Is having separate crankshafts sensible?

[spiralcopter]

The Chrysler XI-2220 sixteen cylinder engine took power to the prop reduction from the center of the engine, essentially dividing it into two V-8s and reducing the torsional loads on the crankshaft. Wright's R-2160 took this one step further, having three separate crankshafts; one per 14 cylinder "unit," connected by seven radially arranged layshafts that took power to the reduction gear. This also reduced torsional loads on the crankshafts and vibration isolated them from each other to some extent.

This all sounds very nice, but I can't help but notice that nobody has really designed engines this way since the 40s. What are the pros and cons of chopping up the crankshaft this way?

 

[MiTasol]

To me it probably depends on what your purpose is. For the Napier H engines there is probably no other practical option as a single shaft would require very long conrods with all the weight and size penalties that would make.

Multiple shafts have the potential to introduce unpredictable vibration which can lead to other problems. For example when they first built the Sikorsky S-58T it had a number of rear fuselages fall off in flight because the vibration from the turbine engine produced a resonance that caused metal fatigue in the attach fittings. Wether that was because of the twin-pac or just the difference between the slow piston engine vibrations and the much higher frequency turbine engine vibrations I do not know.

I would expect the multi shaft piston engines like the R-2160 themselves to have "interesting" vibration characteristics which could exhibit themselves in many different ways in both the engine and airframe.

That said turbine engines started to replace piston engines at the end of ww2 and there have been several very successful twin-turbine engines that used the same concept as several of the multi-shaft piston engines -- that is having the ability to maintain (reduced) power in the event of an engine failure and having the ability to shut off one engine for long range cruise. These days many helicopters have two separate engines feeding a single gearbox which is basically the same concept other than the gearbox being part of the airframe instead of part of the engine.

The two most famous of the turbine engines are the Double Mamba and the PT6T.

 

[spiralcopter]

Interesting point about twin turbines feeding to a common gearbox. As you say, it's quite common in helicopters. I would think that with turbines the vibration problems are much smaller, however, because most of the movement is from rotating components. With piston engines most of the vibration is coming from oscillating components.

 

[MiTasol]

Metal fatigue can equally be caused by high frequency low amplitude vibration which is what happened on the S-58Ts. The secret is in the engine mountings and how they are done which is why they are so complex on American engines. Surprisingly the Brits went with no vibration dampening of any sort on their radial engine mounts and yet did not seem to have any failures in the airframes they were attached to.

 

[tomo pauk]

The Chrysler XI-2220 sixteen cylinder engine took power to the prop reduction from the center of the engine, essentially dividing it into two V-8s and reducing the torsional loads on the crankshaft. Wright's R-2160 took this one step further, having three separate crankshafts; one per 14 cylinder "unit," connected by seven radially arranged layshafts that took power to the reduction gear. This also reduced torsional loads on the crankshafts and vibration isolated them from each other to some extent.

This all sounds very nice, but I can't help but notice that nobody has really designed engines this way since the 40s. What are the pros and cons of chopping up the crankshaft this way?

Short crankshafts are strong crankshafts. Engineering a crankshaft to withstand loads from 8 or 12 cylinders will be easier than designing one for 16 or 24 cylinders.
With a lot of cylinders in quest for great power, it might be prudent to 'split' the load. A 16 or 24 cylinder liquid-cooled engine might have it far easier if it has two crankshafts (Sabre, Fairey 16 and 24 cyl types, V-3420, DB 606/610) than with just one (RR Vulture, Jumo 222).

 

[Engineman]

Short crankshafts are strong crankshafts. Engineering a crankshaft to withstand loads from 8 or 12 cylinders will be easier than designing one for 16 or 24 cylinders.
With a lot of cylinders in quest for great power, it might be prudent to 'split' the load. A 16 or 24 cylinder liquid-cooled engine might have it far easier if it has two crankshafts (Sabre, Fairey 16 and 24 cyl types, V-3420, DB 606/610) than with just one (RR Vulture, Jumo 222).

Remember, the Vulture was four banks of six and the Jumo222 was six banks of four. In both cases, the crankshaft itself was not really the issue, Six inline seems to be workable for a well designed crank, certainly still works for a V12 with good blade and fork rods. However, I believe the problems with producing a reliable big-end on a high output engine with more than two rods on each split big-end are considerable.

Eng

 

[rinkol]

Short crankshafts are strong crankshafts. Engineering a crankshaft to withstand loads from 8 or 12 cylinders will be easier than designing one for 16 or 24 cylinders.
With a lot of cylinders in quest for great power, it might be prudent to 'split' the load. A 16 or 24 cylinder liquid-cooled engine might have it far easier if it has two crankshafts (Sabre, Fairey 16 and 24 cyl types, V-3420, DB 606/610) than with just one (RR Vulture, Jumo 222).

A short crankshaft, all things being equal, will have a much higher resonant frequency for torsional vibrations and generally shouldn't twist as much, as would happen with a long crankshaft.

The amount of twisting over the length of the crankshaft can be a significant issue for valve timing, particularly in engines that drive the camshaft with a gear train at one end of the crankshaft.

 

[MiTasol]

Remember, the Vulture was four banks of six and the Jumo222 was six banks of four. In both cases, the crankshaft itself was not really the issue, Six inline seems to be workable for a well designed crank, certainly still works for a V12 with good blade and fork rods. However, I believe the problems with producing a reliable big-end on a high output engine with more than two rods on each split big-end are considerable.

Eng

Yet radials routinely have 9 cylinders on one big end and in the case of the late R-1820s were putting out around 1,500 hp through one big end.

 

[Engineman]

Yet radials routinely have 9 cylinders on one big end and in the case of the late R-1820s were putting out around 1,500 hp through one big end.

Yes. Some radials use built-up cranks to allow the use of one-piece rod ends but, there are very successful engines with split big-ends, even the huge 4360. However, I think it remains true that designs of many liquid cooled high-output engines that tried to achieve low rotating mass and small engine size with high rpm, failed to achieve a reliable split big-end for more than two rods, other than blade and fork. Interesting question if they could have succeeded with the Vulture, given lots more design and development. Rolls-Royce tried and failed, even by 1944 they were unable to bring themselves to really revisit the problem and designed the Eagle 22 as an H24 with two cranks.

Eng

 

[pbehn]

If you already make V8 engines there are huge savings in joining two together rather than designing and making a new V 12.