Taking advantage of centrifugal forces for oil systemsTags : crankshafts
Crankshafts are at the heart of the vast majority of race engines; only the occasional rotary engine doesn’t have one, and even then it has something that plays a very similar role. The cranktrain, comprising the piston assembly, con rod and crankshaft, converts the reciprocating motion of the piston into a rotary motion that we can use to drive a gearbox or, as we might find in future, an electric generator.
The crankshaft itself rotates in bearings in the crankcases, and at very high speeds in some cases. MotoGP engines rev close to 20,000 rpm, while in the mid-2000s Formula One engines were in the same speed range. I have an old 750 cc motorcycle from 25 years ago that will run happily to 14,000 rpm.
Where we have rotating motion, we have centrifugal forces. We often see these as unhelpful in the context of an oil system as we need to run high oil pressures in order to feed oil into the crankshaft oil drillings, fighting the centrifugal forces that are trying to return the oil to whence it arrived. However, it is possible to take advantage of these forces for the good of the oil system and hence the health of the race engine.
Two problems we might suffer from in a race engine as far as the oil system is concerned are aeration and contamination. Excessive amounts of air in the oil can lead to problems with lubrication, and bearings are especially susceptible to damage through oil that contains too much air. Contamination in the form of solid debris is also particularly damaging to bearings, as well as other components. Anywhere that hard, solid particles can enter a narrow sliding contact presents opportunity for damage. For both these situations, we can use centrifugal forces to help us.
The key is to get the main feed of oil onto the crankshaft centreline. This can be achieved when feeding from a gallery into a main bearing, but it is conventionally done with a nose-feed crankshaft, where the main feed of oil is fed into the non-output end of the crankshaft.
In terms of air separation, we have two options. One is very passive and is by the simple expedient of a small oil drilling on the centreline of the crankshaft, passing into the crankcases; the air, which is forced towards the crankshaft axis owing to its very low density, can pass through without dragging very much oil along. The other option is to use a device to take advantage of the difference in angular momentum between the crankshaft and the oil as it is introduced. This is an active air-oil separator, whose function is the same as that of the simple on-centre drilling, but it should be much more effective.
Regarding solid debris, this is again normally much more dense than the oil. If the oil is moving sufficiently slowly axially, it will have time to pick up enough momentum to force the solid debris to the outside of the drilling. By introducing a ‘dam’ at some point along the cavity into which the oil is introduced, the solid debris should be retained there. A scheme working on the same principle was used in some of the Rolls-Royce Merlin engine variants. It must be said though that trying to include the active separation scheme and the debris centrifuge may not be possible in most engines owing to lack of space.
Written by Wayne Ward