudes at which they achieved the power was also quoted.

For instance a Mk66 was type tested at 2200hp @ 2000ft and 2100hp @ 21000ft (IIRC).

Sid447

29th May 2009, 10:09

Tell the top fuel dragsters that 6000hp is the limit for supercharged engines - since their supercharged engines make 8000hp+.

Yes, and last all of two minutes I believe! If they get that far without a cylinder mis-fire. Running on 85% liquid nitro-methane which gives about twice the power of gasoline means each cylinder consists of almost pure liquid nitro which won't compress if there's no spark.

Nah, Wuzak! there's definately a lack of the ability to breath at higher altitudes due to lack of oxygen!

Over-boosting at low altitudes because of the denser air is the main limiting factor I think.
With a second speed s/c available this enables the aircraft to climb to a higher altitude where the faster speed gear can then pump the cylinders with the less dense air at a greater rate with much less danger of over-boosting and be able to recover most if not all of the power made at it's Full Throttle Altitude the power-plant made at say 5,000' in the lower-speed supercharger gear. (I don't know if this was widely accepted as M.S. gear and F.S. gear?).

45 and 50M modified at factory I would think. They are listed as a production variant in the RR Merlin books by RR themselves. IIRC they were validated by RR for use in both a clipped wing version of a Spitefire 5 (optimised for low-alt operation) and some naval aircraft that had a similar service requirement.
(I'd be amazed if service personnel were able to trim & re-balance a vibration critical component -impeller rotor, with an operational speed of over 20,000 rpm to anywhere near acceptable vibration levels).

I thought it was a actually a different overall size (diameter). A smaller rotor and housing?
Trimming blades on an impeller rotor using an un-modified housing would effectively lower the bosst pressure it was capable of making at the same rpm. i.e. make it less efficient.

:) Ah! In that case, maybe this was done so that 100% throttle could be used at lower altitudes without fear of over-boosting?

p.s. RR made a mini-version of this centrifugal gear driven supercharger for a 1500cc V16 BRM engine in the late 40's. The S/C operated at over 40,000rpm and much an incredible-sounding scream.
If anyone has never heard this it's well worth listening to (for engine junkies like me!) either search for "BRM V16 sound" or use the links on this site:-

http://www.billzilla.org/engread.htm

(Apologies in advance if this is naughty, but it's worth it! :))

Red Admiral

29th May 2009, 11:59

Brake horsepower for supercharged engines includes the losses caused by the supercharger. Typically the propeller is replaced by a friction or water brake to act as the load and the engine runs against this and the power output calculated.

Accessory drives for these sorts of piston engines amount to tens of horsepower at most which would probably be caused by an alternator generating current for radar. Engine pumps don't drain a significant fraction of the overall power.

The difference between bhp and shp available through the reduction gears is likely to be around 5%.

ChrisMcD

29th May 2009, 12:24

[QUOTE=Sid447 IIRC they were validated by RR for use in both a clipped wing version of a Spitefire 5 (optimised for low-alt operation) and some naval aircraft that had a similar service requirement.
[/QUOTE]

IIRC the FAA had 'tired' MkV's rebuilt as Seafires - hence the description; "clipped, cropped and clapped out"

GregP

30th May 2009, 04:43

Wuzak, let's be reasonable.

Top Fuel engines cannot run at full power for even one minute. Their typical full power run is about 5 seconds or less, and many do not make it that far.

That is hardly the engine you would want to fly behind on a 6-hour trip over an overcast, mountainous European countryside. Also, the fuel used cannot even run at 18,000 feet, much less 30,000 feet.

Aero engines need to demonstrate a useful life of at least 25 hours at full throttle and at least 15 minutes at emergency power before being adopted for use. Some makers used slightly different numbers, but the operative fact is life at full power and reliability.

Also, the Allison typically had a BMEP of 325 in later model. Formula 1 only makes about 220. So, let's stay firmly away from automotive engines where, if they fail, you pull over and park. Can't do that in a Mustang, a Spitfire, OR in a Bf 109.

Red Admiral

30th May 2009, 11:52

I'm not sure about the F1 engines being so poor, but its worth noting recent restrictions aimed at limiting power. RRHT has one of the last turbocharged F1 engines, a Cosworth. Its a 1.5L engine that puts out 1000hp giving a BMEP of 769psi. Most powerful V-1710 (apart from the turbocompound) gave 364psi and Merlin 432psi.

I am sure that if RR were able to produce an engine able to run for 15min at 2640hp it is possible to create more power today.

GregP

31st May 2009, 08:18

The BMEP for F1 engines is in the low 200's. not anywhere NEAR 700!

There is no piston engine that must live for longer than 5 seconds that is over 500.

Wherever you got that, it is VERY wrong ... unless it is a 2-second power rating.

I used to run a 450 HP Chevrolet 327 on the street, and it was only only 215. I never hit 6500 rpm for more than 2 - 3 seconds at a time. Fixed it every other weekend. F1 engines do 220 for 2 hours at 19,000 RPM. Those guys are GOOD.

A BMEP of more than 500 would self destruct in no time at all. Never been built!

Check your math VERY carefully. The best mechanics in the WORLD can't do that. Not even close. Lest you ask, I BOUGHT the 450 HP 327 ... I didn't build it, and I verified it on a dyno. last 9 months at mostly low rpm.

Red Admiral

31st May 2009, 10:38

http://i25.photobucket.com/albums/c84/AviationImages/Rolls-Royce%20Vist/CosworthGB_2.jpg

http://i25.photobucket.com/albums/c84/AviationImages/Rolls-Royce%20Vist/CosworthGB.jpg

Pictures of the engine in question.

I am extremely dubious that a souped up Chevy will get anywhere near F1 power levels. Maybe with today's power restrictions.

Actually, one of my friends has a stock Yamaha R6 with about 130hp. This results in a BMEP of about 250psi. No problems with engine breakdowns there.

GregP

31st May 2009, 20:29

Nice looking twin-turbo engine! If you believe the numbers, the BMEP works out as you said. Only in 1985 and 1986 did the twin turbos of 1,000 hp compete, and they broke at an astonishing rate (read that expensive).

