Rotation of piston rings

Piston rings lead a much more active life than might be thought. While we usually imagine that they only reciprocate in a motion defined by the piston, they also rotate during service. Such rotation has been the subject of many experiments and studies over the years, but this is not simply a matter of academic interest; there are implications for engine performance and efficiency.

With few exceptions, race engines use piston rings of the conventional gapped type – that is, there is a carefully controlled gap between the ends of the piston rings. A report by Jung and Jin (1) shows that, at one engine speed, the speed and direction of piston ring rotation depends on load, and it is not necessarily the case that both rings rotate.

Their study, although weighted toward the technique used in the measurements, monitored oil consumption and was able to correlate this with the relative positions of the piston ring gaps. It is perhaps unsurprising to find that oil consumption rose to a maximum when the two ring gaps were aligned, and fell to a minimum when the ring gaps were spaced 180° apart. This makes sense, as the flow losses would be greatest when the ring gaps are as far apart as possible, so for a given pressure difference there would be less flow through the gaps in the rings.

The speed of rotation of piston rings has been measured experimentally. Shaw and Nussdorfer (2) examined the phenomenon on a large engine and found that, at 1000 rpm engine speed, the piston rings were “observed to rotate as rapidly as 1 rpm”. Jung and Jin reported in more detail: on the engine they used, at 4000 rpm and 2 bar bmep, the rings rotated at 0.6 rpm in opposite directions to each other, with the second ring initially oscillating between two positions before finally beginning to rotate continuously.

At higher load, the top ring didn’t rotate, and the second ring rotated at speeds from 0.5 to 3 rpm. On the same engine and at lower engine speed, the top ring simply moved to a given angular position and then remained stationary at 2 bar load. With an increase to 4 bar, there was a change in top ring position but still no continuous rotation. The second ring was also observed to be stationary at this speed in some tests, some of which found the ring gaps aligned – the condition where oil consumption is highest.

It is clear that the piston rings in any engine lead a mysterious life where, depending on the load and speed conditions that apply, they might rotate continually, oscillate between certain positions or remain stationary. Oil consumption is found to vary with ring position and, where the rings rotate periodically, the rate of oil consumption is a function of the rotation period of the rings. Other than pinning the piston rings to prevent rotation (which is commonly done in two-stroke engines), there is little we can do to influence ring position or speed of rotation. It is clear that whatever position the rings are in during the engine build will not be maintained during service. 

References

1. Jung, S., and Jin, J., “Monitoring of Rotational Movements of Two Piston Rings in a Cylinder Using Radioisotopes”, Journal of the Korean Nuclear Society; vol 31(4); ISSN 0372-7327, August 1999

2. Shaw, M., and Nussdorfer, T., “A Visual and Photographic Study of Cylinder Lubrication”, NACA Technical Report no 850, 1946

Written by Wayne Ward