This study is motivated by the current trends in the automotive industry where an increasing level of soot is becoming a challenge to the internal combustion engine (ICE) components. Factors responsible for increasing soot level can be grouped into those explicitly driven by regulations and those driven by technological developments; as well as the requirements of extended drain intervals which lessen the overall maintenance cost of running a vehicle and reduce the environmental impacts of disposed oils.
The research work basically involves a multi-pronged approach to evaluate the behaviour of sooty-oil surrogates (blends of fully formulated engine oil, HX5 SAE15W-40, and carbon black particles) under various conditions, using laboratory specimens and real engine components. Laboratory specimens are appropriate for evaluating systemic changes in various parameters; they, however, tend to be more homogeneous and have smoother surfaces than real specimens [1]. Real component testing affords more realistic surface contacts and contact geometries; and therefore loading conditions.
Increasing soot-loading affects virtually every component involved in the combustion process; the most vulnerable are, however, the valve-train components. Consequently, the focus of the research was on the components within the engine's valve-train related specifically to the diesel engines.
Based on the specific component simulations that were proposed, two standard commercial tribometers were used; namely, the High Frequency Reciprocating Rig (HFRR-Plint TE77) and the Mini Traction Machine (MTM-Plint TE54). Two others were designed and built as part of this research; these are: a Pin-in-Bush (PIB) Rig and a Chain Rig.
The generic tests involving the basic geometric contacts, non-conformal point (ball-on-flat) and non-conformal line (ring-on-ring), were designed to mimic specific contact conditions in an internal combustion engine (ICE). The elephant’s foot/valve tip contact of the automotive engine valve train is perfectly simulated by the ball-on-flat test with a small stroke length; while ring-on-ring test undergoing sliding and rolling concurrently perfectly mimics the valvetrain’s cam lobe/roller follower contact geometry.
The pin-in-bush conformal contact reciprocating sliding rig was designed to have conformal interacting surfaces which provides area contact for the pin and bush moving with sliding action on the lubricant film at the interface. The rig was also adapted, with minimum modifications, for the chain rig which was used for the real engine component test; specifically, Mercedes Benz M271 timing chain and sprocket. The results obtained from the chain were compared with a similar component that has undergone about 206,000 kilometres in a real engine.
The results obtained in this study are comparable with other reported studies using carbon black to mimic engine soot. Although loads of data were generated in the course of various tests and post-test analysis, space limitation would not allow all of these to be presented. For the quantitative (friction coefficients, viscosity, wear volume and roughness profile) results, averages of the measured values were determined and used to present the results in various formats. However, only few of the qualitative analysis (microscopy, SEM, ContourGT and Alicona) results were presented in this thesis.
Generally, the behavioural pattern of increasing volume of wear, viscosity, frictional force and coefficient of friction with increasing carbon black content remain essentially consistent with all the tests carried out under this study. However, the surface roughness revealed a somewhat smoother surface profile for moderate carbon black content (3-5wt%CB), especially at moderate normal load. Also, traction coefficient values decreased progressively and consistently with increasing carbon black content for mixed lubrication tests.
Wear analysis revealed that the carbon black contamination effects in the contact are substantial at high concentration levels, high temperature and high normal loads as wear scar volume increased with these parameters. Evolution of the wear test further reveals that the progression of wear is a function of time. The severity of wear is more pronounced in the ball-on-flat tests operating under boundary lubrication as compared to the ring-on-ring (mixed lubrication) tests. Though, both contacts are non-conformal, but the load distribution was over a wider surface area with a line contact as against what happens with point contact, where a small area of contact carries the full load.
Observed wear mechanisms can be interpreted as mild three-body abrasive wear at moderate carbon black contents (3-5wt%CB) where the well-dispersed carbon black particles freely roll and slide between the contacting specimens. While at higher carbon contents (7-12wt%CB) where agglomeration is more likely, the resultant wear mechanism is metal-to-metal contact due to starvation of lubricant which access into the contact is restricted by the carbon particle agglomerates, and thus resulting in two-body abrasion. Another possible mechanism at higher content is that some of these hard particles get into the contact zone where they become squeezed and get embedded into the surfaces. The embedded hard particles burrow through the contacting surfaces, forming grooves along the sliding direction. This is also classified as two-body abrasion.
For the mini-traction machine (MTM) test, there was also cyclic stress-induced surface fatigue due to the combined effects of high contact pressure resulting from Hertzian line contact and high load and occurrence of fatigue-induced incipient damage in the subsurface, along the plane of maximum shear stress.
The real engine component tests revealed multiple effects on the contacting bodies. These range from sliding and rolling actions between the sprocket teeth and the chain rollers along with the carbon black particles. There was also the effects of the impact stresses induced by the collision of chain with the sprocket teeth during engagement. The emerging results from various post-test analysis have also revealed the damaging effects any particle infiltration into the contact between various moving parts of the timing chain system can cause.
The analysis of the ‘L’ (denoting the initial of the owner of real engine components) vehicle components also gives some valuable and instructive indications of possible damages that prolonged usage can cause to a component, particularly timing drive components which constitute parts of the most durable and lifetime components in an internal combustion engine (ICE).
The novel idea on the use of ultrasound technique to measure the instantaneous film thickness (between the contacting specimens) of selected sooty-oil surrogates also recorded a significant level of success in terms of the obtained results which are comparable with analytical results in behavioural trends and numerical values. Notably, being a pioneering move towards determining the real-time film thickness of soot-contaminated oil, this indicates a potential for future research.