In those days, the qualifying engines made about 1,000 and lasted only for a few laps during qualifying for the race. Many did not even last for qualifying, but enough did to produce a starting grid.

Once qualifying was over, the team switched back to more reliable engines for the races. They made about 700 - 800 hp and turned about 11,500 rpm. There were NO production engines anywhere NEAR that level ever sold to the public. Who wants an engine with a life measured in minutes?

It was so expensive to run that Formula 1 went back to 3.0 or 3.5 liter engines after 1987 (also DURING 1987) and stayed there for the next 20 years. A Formula 1 engine today displaces 2,400 cc, turns a regulated maximum of 19,000 rpm, and produces about 780 - 800 HP for a BMEP of 227 at the 800 HP output.

As an interesting aside, my quoted BEMP limit of about 225 for piston engines is, of course, for normally aspirated engines. Your selected Cosworth F1 powerplant above is a twin turbo and the 225 BMEP limit obviously does not apply. As an aside, the WWII aero engines were ALL supercharged, and some had either a second stage supercharger or a turbocharger feeding the built-in supercharger. The normally-aspirated BEMP limit does not apply.

As for a small-block Chevy rivaling the BMSP of Formula 1, they don't. According to the formula for BMEP, 450 HP at 6,700 rpm from 327 cubic inches produces a BMEP of 162. If you own one, you don't run it at 450 HP for any length of time. Maybe for about 12 seconds going down a quarter mile dragstrip. Most of the time, the engine is making less than 100 HP crusing around at 2,000 rpm or less.

My 2002 Camaro SS makes 380 HP at the crank today from 346 cubic inches at 6,000 rpm for a BMEP of 144.9. The 380 HP turns into about 325 HP at the rear wheels. I only hit 6,000 rpm every once in a while, and only for a brief acceleration lasting a few seconds.

For US $10,000, I can easily modify ot to turn out about 550 HP at 6,200 rpm from the same displacement. The BMEP works out to 203, close to that of a NASCAR engine. If I were to do that, which I am not going to do, I could not afford to turn it at that rpm very often because of the extreme possibility of damage. Still, it would survive a while. I LOVE powerful engines, but am unwilling to fund one at this time that runs more than about 450 HP.

In Formula 1, on the other hand, they run them at BMEP values of about 225 for hours at a time. In fact, the engines they qualify with must also last for the race and beyond.

Now back to aero engines ... I don't know of anyone who wants an engine in a heavy piston-powered aircrfaft that has a life measured in minutes. Most of the great aero engines of WWII could be run at BMEP values of about 325 to 400 for very brief periods if time, usually for 5 minutes or less. Some could survive for 15 minutes or more, but were scrap when they landed back home.

The Reno engine I was talking about before is an Allison that can be made to produce 2,950 HP at the prop at 3,200 rpm from 1710 cubic inches. That works out to a BMEP of 427. It is possible, but the danger of engine failure is definitely high, and the engine would likely need overhaul after only a short time. A typical Reno Unlimited race lasts for about 7 to 9 minutes, depending on how many of the really fasy guys break, and the engines will usually need maintenance after a race. "Maintanance" can very easily include "parts," sometimes a lot of parts.

None of the Reno engines would be suitable for a bomber escort mission unless they cruised at 850 HP or so. If they did that at, say, 2800 rpm, the BEMP works out to 140 and they might easily last for the mission and beyond.

Last, I apologise for not making it very clear that the BEMP limits I was talking about were for normally aspirated engines. Since many engines are NOT nromally aspirated, your obvious choice of a twin turbo makes BMEP seem to be a nebulous measurement. Nothing could be further from the truth.

There are definite BMEP level limits for supercharged, turbocharged, and turbo-supercharged engines. This subject could eaily blossom into an entire forum. I will decline to start the thread since passion runs high on the subject.

However, most of the Merlins that are making more than 2,600 HP at the propeller at Reno are running Allison G-series rods. They only made about 750 Allison G-series engines and the supply of G-series rods is very limited. You can still get them, but each time you do, the population dwindles. There is nobody willing to make a new production run of Allison G-series rods, due to product libility, so there is a definite life on racing Merlins and racing Allisons.

I might be wrong, but I doubt the ability of the teams to increase the Unlimited Gold speeds much beyond 500 mph, and they will not be running these engines for more than about another 20 - 25 years at Reno without an injection of new parts, including new engine cases, rods, bearings, cylinder liners, etc.

Wuzak

1st June 2009, 03:44

Wuzak

1st June 2009, 03:55

Nah, Wuzak! there's definately a lack of the ability to breath at higher altitudes due to lack of oxygen!

The lack of oxygen is due to a lack of air.

Over-boosting at low altitudes because of the denser air is the main limiting factor I think.
With a second speed s/c available this enables the aircraft to climb to a higher altitude where the faster speed gear can then pump the cylinders with the less dense air at a greater rate with much less danger of over-boosting and be able to recover most if not all of the power made at it's Full Throttle Altitude the power-plant made at say 5,000' in the lower-speed supercharger gear. (I don't know if this was widely accepted as M.S. gear and F.S. gear?).

45 and 50M modified at factory I would think. They are listed as a production variant in the RR Merlin books by RR themselves. IIRC they were validated by RR for use in both a clipped wing version of a Spitefire 5 (optimised for low-alt operation) and some naval aircraft that had a similar service requirement.
(I'd be amazed if service personnel were able to trim & re-balance a vibration critical component -impeller rotor, with an operational speed of over 20,000 rpm to anywhere near acceptable vibration levels).

I thought it was a actually a different overall size (diameter). A smaller rotor and housing?
Trimming blades on an impeller rotor using an un-modified housing would effectively lower the bosst pressure it was capable of making at the same rpm. i.e. make it less efficient.

:) Ah! In that case, maybe this was done so that 100% throttle could be used at lower altitudes without fear of over-boosting?

Yes, the 45M and 50M were modified at the factory, not in the field. The M suffix is Modified. As far as I am aware the impellers were reduced in size but the housings remained the same.

