Piston ring stability considerations are also important in maintaining low oil consumption. It is crucial for the top piston ring to remain stable and not to flutter in its groove. This is illustrated in below figure that shows piston ring motion and inter-ring pressure traces measured on a running engine for two different second ring profiles. The only difference between the two rings is a small internal chamfer on the second ring which gives the ring some positive twist and stabilizes the ring.
In the case of the non-chamfered second ring, the top ring remains on the lower groove face over most of the stroke except under inertia loading just before TDC exhaust. In the chamfered case, the pressure in the second land can build up to the point that when it exceeds the cylinder pressure during the end of the expansion stroke and during the exhaust stroke, it allows the top ring to rise in its groove and for gas to flow from the second land back into the combustion chamber. This gas flow carries lubricating oil along with it and in this case, doubles oil consumption.
Oil consumption, Blowby, Piston Ring Motion and Second Land Pressure
6 cylinder turbocharged diesel engine, 150 hp @ 2400 rpm
In above figure, it should be noted that in the case of the plain second ring design with lower oil consumption, the second ring is unstable and flutters in its groove. This second ring flutter can be beneficial from an oil consumption perspective in that it prevents the build-up of pressure in the second land by venting it to the crankcase.
Ensuring that second ring is unstable and flutters during the expansion stroke is one approach that can be used to stabilize the top ring and keep oil consumption low. This approach can increase blowby.
