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The friction mean effective pressure (fmep) contribution of piston assemblies running under motored engine conditions in cylinders with plateau or laser honed bores has been investigated. Results have been obtained for engine speeds in the range 400-2000 r/min and temperatures from −20 to around 60 • C. The fmep of the piston assembly is observed to depend most directly on oil viscosity evaluated at the temperature of the cylinder wall at mid-stroke position. The variation of fmep with viscosity provides a basis for comparing the effects of surface finish or piston design modifications. Using liners with fine honed bores reduced piston assembly friction by between 3 and 20 per cent relative to results for pistons running in plateau honed bores. Piston assembly friction was reduced by between 5 and 38 per cent through design changes to the pistons.

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Friction data have been acquired from motored engine tests on four designs of light duty automotive diesel engines. The engines were all four-cylinder designs with a swept capacity around two litres (1.8l-2.2l). These are typically used in the small and medium car sectors of the European market. The test programme was aligned to cold start and warm-up conditions. The data cover temperatures at the start of motoring of −20°C and above, and motoring speeds covering 200 rev/min – 1000 rev/min. Most tests were carried out using 10W/30 oil. The breakdowns separated piston assembly, crankshaft assembly, valve train and auxiliary component contributions to friction mean effective pressure (fmep), from −20°C upwards, under quasi-steady thermal conditions. Under these conditions, during warm-up the variation of engine fmep exhibits a simple power law dependence on oil viscosity evaluated at the current bulk oil temperature. The dependence of component contributions on oil viscosity has been examined. The empirical coefficients and functions used in the engine friction model developed by Patton, Nitschke and Heywood (SAE 890836) have been modified to give an improved fit to the low speed, low temperature test data, whilst limiting the effect on predictions for fully-warm conditions. The quality of the predictions is illustrated. The degree to which the effect of design detail can be predicted and the scope to reduce friction levels through design are discussed.

An experimental study has been carried out to examine the influence of ring tan load and piston skirt modifications on piston assembly friction under motored engine conditions for initial temperatures of -20, 0 and 30 o C and motoring speeds within the range 400 to 2000 rev/min. The study has been carried out using the block, crankshaft and pistons of a 2.4l, 4 cylinder diesel engine with a bore and stroke of 89.9mm and 94.6mm respectively. The pistons examined are typical of current designs for light duty diesels. A range of ring pack and piston skirt modifications have been tested, in each case as part of a complete piston assembly. The first changes produced reductions in fmep of between 5% and 38%. The reduction was due to improved skirt and ring pack designs in equal measure, each giving improvements of up to 20%. From this baseline eliminating the tan load of the piston rings was projected to give a further reduction in fmep of between 10% and 20%. Increasing the piston diametric clearances by close to the difference between minimum and maximum grade clearance gave smaller improvements of between 2% and 10% relative to the baseline, and reducing skirt roughness to perfectly smooth gave a projected reduction of typically 4%, and more at low oil viscosities.