Yes, it enabled the pilots to use full throttle at a lower altitude, meaning that the supercharger was doing less work to get the boost. This was great for low level work but severly impaired altitude performance.

p.s. RR made a mini-version of this centrifugal gear driven supercharger for a 1500cc V16 BRM engine in the late 40's. The S/C operated at over 40,000rpm and much an incredible-sounding scream.
If anyone has never heard this it's well worth listening to (for engine junkies like me!) either search for "BRM V16 sound" or use the links on this site:-

http://www.billzilla.org/engread.htm

(Apologies in advance if this is naughty, but it's worth it! :))

The BRM V16 makes a truely wonderous noise, well worth listening to. I have a book about the development of the BRM, and it makes mention of RR's role. IIRC it says that some of RR's recommendations on the s/c were not followed due to ignorance or lack of money. One of these suggestions would have made the engine better at lower rpm.

Wuzak

1st June 2009, 03:59

p.s. RR made a mini-version of this centrifugal gear driven supercharger for a 1500cc V16 BRM engine in the late 40's. The S/C operated at over 40,000rpm and much an incredible-sounding scream.
If anyone has never heard this it's well worth listening to (for engine junkies like me!) either search for "BRM V16 sound" or use the links on this site:-

http://www.billzilla.org/engread.htm

(Apologies in advance if this is naughty, but it's worth it! :))

btw, the BRM supercharger was a 2-stage gear driven supercharger.

http://en.wikipedia.org/wiki/BRM_British_Racing_Motors_V16

The contemporary Alfa Romeo 158 (actually a 1939 design) used a Roots type supercharger.

Wuzak

2nd June 2009, 02:17

btw, the BRM supercharger was a 2-stage gear driven supercharger.

http://en.wikipedia.org/wiki/BRM_British_Racing_Motors_V16

The contemporary Alfa Romeo 158 (actually a 1939 design) used a Roots type supercharger.

Doing a bit more reading, I found that RR had suggested adjusable guide vanes either before the first stage or between the stages of the supercharger in order to get a better spread of boost across the rpm range. BRM's chief engineer thought that was too complex, so it was never fitted to the V16, though RR made the adjustable guide vane for the inlet.

Centrifugal superchargers were rare in road racing until the adoption of turbochargers because they tended to only make boost at the upper end of the rev range.

Some more interesting supercharged cars:
The Alfa Romeo 158 started of with a single stage roots type supercharger in 1938. During the war the engine's designer, Colombo, designed a two stage system of roots superchargers.

The Merecedes Benz M154 (fitted to the W154 GP car) was fitted with two equal size roots superchargers working in parallel for 1938. The design was modified in many ways for 1939's M163, including the change to two stage supercharging. Mercedes found that they got 7% more power with 10% less boost at lower engine rpm.

Also, the Maserati 4CL had twin stage roots type supercharging.

Most of these had MAPs of about 2.5-3 atm (absolute), whilst the BRM had a MAp of around 4.5-5 atm (abs).

Lightning

2nd June 2009, 18:35

Hi Greg,

So, let's stay firmly away from automotive engines where, if they fail, you pull over and park. Can't do that in a Mustang, a Spitfire, OR in a Bf 109.

Or, if you're flying a P-38, you just feather the engine and return to base. :D

Regards,
Lightning

Lightning

2nd June 2009, 18:56

As I understand it, the Oxygen content of air remains pretty much 21% for the practical altitude of most WW2 aircraft.

The reason why mountain climbers can have difficulty at altitude is not because a lower percentage of oxygen in the air, but that their is less air to breathe.

This is true. One half of the atmosphere lies below 18,000 feet, so a liter of air at that altitude contains one half the amount of oxygen that a liter of air at sea level. Your lungs have the same volume at altitude as at sea level, so a lung full of air at 18,000 feet contains only one half as much oxygen.

Per Sid447:

Nah, Wuzak! there's definately a lack of the ability to breath at higher altitudes due to lack of oxygen!

Explained above. Also, at 18,000 feet, the partial pressure of the oxygen is only half that at sea level, and that is also the problem.

But, as Wuzak stated, the 21% figure pretty much remains constant as altitude increases.

Regards,
Lightning

GregP

3rd June 2009, 02:41

It's probably 21% in space, too. Since there is no air in space, 21% of zero is still zero. :D

Lightning

4th June 2009, 16:55

Hi Greg,

It's probably 21% in space, too. Since there is no air in space, 21% of zero is still zero. :D

Wow! Even a P-38's turbosupercharger would have a hard time achieving sea level performance under those conditions! :D

Regards,
Lightning

GregP

5th June 2009, 05:02

C'mon Lightning, all you need is a small impulse engine, a warp core drive and, probably, a pressurized cockpit!

SURELY a P-38 can handle THAT.

Warp Factor 2, Mister! Make it so. Engage!

Lightning

6th June 2009, 22:19

Hi Greg,

C'mon Lightning, all you need is a small impulse engine, a warp core drive and, probably, a pressurized cockpit!

SURELY a P-38 can handle THAT.

Warp Factor 2, Mister! Make it so. Engage!

Greg, you sneaky devil! You penetrated Kelly Johnson's inner sanctum! Those were the exact modifications that were slated to go into the P-38N. Only the end of the war kept it from happening.

Regards,
Lightning

Ricky

9th June 2009, 10:11

Hi Greg,

Greg, you sneaky devil! You penetrated Kelly Johnson's inner sanctum! Those were the exact modifications that were slated to go into the P-38N. Only the end of the war kept it from happening.

Regards,
Lightning

Was that the model initially fitted with an arrestor hook, and trialled on a certain aircraft carrier beginning with the letter 'E'?:p

Lightning

9th June 2009, 18:13

Hi Ricky,

Was that the model initially fitted with an arrestor hook, and trialled on a certain aircraft carrier beginning with the letter 'E'?:p

Why yes, it was. Kelly Johnson was, after all, a very enterprising individal. :)

Regards,
Lightning

GregP

10th June 2009, 03:56

Fascinating ...

Sid447

16th June 2009, 09:28

I'm unsure of the calculation,..........

