The tribological effects of soot contaminated lubricants on engine components

The work in this thesis investigated the effects of soot contaminated lubricants of engine components. A thorough background review demonstrated that soot related wear is a major issue for the automotive and lubricants industry. A detailed analysis of the physical properties and effects of soot contaminated lubricants highlighted that there are associated effects that will tend to reduce engine perfonnance and efficiency. The actual wear that was produced is studied for the area of the engine which is most significantly affected by soot contamination, the valve train. A typical elephant's foot to valve tip contact, known to suffer from soot related wear, fonned the basis of the specimen ball-on-flat reciprocating wear testing. A standardised wear test rig and method were developed for the testing, which investigated different lubricants (base and fonnulated oil), increasing levels of soot contamination (using carbon black as a soot surrogate) and different temperatures. The results demonstrated significant increases in wear with increasing level of carbon black contamination. Furthennore, increased temperature tended to increase the amount of wear produced and using a fonnulated lubricant reduced the amount of wear measured when compared to tests using base oil. Significant changes in wear mechanism were visible from surface imaging ofthe wear scars, showing initially lubricated sliding, which changes to abrasion at higher contamination levels, and then shifting to contact starvation as the contamination significantly reduces the flow of lubricant into the contact at very high contamination levels. Significantly, the severity of the effects of soot contamination was due to the lubricant film thickness in the contact and therefore ultimately the lubrication regime within which it is operating. Where boundary lubrication is significantly affected by soot contaminated lubricants, but these effects are reduced when the contact operates under elastohydrodynamic (EHD) lubrication conditions. A region of the engine that is significantly affected by soot contaminated oil is the contact between the piston and the cylinder wall or liner. This is mainly due to the component's proximity to the area where soot in produced. Wear will occur in this highly sensitive region due to the breakdown of the protective oil film, which will occur when the lubricant is contaminated with soot. A method of analysing the effect sO~as been investigated 'using an ultrasonic technique to measure the oil film between a piston and a cylinder wall or liner. The ultrasonic oil film thickness measurement technique (developed at the University of Sheffield) has been used in many static and dynamic applications; therefore an initial trial was performed on a single cylinder engine, demonstrating some success. A motored test engine was then built on which to develop the technique and to provide a platform for future research. The test engine demonstrated potential for future success, as a variety of ultrasonic transducers were tested. Testing also highlighted that significant future work, regarding reducing electrical interference, improving the data processing electronics and further investigations into the different types of transducer is required to improve the technique.