1. Stresses on lubricants

Of all mechanisms, reciprocating internal combustion engines are undoubtedly the most difficult to lubricate. In fact, these machines contain virtually every mechanical component in existence: piston-cylinder linear guides and valve stems-valve guides, cylindrical plain bearings, thrust bearings, cam-pusher or cam-tilt actuator assemblies, gears, toothed wheel chains, ball or roller bearings, hydraulic backlash-compensating systems and camshaft phasing actuators, oil pumps, valves, etc. What's more, these various components operate under extremely variable tribological conditions. Contact pressures can reach 1 GPa, sliding speeds vary from zero to several tens of meters per second, and oil temperatures range from ambient (down to – 35 C) to as high as 300 C in the hottest parts of the engine. All lubrication regimes are therefore encountered in engines (hydrodynamic, elastohydrodynamic,...