.....People are using for B.M.E.P.
I always understood this was maximum torque in Lbs-ft (or ft-lbs for the hair-splitters here) multiplied by the constant number of 2473 then divided by the cubic capacity.

i.e. Tq x 2473 / cc = BMEP.

p.s. Which is exactly in line with GregP's posting.

p.p.s. If an R1 makes a BMEP of greater than 200psi it wouldn't be any road bike version.
Maybe the V8 version built for the Caterham Levant maybe (2 x R1 cylinder blocks on a common crankcase and supercharged for 500hp).

http://www.youtube.com/watch?v=r8iPMPwQyWU

http://www.youtube.com/watch?v=-Zxdc1JsBgY

Wuzak

17th June 2009, 02:19

http://en.wikipedia.org/wiki/Mean_effective_pressure

GregP

17th June 2009, 06:42

The equations I am using are as follows:

Torque (pound-feet) = (HP * 5252) / RPM

BMEP (for a 4-stroke) = (150.8 * Torque) / Displacement (cubic inches)

For a 2-stroke, the 150.8 becomes 75.4

Of course, they are easy to convert to metric units. These are classic equations. For a normally aspirated engine, values over 200 to 210 are very suspect unless you are in the business of racing. Where I got into trouble below was forgetting that these values were for NORMALLY ASPIRATED engines.

Engines with boost can easily exceed BMEP values of 200, as has been amply demonstraed in this thread. The maximum BMEP for a boosted engine is probably in the neighborhood of 1500 or more, but that would be for a supercharged drag race V-8 putting out 8,000 HP from 500 cubic inches at 8,000+ RPM.

The engine life of these drag racing powerplants is measured in seconds and thus they are not suitable for flying unless you crazy.

BMEP values in the neighborhood of 350 to 450 have been achieved by multi-stage, multi-speed supercharged aero engines in the 1700 cubic inch range (Merlins and Allisons). These BMEP values, while possible, are not very healthy for the engine and were mostly at what was called "War Emergency Power" in WWII.

During wartime, these engines were limited to 12 to 15 inches of boost early in the war with 87-Octane gasoline. This climbed to 40 to 60 inches of boost late in the war with 120+0Octane gasoline and 2-stage superchargers and even turbo-superchargers. At Reno, it is not unusual to see a Merlin at 140 inches of boost. I leave you to calculate the power produced, but please be reminded that the second stage supercharger, at race power, will absorb 800 HP minumum.

I have seen 3,500 HP claimed, but that is before the 800 HP is deducted for the second stage supercharger. A single-stage Allison, built correctly, at 110-inches of boost, would probably put out 2900 to 3000 horsepower to the propeller. It may happen in the next year or two. We'll see. If it does, it will break a long string of victories for radials and Merlins, and would be a very welcome (at least by me) change.

The thing is, the world is running out of G-series Allison rods. All the real rat and mouse motor Merlins use G-series Allison rods for added strength. Unless someone makes some new ones, they will run out eventually.

Wuzak

18th June 2009, 07:54

During wartime, these engines were limited to 12 to 15 inches of boost early in the war with 87-Octane gasoline. This climbed to 40 to 60 inches of boost late in the war with 120+0Octane gasoline and 2-stage superchargers and even turbo-superchargers. At Reno, it is not unusual to see a Merlin at 140 inches of boost. I leave you to calculate the power produced, but please be reminded that the second stage supercharger, at race power, will absorb 800 HP minumum.

I have seen 3,500 HP claimed, but that is before the 800 HP is deducted for the second stage supercharger. A single-stage Allison, built correctly, at 110-inches of boost, would probably put out 2900 to 3000 horsepower to the propeller. It may happen in the next year or two. We'll see. If it does, it will break a long string of victories for radials and Merlins, and would be a very welcome (at least by me) change.

The thing is, the world is running out of G-series Allison rods. All the real rat and mouse motor Merlins use G-series Allison rods for added strength. Unless someone makes some new ones, they will run out eventually.

A Merlin running on 87 octane fuel had a maximum boost of about +12psi.

Again you surmise that the 3500hp is sans supercharger power. I doubt that it is...

In other words I think the Merlin is making 4300hp gross and 3500hp nett. I would argue that race shops would not test the supercharger separate from the engine. When rating engines RR ran the engine as a complete unit, and took it out from either the crankshaft (ie not including the reduction gear) or the prop shaft.

Remember that the RM.17SM

Wuzak

18th June 2009, 08:56

I ran some numbers....

The RM.17SM Merlin ran a corrected (for ambient conditions) 2620hp @ 3150rpm in 1944. Undoubtedly this number was at the prop shaft or crank, the engine in unit with the supercharger and the supercharger hp not measured separately (and not added to the engine hp to give an inflated number).

This was done at 36psi boost (103 inHg MAP), and gives a BMEP of 2755kPa/399.8psi.

If we assume the 3500hp Reno engine is a nett hp, the engine running at 3400rpm, we get a BMEP of 3410kPa/494.8psi. That is a 23% increase in BMEP for a 36% increase in boost.

If we assume that the 800hp is subtracted, leaving the Reno Merlin with 2700hp @ 3400rpm we get a BMEP of 2631kPa/381.7psi, a 5% loss in BMEP for a 37inHg increase in MAP!

Again, I reiterate, at these boost pressures the two stage supercharger is more efficient than a single stage supercharger.

Rolls Royce's single stage superchargers were more efficient than Allisons, and their two stage superchargers more efficient at these pressure ratios than their single stage supercharger.

I'd also remind you that the RR twin stage supercharger involve no more mechanical losses than the single stage, as like the latter the twin stage was mounted on a single shaft fed by a gear drive system. The pumping losses were minimal as the first stage fed directly into the second stage, with carefully designed volute and casing.

Wuzak

18th June 2009, 09:02

A single-stage Allison, built correctly, at 110-inches of boost, would probably put out 2900 to 3000 horsepower to the propeller. It may happen in the next year or two.

FWIW, the 2900hp V-1710 at 3400rpm gives a BMEP of 2724kPa/395psi, whilst at 3000hp the BMEP would be 2818kPa/408.8psi.

The thing is, the world is running out of G-series Allison rods. All the real rat and mouse motor Merlins use G-series Allison rods for added strength. Unless someone makes some new ones, they will run out eventually.

I should think that making new rods would be relatively simple. Even making stronger ones, as there are many rod manufacturers capable of that. The more concerning factor, I would guess, is running out of crankcases, blocks and heads for all WW2 aircraft engines.

