How many rings to use?
If we look at sectioned drawings of some old piston engines, we might be able to count many more piston rings being used per piston than we would find these days. We might see multiple rings placed above the piston pin (as we would find now) but these might also be augmented by one or more rings below the piston pin, placed towards the bottom of the piston skirt. Cylinder sealing was much worse in years gone by – oil leakage past the rings was very bad, and rates of blow-by of combustion products was similarly dire.
Nowadays, in a four-stroke engine, we would typically find no more than three piston rings used on any piston, whether it be for racing or for passenger car use. The reason which has driven the use of fewer piston rings is friction, but what has allowed it is better sealing between rings and cylinder bores. The friction resulting from using an extra piston ring to improve cylinder sealing is significant, and harms engine performance and efficiency. With improved sealing behaviour owing to a better understanding of piston rings, piston design and cylinder bore surfaces, we can often manage happily with three rings for most applications. Where a ring is deleted, it is one of the compression rings – in almost all circumstances, an oil control ring is required in a four-stroke engine. There are some heavy-duty diesel applications though that still use three compression rings and an oil control ring.
Knowing that there is a gain to be had from deleting a piston ring in terms of friction is reason enough to dispense with one of them where possible, but the benefits go further. One obvious benefit is that the piston can be developed to be lighter, as we require less depth of material in which the ring grooves are cut. Taking this a step further allows us to decrease the distance between the piston pin bore and the crown. In order to maintain the same compression ratio, we might choose to use a longer con rod, but this also improves efficiency owing to lower rod articulation angles and therefore lower piston thrust forces.
The lighter piston and lower rod articulation means that reciprocating forces are reduced, and this can mean an efficiency improvement as bearing loads will be lower. The con rod, being less highly stressed, can be re-optimised based around a lighter piston to give a mass reduction or a possible gain in durability.
The gains don’t stop here, as it might be possible to reduce the counterweight mass on the crankshaft, leading to lower mass and inertia. In lowering the distance between the pin bore and the crown, we could alternatively have opted to keep the existing rod and choose to reduce the distance between the crankshaft bore and the top of the cylinder block.
By the simple expedient of reducing the number of piston rings, we can make quite significant differences to engine efficiency and mass. Along with the reduction of reciprocating valvetrain mass, piston assembly mass reduction is one of the more potent tools for making the whole engine lighter, as it opens the way for mass reduction in other components. So, where sealing is adequate and durability is sufficient, there is a real incentive to use fewer piston rings. The same logic also applies to two-stroke engines – racing two-strokes generally use only a single compression ring, as oil control rings are not necessary in a conventional two-stroke engine.
Written by Wayne Ward