Adding cylinders to existing engine designs

[PMN1]

How much time and effort would it take to add extra cylinders to an existing engine design, for example what if you wanted an 18-cylinder version of the Hercules, what would you need to do?

[ChrisMcD]

How much time and effort would it take to add extra cylinders to an existing engine design, for example what if you wanted an 18-cylinder version of the Hercules, what would you need to do?

Call it a Centaurus!

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

Bristol tended to start from an existing engine and the Centaurus started as an 18 cylinder Hercules - which in turn was a 14 cylinder twin row version of the 9 cylinder single row Perseus.

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

Bristol tended to drop cylinders when they went to two rows - so you could argue that the Centaurus was a twin row Perseus, but it was obviously more complex than that since they took so long to sort out the lubrication problems on the Centaurus.

[PMN1]

Call it a Centaurus!

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

Bristol tended to start from an existing engine and the Centaurus started as an 18 cylinder Hercules - which in turn was a 14 cylinder twin row version of the 9 cylinder single row Perseus.

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

Bristol tended to drop cylinders when they went to two rows - so you could argue that the Centaurus was a twin row Perseus, but it was obviously more complex than that since they took so long to sort out the lubrication problems on the Centaurus.

Ahh yes but different cylinder size, now what changes would be needed for an 18-cylinder Hercules???

[ChrisMcD]

Ahh yes but different cylinder size, now what changes would be needed for an 18-cylinder Hercules???

Bristol were great believers in evolution. They made a bigger engine by adding extra cylinders of the same bore and increasing the stroke - presumably because they had the space.

Same bore - 5.75 in

Different stroke; Hercules 6.5 in, Centaurus 7 in

More cylinders; Hercules 14, Centaurus 18.

So more swept volume; Hercules 2,360 sq in, Centarus 3,270 sq in

And more power: Hercules 1,615 hp, Centaurus 2,500 hp

Obviously there is more than that but it works for me

[Wuzak]

How much time and effort would it take to add extra cylinders to an existing engine design, for example what if you wanted an 18-cylinder version of the Hercules, what would you need to do?

You'll need a new crankcase, with 18 holes instead of 14, and probably a bigger size. You will have to change the timing system - in a Hercules it means adding skew gears for the extra cylinders, as well as their drive trains. It might even mean a move from a front driving system (all 14 Hercules cylinder sleeves were driven from the front of the crankcase) to one front and rear driving system (I believe in the Centaurus the rear sleeves were driven from the rear of the crankcase, the front sleeves from the front). If you have a poppet engine, then you would need to make new cam rings or drives (if OHC).

You will need a new ignition system, to cope with the additional cylinders and the extra timing. You would also require a new supercharge volute with 9 instead of 7 outlets, possibly a new supercharger section to cope with increased air flow, including a resized throttle body.

You may need a new crank, depending on stresses and vibrations, definitley a new master rod, and possibly new slave rods. Exhaust system would need to be modified to suit the extra cylinders.

So, quite a bit. Still, easier than making a V16 from a monobloc V12....

[Kutscha]

Adding to Wuzak's post.

http://img.villagephotos.com/p/2005-12/1114844/Bristol-1.jpg

from G. White's engine book

[ChrisMcD]

AFAIK the main problem with the Centaurus was getting the lubrication sorted, and it was always a 'smokey' engine.

But does anyone have more on it than that?

My reference books simply say that Bristol were told to concentrate on getting production of the Hercules sorted and to give development of the Centaurus a low priority.

[PMN1]

What impact would the Hercules being an 18-cylinder engine have - the Stirling, Beaufighter, Wellington and Halifax have around 375hp per engine extra from the start but higher fuel burn.

The Sunderland becomes more like the Seaford.

I would say there is still enough of a gap in the power range between an 18-cylinder Hercules and the Centaurus for the Centaurus to still be developed.

[Wuzak]

The 18 cylinder version of the Hercules - let's call it the Hercules 18 - has 93% of the capacity of the Centaurus.

Based on the BMEP of the Hercules XVI (1362kPa, 1650hp @ 2800rpm) the Hercules 18 would produce 2120hp @ 2800rpm. It maintains the hp/l of the Hercules XVI (obviously).

The Centaurus II had a higher BMEP of 1558kPa and gave 2520hp @ 2700rpm, the rpm being more restricted due to the extra stroke.

Using the Centaurus's BMEP therefore gives the Hercules 18 2430hp, and a higher specific power (48.7hp/l as compared to 47.0hp/l for the Centaurus).

I'd say that while the hp of the Hercules 18 would fall between the Hercules and the Centaurus it would be of not practical benefit to the war effort. Better develop just the Centaurus than both it and a Hercules 18 - which would be similar in weight and size.

Perhaps better to sort out the cooling issues of the Taurus and make 21 or 28 cylinder versions of them. A 28 cylinder version of the Taurus would weigh approximately the same as a Centaurus, and with similar BMEP to the Centaurus would pump out slightly more power. It would be longer, but significantly smaller in diameter to either the Centaurus or Hercules.

[Nick Sumner]

To divert this thread slightly how about the DB 609, essentially a DB 603G with four more cylinders to make it a V16 rather than V12.

I assume the cooling system would have needed to be larger but apart from an extra 3mm on the bore everything else is similar even crank rotation which we might expect to be lower is in fact slightly increased (2700rpm to 2800 rpm).

[Wuzak]

The problem with a V16 is the length of the crank. If DB took some or all of the drive from the centre of the crank they would reduce the problems of the long crank, effectively making the engine two V8s.

I believe that is what Chrysler did for the IV2220.

I'd also imagine a V16 would be a 90° instead of the preferred 60° for the V12.

[Kutscha]

http://img.villagephotos.com/p/2005-12/1114844/File0002-1.jpg

[Wuzak]

That section clearly shows the drive for the prop coming from the centre of the crankshaft, thereby alleviating the problems asociated with V16s as opposed to V12s - their excessively long cranks.

The caption of that picture mentions the long length of the IV-2220, as compared with the Griffon, which was almost the same capacity.

There were a couple of factors here, not least of which is that the bore of the IV-2220 was nearly as big as the Griffon's (5.8"/147.3mm vs 6.0/152.4mm), but requiring eight such cylinders per bank compared with six. I think the central geartrain would also increase the length of the engine.

Another reason for the length is that the Chrysler, like all "hyper" engines, has separate liquid cooled cylinder construction - which means increased bore spacing. Making monoblock cylinder nbanks and heads would reduce the bore spacing, and probably reduce the length by several inches - but the engine would still be long.

Mind you, the Eagle22 was longer, mostly because ofits nose case arrangement.