GregP

19th June 2009, 03:16

Hi Wuzak,

I am aware of the BMEP numbers. And if you think stronger rods can be made then, by all means, do it. There is a very small but persistent market.

I think they can be made, too. The things is, it would take some engineering and metalurgy work ... and the market cannot or will not fund the development. You could even try to copy the Allison G-Series rods. Again, the metalurgy has been lost, and where will the money come from?

As for the HP to the prop, I'd love to see one of these modified Reno racers fitted with a stock engine that had been on a dyno. Then you could run it at full power and measure the speed ... then fit the race engine and measure the speed, and back into the HP at the prop with the drag equations since whatever drag is there would be the same unles you modified the airframe.

Unfortunately, there is nobody with the money or inclination to do this. I don't believe there is a 3,700 HP Merlin turning a Reno propeller ... or even a 3,500 HP Merlin doing the same. I might be wrong.

To get there, they would have to turn the rpm at a rate that would put the prop tips into the supersonic range, and then the plane would slow down rapidly while simultaneously creating a horribly loud sound ... kind of like a T-6at full fine pitch at takeoff.

You may argue that there is new gearing in the engine cases, but there is nobody out there making gear reduction drives for Merlins or Allisons, so we are stuck with the WWII gearing. You can change it within the avilable gears, but that's about it.

Ditto the prop blades, though these CAN be recontoured and made shorter if you turn the rpm up. It would be illuminating to measure the drag coefficient of Strega. Maybe Tiger has done that ... I'll ask and report back when Steve Hinton Jr. is around the museum. He had Strega up at Reno last weekend for pylon school, or was supposed to have had it there, and may not be back by Saturday. Heck, maybe they know the drag coefficient of Dago Red. Again, I'll ask when the opportunity presents itself.

Either of the teams might answer or might tell you some bogus numbers. It depends on the person you ask and what they feel you might do with the answer. After all, they all want to win and to deny their opponents the same.

Wuzak

19th June 2009, 05:54

Hi Greg,

I am aware of the BMEP numbers. And if you think stronger rods can be made then, by all means, do it. There is a very small but persistent market.

I think they can be made, too. The things is, it would take some engineering and metalurgy work ... and the market cannot or will not fund the development. You could even try to copy the Allison G-Series rods. Again, the metalurgy has been lost, and where will the money come from?

One thing that has improved in the past 60+ years is metallurgy. I'm sure if someone paid enough cash existing top end con-rod manufacturers will be happy to engineer and build a set or three....

Hi Wuzak,
As for the HP to the prop, I'd love to see one of these modified Reno racers fitted with a stock engine that had been on a dyno. Then you could run it at full power and measure the speed ... then fit the race engine and measure the speed, and back into the HP at the prop with the drag equations since whatever drag is there would be the same unles you modified the airframe.

Unfortunately, there is nobody with the money or inclination to do this. I don't believe there is a 3,700 HP Merlin turning a Reno propeller ... or even a 3,500 HP Merlin doing the same. I might be wrong.

Or just stick the race engine on the dyno....

To get there, they would have to turn the rpm at a rate that would put the prop tips into the supersonic range, and then the plane would slow down rapidly while simultaneously creating a horribly loud sound ... kind of like a T-6at full fine pitch at takeoff.

You may argue that there is new gearing in the engine cases, but there is nobody out there making gear reduction drives for Merlins or Allisons, so we are stuck with the WWII gearing. You can change it within the avilable gears, but that's about it.

The hp quotes I have seen from Merlins claim it at 3400rpm. Late WW2 Merlins were cleared for operation at 3300rpm for periods of 15 minutes. Many Allisons were cleared for 3200rpm and I think the later ones 3400rpm.

I don't think that rpm is the solution teams have gone for - the answer is boost. There will be a point where the extra hp from the extra boost is negated by the extra power required to drive the supercharger. Who knows what that is - maybe 120inHg, maybe moe than 140inHg MAP?

But, if an Allison running 110inHg MAP can make 2900-3000hp at the prop I'm sure that a Merlin running 110inHg MAP would be the same, if not more. And it will produce more power until the increase in hp is cancelled by the supercharger drive hp.

The claim of 3500hp (3800hp I saw for one) may be inflated, by I have no doubt they are claiming the hp at the prop.

Wuzak

19th June 2009, 05:59

I was reading an article in an old copy of Torque Meter, which is produced by the Aircraft Engine Historical Society (http://www.enginehistory.org/) which mentions the situation regarding new built Merlins.

Apparently some of the castings are incredibly difficult to make, particularly the head casting. Imor co-founder Paul Morgen had managed to obtain the original casting patterns, and had planned to reproduce Merlins - and if anybody could it would be Ilmor. Unfortunately Paul Morgan died, and the patterns destroyed by Ilmor.

GregP

19th June 2009, 08:04

Wuzak, I don't think the Merlin is better than an Allison at all. All the Merlin had was a better high supercharger system; and that was due to the U.S. war materiel board deleting the turbocharger from the Allison.

If the Merlin had its high altitude supercharger deleted, it would perform about the same as the Allison.

If the Allsion were to be fitted with the Merlin boost system, it would perform about the same as the Merlin. I like to think slightly better since the early high-altitude Allisons, which were fitted with the high-altitude boost systems, were as good as or better than the early Merlins, which didn't have the high-altitude systems yet. Rolls-Royce got there quickly, however. They had a genius supercharger designer. Wish Allsion could have hired him for a short time to help out a bit.

In point of fact, Wuzak, the Allison supercharger is not the same as the Merlin supercharger. Fitted with a 10.5-inch impeller instead of the standard 9.5-inch unit which was designed for the turbocharger application, the Allison can make about 3,000+ HP at low altitudes. With a turbocharger, it can make 2,800 at 35,000 feet, which is better than ANY Merlin.

If you check, the late model Allisons made more HP than the late model Merlins. But few were built due to the arrival of jet engines.

In Europe, where they run Allisons (and Merlins) in tractor pulls today, they are turning them at 4,400 rpm for short periods RIGHT NOW and are making more HP than the Merlins. They are doing so well that the European tractor pull consortium is trying to limit the displacement to 1650 cubic inches, forcing some to go to Merlins.

