The Effect of Oil Supply on Piston-ring Lubrication in an Internal Combustion Engine

Abstract

A drive for efficiency and tightening emissions legislation are imposing increasing constraints on the piston-ring lubrication of large marine internal combustion engines, motivating engine manufacturers to invest in improved lubricating systems. In order to enable a new generation of systems to be developed more understanding is needed of the interaction between lubricant supply, oil-film thickness, and the development of the lubricant film on the cylinder liner.
The methods used for delivering lubricant to the ring pack in large marine diesel engines are focussed on improvements in timing, metering, and spread, but there is still a reliance on lubricant transport to distribute lubricant to many areas of the cylinder liner away from the area of injected lubricant. With corrosive and abrasive wear being consequences of inadequately balancing the lubrication needs of the piston-rings, it is important to understand how lubricant supply influences oil-film thickness and lubricant transport both near
to, and more distant from the area of injected lubricant.
To address these issues a test apparatus has been developed to allow delivery of a metered dose of lubricant to selected regions of the cylinder. Capacitance based transducers were used to measure oil-film thickness over a number of cycles as the lubricant film develops. Investigations into the response of the lubricating film following a change in lubricant supply were conducted, and a method of evaluating the rate at which lubricant is transported using oil-film thickness data has been devised, and used to map lubricant transport on the liner at a number of lubricant supply rates. A two-dimensional piston-ring lubrication model has been developed and validated, with oil-film thickness predictions within 30% of experimental data in the top half of the stroke. The model is capable of predicting the development of the lubricant film over a number of cycles following the delivery of a non-uniform oil-film to the cylinder.
Oil-film thickness has been observed to follow a first order response with time constants in the range 10 to 20 seconds (20 to 40 cycles) following a change in lubricant feed-rate. The response is due to circumferential lubricant transport, and follows a fast response resulting from axial lubricant transport. The time constant is not dependent on lubricant supply but does vary over the stroke. By characterising the relationship between oil-film thickness and lubricant injector feed-rate a system model for oil-film thickness as function of lubricant
supply can be determined allowing an oil-film thickness control system to be developed and reducing risk of over- and under-lubricating during fuel switching. The lower limit of lubricant supply before the onset of corrosive wear or scuffing can be found leading to reductions in oil consumption.