Fluid Flow and Lubricant Degradation in the Piston Ring Pack

R.J. Gamble,a C.J. Hammond,b J.R. Lindsay Smith,b M.Priest,a M.S. Stark,b* R.I. Taylor,c


a Department of Mechanical Engineering, Leeds University, Leeds, LS2 9JT, UK

b Chemistry Department, University of York, York, YO10 5DD, UK

c Shell Global Solutions (UK), Cheshire Innovation Park, Chester, CH1 3SH, UK


Engine lubricants are having to withstand ever harsher operating conditions as power densities increase and oil drain intervals are extended. These trends, coupled with moves to reduce sulphur and phosphorus levels in lubricants, are likely to cause an increase in the oxidative stress experienced by engine lubricants.

To aid understanding of the lubricant degradation process, a computer model of a two-reactor system has been developed that includes hydrocarbon oxidation chemistry and fluid flow between a large, relatively cool, sump and a small, hot zone that represents the top piston ring area.

Key variables required to parameterise the model are the flow rate between the reactors, the length of time the oil stays in the reactors, and the reactor temperatures. The residence time for a lubricant in the piston ring pack was measured for a single cylinder Hydra gasoline engine. This was achieved by adding a chemical marker (octadecane) to the sump, while monitoring the time it took for the marker to appear in oil samples extracted directly from the top ring groove. The oil flow rate from the piston ring pack to the sump was measured by comparing the level of oxidation in the oil samples extracted from the ring pack with the rate of increase of oxidised products in the sump. These measured lubricant flow parameters are compared with simulations using a detailed tribological model describing fluid flow in the ring pack.

The oxidation mechanism for hydrocarbon-based lubricants describes the formation of peroxides, ketones and carboxylic acids. Preliminary work is presented comparing lubricant oxidation in the single cylinder engine with simulations using the two reactor oxidation model.

Moray Stark