Unfortunately for the European tractor pull consortium, all you have to do is fit a crank with a slightly shorter throw (very difficult) or fit a cylinder liner with a slightly smaller bore (easy), and you will be at 1650 cubic inches.

If you are a European tractor puller, call Joe Yancey and he can set you up for it RIGHT NOW ... and you can continue beating the Merlin tractors with your reliable, long-lasting Allison engines. Nothing at all wrong with Merlins ... I like them. But there is nothing wrong with an Allison either that can't be fixed today, and fixed more cheaply than fixes for a Merlin (rods at minimum ... hey, if you fix 'em, you use Allison rods anyway).

Cut to aviation. Give the argument a rest for 5 years and we should see an Allison victory at Reno. If not, then maybe the arguement will continue ... as it has for the last 60+ years.

But ... you could be right. Maybe Merlins ARE better. I love 'em, but I don't think either one is significantly better than the other when the sins of the past are rectified. Rolls Royce DID the fixes, Allsion DIDN'T ... at least in volume, but they are available today and the engines are very much equivalent today when configured for the same task at the same altitude ... except the Allison is stronger in its rods and internal items ... except for the supercharger quill shaft. That has a fix, too. Call Joe Yancey, get the fix.

After that, it is up to you whether you want to race or just fly a fighter around. If I had one, I'd fly it around normally unless I ahd a sponsor. Then I'd race it.

I am assuming that if YOU had a Merlin powered unit of some sort, you'd fly it around rather than race it. I might be wrong. You COULD be a speed demon. From reading 5+ years of your posts, I assume you'd run the crap out it at LEAST once, just to be sure, then fly it conservatively. If true, I hope it wouldn't break on you ... better to fly airshows with it and be content with 250 knot cruising speeds.

Wuzak

19th June 2009, 10:10

Late war Allisons were around 2200hp (twin stage supercharger, or withturbocharger installation), or roughly the same as late model Merlins. Merlin 66s were rated at 2000hp at sea level from 1943, and 2200hp by the end of the war.

The turbocompound Allisons were capable of 3000hp, but not the single stage, twin stage, or turbocharged versions (well, not 60 years ago). But that is something else again. I wonder if anybody has considered recreating one of them.

btw, I am not necessarily saying Merlins are better. Just suggesting that if you can get 3000hp from an Allison with a MAP of 110inHg that a Merlin would probably match that at similar MAP, and definitely when running more boost. You suggest that the Merlin would be making 2700hp (3500hp-800hp) at 140inHg. They were basically making that at 100inHg in 1944.

Also, I am suggesting that the twin stage supercharger from the Merlin was better than the single stage supercharger from the Allison.

If the Merlin had its high altitude supercharger deleted, it would perform about the same as the Allison.

The single speed single stage Merlins did perform much the same as the equivalent Allisons - but they were very early Merlins.

Wuzak

19th June 2009, 10:13

Cut to aviation. Give the argument a rest for 5 years and we should see an Allison victory at Reno. If not, then maybe the arguement will continue ... as it has for the last 60+ years.

We may see that victory, and it may be due to many reasons other than power - Mustangs, for instance, were sliprier with Allisons than Merlins, Allisons are lighter, etc.

Then again, someone may cotton on to making power from Griffons.

Lightning

19th June 2009, 18:46

Hi Guys,

During wartime, these engines were limited to 12 to 15 inches of boost early in the war with 87-Octane gasoline.

A Merlin running on 87 octane fuel had a maximum boost of about +12psi.

Greg says "12 to 15 inches" while Wuzak says "12 psi. Keep in mind that 15 inches of murcury (in Hg) equals about 7.35 psi or about 1/2 atmosphere.

Again you surmise that the 3500hp is sans supercharger power. I doubt that it is...

In other words I think the Merlin is making 4300hp gross and 3500hp nett. I would argue that race shops would not test the supercharger separate from the engine. When rating engines RR ran the engine as a complete unit, and took it out from either the crankshaft (ie not including the reduction gear) or the prop shaft.

I look at the engine as a "black box" with the end of the crankshaft sticking out. The supercharger is part of the engine (i.e. not an accessory] and is therefore included inside the box. The power produced inside this black box is measured at the protruding shaft. This means that the engine's output power includes all the gains and losses occurring inside the box before the power at the crankshaft is measured. These gains and losses include the power absorbed by the supercharger as well as the increase in power produced as a result of it.

When the power absorbed by the propeller reduction gearing is subtracted from the output power of the "black box," the result is the power delivered to the propeller.

Your thoughts?

Regards,
Lightning

Wuzak

20th June 2009, 07:24

Hi Lightning,

Hi Guys,

Greg says "12 to 15 inches" while Wuzak says "12 psi. Keep in mind that 15 inches of murcury (in Hg) equals about 7.35 psi or about 1/2 atmosphere.

Boost is measured (by the British in WW2) in psi gauge (psi(g)). ie the pressure of the air above atmospheric conditions. The US generally measured Manifold Absolute Pressure (MAP) in inHg (absolute pressure), but I guess it also could mean gauge.

I look at the engine as a "black box" with the end of the crankshaft sticking out. The supercharger is part of the engine (i.e. not an accessory] and is therefore included inside the box. The power produced inside this black box is measured at the protruding shaft. This means that the engine's output power includes all the gains and losses occurring inside the box before the power at the crankshaft is measured. These gains and losses include the power absorbed by the supercharger as well as the increase in power produced as a result of it.

When the power absorbed by the propeller reduction gearing is subtracted from the output power of the "black box," the result is the power delivered to the propeller.

Your thoughts?

Regards,
Lightning

I agree. Whilst Rolls Royce had the capability and will to test the supercharger separate from the main engine and check the hp required to run it under certain conditions I doubt that any modern day tuner would bother or care. It is the output to the crank or prop that they care about. The question is whether they measure horspower at the crank or at the prop shaft - you would have to ask the guys that built them.

Lightning

21st June 2009, 20:28

Hi Wuzak,

Boost is measured (by the British in WW2) in psi gauge (psi(g)). ie the pressure of the air above atmospheric conditions. The US generally measured Manifold Absolute Pressure (MAP) in inHg (absolute pressure), but I guess it also could mean gauge.

