Most studies of engine friction have been carried out at fully-warm operating conditions. Relatively little attention has been given to frictional losses when the engine is running cold, although these can be considerably higher and have a strong influence both on cold-start characteristics and fuel consumption during warm-up. The losses which effect the indicated load on the engine are rubbing losses and loads associated with driving auxiliaries. The equivalent frictional mean effective pressures (fmep) are generally highest during the first seconds of engine operation. These decay rapidly onto a characteristic variation which depends upon oil viscosity, and which fmep follows throughout the warm-up period. The oil viscosity can be evaluated at the bulk temperature of oil in the sump or main gallery.
Breakdown motoring tests have been carried out on a series of diesel engines to examine how the friction contribution of various sub-assemblies in the engine contribute to the total and how this varies with temperature and speed. Tests were carried out using a compact cold cell and engine motoring facility. The engine was cold soaked to a target test temperature and then motored to a target speed and the variation of motoring torque recorded. Sets of tests were carried out at several stages of breaking the engine down. This enables the contributions due to the valve train, piston and big end assembly, crankshaft, fuel injection pump, and auxiliary load to be determined.