The effects of engine oil rheology on the oil film thickness and wear between a cam and rocker follower

摘要

Continued interest in energy conservation and carbon dioxide emissions has resulted in enhanced opportunities for development of fuel efficient lubricants. This fuel efficiency has been achieved to a large extent by reducing viscosity as far as volatility and lubrication requirements allow. There has been much industry activity to assess fuel efficient lubricants without compromising engine durability. One area of potential durability concern is that of the overhead camshaft (OHC) rocker follower configuration widely used in modern passenger car engines. A motored cylinder head from an industry standard wear test having an OHC rocker follower configuration, has been instrumented to measure oil film thickness (OFT) in an exhaust valve contact by means of an electrical capacitance technique. OFT measurements over the whole of the active part of the cam cycle are presented using both single and multigrade oils based on a variety of commercially available viscosity index improvers at an operating temperature of 100 deg C. Using single grade oils, the measured OFT over the cam nose region was only slightly dependent on the lubricant viscosity for all test conditions. Under steady-state motored conditions metallic asperity contact was absent and the surfaces were separated by an electrically insulating film which was present at all times. These observations would suggest that the dominant contribution to measured OFT is due not to viscosity but to the presence of anti-wear films on the metallic surfaces. The effect of adding VIIs to single-grade base oils is to enhance the OFT in the predominantly elastohydrodynamic lubrication regimes in the cam flank regions of the cam cycle. However, little OFT enhancement could be found in the heavily loaded regions outside the cam flank regions when using multigrade oils. There is evidence that at higher camshaft speeds the measured film is composed of surface film augmented by a small hydrodynamic contribution, however, it would appear that the surface films laid down under lower speed conditions are more tenacious than those deposited under thicker film operation. Observed wear maxima on the cam follower surface occurred as predicted at positions associated with extended durations of contact with the cam. An apparent thinning of the film occurs (below that observed with single grade oils) when using lower viscosity multigrade oils, around the same positions in the contact cycle.