Internal combustion engines experience the majority of their wear when they are started. The three main reasons for this wear are:
- inadequate anti-wear additives in the oil
- inadequate oil flow
- gasoline contamination
In an engine, there are 4 regimes of lubrication and moving parts within an engine pass through these regimes starting from stopped to operational speed. See Lubrication for more information. Also see Car Engine Oil Lubrication Automotive Appreciation video.
- Boundary Lubrication
- Mixed Lubrication
- Elasto Hydrodynamic Lubrication
- Fluid Film Lubrication
- Hydrostatic Lubrication
- Hydrodynamic Lubrication
Of specific concern for older flat-tappet engines is minimizing wear on the camshaft lobes and lifters as well as on the crankshaft bearings during the initial Boundary Lubrication. Before the engine is started, there is obviously no motion between the component surfaces. As the camshaft begins to rotate, the tappet starts to slide along the surface of the lobe. At the initial startup, the lack of any oil flow causes these surfaces to be in the boundary lubrication regime. In the boundary lubrication regime, there is insufficient oil film to keep the parts separated, and contact between the cam and the tappet will occur. With oil flow and once rotating quickly enough, the wear surfaces are separated by the oil wedge formed by Hydrodynamic Lubrication. See Effects of Shearing for more information about weak oil film.