I know what you're saying, and I agree. My point was that the 12 in.Hg stated by Greg and the 12-15 psi you gave in your post were, in reality, quite far apart.

My take:

Gauge Pressure (psig) = Pressure Above Ambient Pressure

Absolute Pressure (psia) = Actual Pressure (i.e. referenced to a perfect vacuum)

"Boost": Pressure in Excess of Ambient Pressure (again, psig)

At sea level, ambient pressure equals 14.72 psi (standard atmosphere) which is 29.92 in.Hg.

The British used the term "boost" (i.e. psig). The US used absolute manifold pressure (or MAP).
At sea level, 15 psi of boost equals (14.72 psi + 15.00 psi) = 29.72 psia

The US system would express this same condition as:

15 psig = (29.92 in.Hg + 30.49 in.Hg)----(since 15.00 psi equals 30.49 in.Hg)
= 60.41 in.Hg

To put the above in simple terms:

A stated boost of 15.00 psi in the British system is the same as a manifold pressure 60.41 in.Hg in the US system. These numbers are nowhere near each other.

Regards,
Lightning

Lightning

21st June 2009, 20:33

Hi, again,Wuzak,

The question is whether they measure horspower at the crank or at the prop shaft - you would have to ask the guys that built them.

I believe that the power measured at the crankshaft is brake horsepower while that measured at the propeller shaft is shaft horsepower.

Regards,
Lightning

Kutscha

21st June 2009, 20:52

Brake / net / crankshaft horsepower (power delivered directly to and measured at the engine's crankshaft)

minus frictional losses in the transmission (bearings, gears, oil drag, windage, etc.), equals

Shaft horsepower (power delivered to and measured at the output shaft of the transmission, when present in the system)

minus frictional losses in the universal joint/s, differential, wheel bearings, tire and chain, (if present), equals

Effective, True (thp) or commonly referred to as wheel horsepower (whp)

Lightning

21st June 2009, 21:10

Shaft horsepower (power delivered to and measured at the output shaft of the transmission, when present in the system)....

In the present case, the propeller-speed reduction gearing can be considered to be the transmission. Its output shaft is the propeller shaft.

Wuzak

22nd June 2009, 05:55

Brake horsepower refers to horsepower as measured on a brake dynamometer.

What is and isn't included in the power depends on the standard being used specifies.

Shaft horsepower, I believe, is specifically related to gas turbines - ie turboshafts and turboprops. Shaft horsepower is the direct power from the output shaft of the engine (maybe at gearbox for turboprops). Turboshafts and turboprops also often are rated as effective hp (ehp) which seeks to account for the residual thrust of the turbine. This is usually rated at a certain speed (hp from thrust is related to speed).

Lightning

25th June 2009, 18:56

Hi All,

When the power of an engine is specified, it is the power of the stand-alone engine. It is specified as having a certain power at a specified rpm. That rpm is of the engine crankshaft--not the propeller. In airplanes having different propeller diameters, airfoil sections, numbers of blades, etc, etc, the optimum prop speeds would not be the same. The output power of the same engine, as used on those different propellers, however, would still be the same at that specified engine rpm, even though the prop speeds would be different.

Regards,
Lightning

Kutscha

25th June 2009, 20:58

The power is at the prop shaft!

Lightning

26th June 2009, 18:32

The power is at the prop shaft!

Quoting wikipedia:

"Allison V-1710

'The engine was constructed around a basic power section from which different installation requirements could be met by fitting the appropriate Accessories Section (sic) at the rear and a tailored reduction gear for power output at the front.... It allowed a variety of propeller drives and also remote placement the reduction gear.

'The P-39, P-63, and XB-42 installations used V-1710-E series engines that exchanged the integral reduction gear for an extension shaft that drove a remotely located reduction gear and propeller. Aircraft such as the P-38, P-40, P-51A, and P-82 used close-coupled propeller reduction gears, a feature of the V-1710-F series."

The engines on this list of airplanes would not have had different stated horsepowers depending on which reduction-gear installations were used for different propellers and whether the reduction gear in question was a close-coupled or a remote type. The power would be stated as xxxx hp at 3000 (crankshaft) rpm. How that power was distributed beyond the crankshaft was another matter.

Of course there is no rule that prevents specifying the power at the propeller shaft, but this would not be the power output of the engine proper. In such a case the power take-off point should be stated.

GregP

27th June 2009, 03:05

As far as I know, all power output for WWII engines was at the propeller shaft, not the engine crankshaft. Changing the gearing would change the power delivered to the propeller shaft.

Wuzak

27th June 2009, 08:55

As far as I know, all power output for WWII engines was at the propeller shaft, not the engine crankshaft. Changing the gearing would change the power delivered to the propeller shaft.

If it is a change of gear ratio using the same number of reduction gears then the change would be small.

GregP

28th June 2009, 10:14

Hi Wuzak,

Yes, it would be small ... maybe. A Merlin mostly had 0.447 or 0.420 gearing in many vairants. All the 0.420 variants had more torque (HP) than the 0.447 variants because they were turning slower. Naturally, they had the same size or larger props than the others.

The engines, at the crank, mostly had about the same power, depending on the Octane gas used, and the supercharger gearing installed. The HP, actually torque, to the prop could be varied with gearing as long as the prop tips didn't go transonic or supersonic.

Wuzak

29th June 2009, 02:49

GregP

29th June 2009, 05:01

Hi Wuzak, Horsepower = (Torque x RPM) / 5252

If one changes, the other does, too, by definition ... they are linked mathematically.

Wuzak

29th June 2009, 05:56

Hi Wuzak, Horsepower = (Torque x RPM) / 5252

If one changes, the other does, too, by definition ... they are linked mathematically.

Yes, so long as the rpm stays the same.

If we ignore losses in the reduction gear then the power at the prop shaft is the same as the crankhaft. The Torque at the prop shaft will be proportionally increased by the gear ratio, whilst the rpm at the prop shaft will be proportionally decreased by th egear ratio.

So, assume 1600hp @ 3000rpm at the crankshaft.

