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The widespread use of overhead camshaft (OHC) rocker-follower valve-train configurations in current automotive engines allows a more compact cylinder head design and improved valve operation. Unfortunately, this valve train configuration can be difficult to lubricate, as evidenced by a number of wear problems occurring in service. As a consequence, there have been a proliferation of industry standard wear tests. Little work has been published on the rheological behaviour of the lubricant in these severe non-conformal contacts. A motored cylinder head utilising a cam and rocker-follower-valve train configuration has been instrumented in order to measure the oil film thickness (OFT) in an exhaust valve contact by means of an electrical capacitance technique. The experimental apparatus and data acquisition system are described, together with the subsequent data processing.

The viscosity of 10W/40 motor oils formulated with different viscosity index (VI) improvers has been measured at pressures up to 200 MPa (2000 bar) over a wide temperature and shear rate range. The response of viscosity to pressure was found to depend on the chemical nature of the VI improver at both low and high shear rates. As a consequence, the ranking of the 150°C viscosities of polymer-thickened oils can be different at high pressures to that observed in conventional atmospheric pressure viscometers. These effects of VI improver type on high pressure viscosity appear to be relevant to wear in journal bearings of fired engines; thus in experiments carried out in the ALI Bearing Distress Test, bearing weight loss of multigrade oils containing different VI improvers correlated better with high pressure, high shear viscosities than with high shear viscosities measured at atmospheric pressure.

Piston ring wear, bearing wear and fuel consumption have been measured in European engines run under fully warmed-up conditions at steady speed and load. The engines were lubricated with motor oils which were formulated from a range of types and concentrations of VI improvers and base oil blends. The viscosities, viscosity/temperature relationships and viscosity/shear-rate characteristics of these formulations varied widely. The results for all the oils tested showed a poor correlation with their low shear-rate viscosities measured at 210°F or 150°C but a good correlation with their viscosities measured at shear rates of 105 to 106 sec-1.

Hot-weather driveability performance is influenced by vehicle design, ambient temperature and fuel volatility. For individual markets, the gasoline producer can control this aspect of performance by adjusting the fuel volatility level to meet the requirements of the car population under the seasonal ambient temperature conditions. The volatility expression that has been successfully used for several years to control hot-weather iriveability is (RVP + 0.7E70). A standard CBC test procedure is used by a volatility data-sharing group of European oil companies to characterise the performance of modern car models available in European markets. The data thus produced are used to determine the hot-weather driveability performance of European car populations. The data reveal that the performance of these car populations has improved significantly over recent years, although some modern car models with high under-bonnet temperatures show some deterioration.