That equates to a torque of 3799Nm

With 0.420 gearing the power remains at 1600hp, but the shaft rpm is reduced to 1260rpm and the torque increased to 9046Nm.

With 0.447 gearing the power remains at 1600hp, but the shaft rpm is reduced to 1341rpm and the torque increased to 8500Nm.

The gear reduction set would not be 100% efficient, and would in fact be around 90% efficient. But since the change in gear sizes is not that big, and the quality of the gears is (I presume) the same then the differece in efficiency between the two gearsets would be extremely small if at all measureable.

Now, in the 130 series Merlin the RH tractors had 0.420 gearing and the LH tractors had 0.422 gearing, achieved by using an idler gear. In this case the LH tractor will have less power than the RH tractor because there is effectively 2 gearsets in the reduction as opposed to one.

GregP

29th June 2009, 08:32

Actually, HP, torque, and RPM are all accounted for in the equation, so they are linked whatever the RPM does by the equation.

Still, some basic physics tells me that the we can multiply the torque with gearing, but the RPM drops if we do ... and the power stays the same through the equation, not because the RPM is the same or changes, but due to the math relationship.

The real job of the gearing is to make sure the prop tips don't go supercsonic ... or even transonic. So, an 11-foot 4 inch prop neds to revolve at certain RPM in order not to have the tips go transonic or supersonic.

Lightning

29th June 2009, 19:03

Hi Greg,

As far as I know, all power output for WWII engines was at the propeller shaft, not the engine crankshaft. Changing the gearing would change the power delivered to the propeller shaft.

Yes, it would change the power delivered to the prop shaft after losses through the reduction gearing, but it would not change the power developed by the engine.

This is the point I was making in postings #106 and #108. It is presented clearly on page 99 of Francis Dean's America's Hundred-Thousand:

"The output power of an engine was defined by pressure in the intake manifold leading to the cylinders, and the speed of rotation of the output shaft."

Since the maximum power in WWII engines was specified at a typical 3000 rpm, the output shaft to which he refers was certainly not the propeller shaft.

Take the example of a given engine in a given airplane. With all accessories turned off, run the engine up to full power (i.e. specified manifold pressure and rpm) and measure (or compute) the power at the propeller shaft. Now, at that manifold pressure and crankshaft rpm (i.e. maximum engine power output), turn on all power-draining accessories--lights, de-icers, fans, radios, hydraulic pumps, etc., etc. For good measure, lower the landing gear and flaps. (All of this is assuming that maximum allowed loads are observed.)

At the point of maximum accessory power-drain, engine rpm will drop. If manifold pressure is maintained at the original specified value, the only way to re-establish original rpm is to reduce the power to the propeller by reducing prop pitch--that is, to reduce the load on the engine caused by the propeller. When this is done, the engine is back to producing maximum power at the specified manifold pressure and rpm, but the power delivered to the propeller is less than before.

The point here is that you have two different power levels being delivered to the propeller, but the engine is producing its specified maximum power in both cases. This power is the stated maximum for that engine and is the power at the crankshaft.

One final point: If the stated power of WWII engines was the power delivered to the propeller shaft, how would that power be specified in the case of contra-rotating propellers? If the same engine were to be used to drive a single propeller in one case and a contra prop in another, the power to the props would be quite different, but the stated power of the engine would be the same in either case.

Regards,
Lightning

GregP

30th June 2009, 03:31

C'mon Lightning, 1600 HP clockwise coupled to 1600 HP counterclockwise equals no power at all!

Everybody knows THAT! The entire thing screws itself into a twisted mess.

Seriously, the entire WWII methodology of specifying power is quite suspect. They quote the power at the crankshaft for the engine at engine RPM, but you can't access the crankshaft ... all you can access is the propeller shaft. So, you only have access to the power at the prop shaft, which spins much slower than the engine. On top of that, most WWII props were, maybe, 85% efficient.

It is things like this that make us all pull our hair out ... like quoting boost pressure in psig, or manifold pressure in psia ... or in technical atmospheres (ata). It SHOULD be quoted in inches of 88-proof Jack Daniels blended whiskey aged to at least 12 years with no ice in the mix!

Of course, in the Soviet Union, it was probably quoted in millimeters of Yak piss taken from a mature feral Yak fed only on steppe-grown grain and chased at least 2 kilometers with a horse or a captured German Kubelwagon.

Lightning

30th June 2009, 18:48

Hi Greg

C'mon Lightning, 1600 HP clockwise coupled to 1600 HP counterclockwise equals no power at all!

Everybody knows THAT! The entire thing screws itself into a twisted mess.

No wonder my model airplanes never got beyond maiden flight! :D

Seriously, the entire WWII methodology of specifying power is quite suspect. They quote the power at the crankshaft for the engine at engine RPM, but you can't access the crankshaft ... all you can access is the propeller shaft. So, you only have access to the power at the prop shaft, which spins much slower than the engine.

On the P-39 and the P-63, there was a 10-foot, two-section extension on the crankshaft connecting it to the remotely located propeller speed reduction gear. In this configuration, access to the crankshaft was quite feasible. This was also true for the XB-42 (although I'm not sure about the extension-shaft length).

Also, the reduction gear was not necessary to the running of the engine itself. It can really be thought of as an accessory. It was not even required to be bolted directly to the engine as is demonstrated in the above-named aircraft. The engine, sans reduction gear, could have easily been run up on a test stand at the factory in order to directly measure its power output at the crankshaft.

On top of that, most WWII props were, maybe, 85% efficient.

That does not change the amount of power being delivered to the propeller, only the percentage of that power that was transmitted to the air to provide actual propulsion.

It is things like this that make us all pull our hair out ... like quoting boost pressure in psig, or manifold pressure in psia ... or in technical atmospheres (ata). It SHOULD be quoted in inches of 88-proof Jack Daniels blended whiskey aged to at least 12 years with no ice in the mix!

Of course, in the Soviet Union, it was probably quoted in millimeters of Yak piss taken from a mature feral Yak fed only on steppe-grown grain and chased at least 2 kilometers with a horse or a captured German Kubelwagon.

You've got me there! I bow to your impressive knowledge of units of measure--both foreign and domestic. :D

Regards,
Lightning