The lubrication of internal combustion engines began with the need to prevent moving parts from damaging each other. As one surface slides or rolls against another, extreme temperatures are generated in the very small regions in which the high points of one surface contact those of the other. Metals melt and fuse, forming micro-welds. These welds are broken by the further motion of the parts, generating surface damage and liberating wear particles.
Even a tiny amount of lubricant can prevent most of this by forming a film which separates the parts very slightly. This greatly extends parts life. One example is wheel bearings, which because they turn slowly and generate little heat can be lubricated by grease. Grease is a mixture of a soap, which provides structure, and oil, which is the lubricant. The gearboxes of many motorcycles through about 1935 were likewise lubricated by grease. As friction and parts motion generated heat, oil melted out of the grease to form lubricating films.
As machine parts move faster, the ability of grease to adhere to them disappears and oil must be conveyed to where it is needed in some other way. The earliest engines were lubricated by 'splash' - pouring a liter or less oil into the crankcase and allowing the motion of the rotating crank to throw it everywhere. Small guides were often cast into the inner surfaces of the crankcase to channel oil to holes that led to the main bearings. As oil made its way past piston rings and thence out of the engine, more had to be provided from a small tank by a hand pump. This was operated by a vigilant rider who looked back often. If there were no smoky trail, the engine needed a shot of oil. Much railroad technology appeared in early engines, so connecting-rod big-end bearings were 'brasses' just as used in locomotive side-rods. This worked only until splash oiling could no longer support the load. As rolling-element bearings could survive on very little oil, these were widely adopted to allow the higher engine performance that higher revs made possible.
Oil supports the load between moving parts by its viscosity – the 'internal friction' that prevents oil from being instantly squeezed out from between loaded surfaces. But as oil temperature rises, viscosity drops, so the hotter the oil, the less load it can carry. This is the direct opposite of what engines need, for as they make more power, they also make more heat. Now something has to be done to keep the oil cool enough to do its job. That something already existed in automobiles – a pumped circulating oil system. Oil from a remote tank or from a sump beneath the engine was picked up by a pump and sent through pipes or drillings to where it was needed. Oil flung from moving parts moved by gravity to a low point from which it was once again picked up by the pump.
For some years the mere residence of the oil in sump or tank provided sufficient cooling to keep oil temperature within reason, but in 1934 Benelli patented an oil-to-air cooler mounted on a remote oil tank. In some modern engines, oil circulates through an oil-to-coolant heat exchanger that sends excess heat from the oil to the coolant radiator.
When plain journal bearings gradually replaced rolling bearings in motorcycle engines after 1969, pumped oil had to do more than just lubricate. Viscous friction at high rpm made bearings hotter and hotter so oil was pushed through them in increased volume to also act as a coolant, controlling bearing temperature.
Most motorcycle engines employ a normal automotive oil circulation. A main oil gallery runs parallel with the crank, and the main bearings are supplied from it. Oil is tapped from main bearings to enter drillings in the crank which carry it to each crankpin, lubricating the con rod big-ends.
At very high rpm it becomes increasingly difficult to push oil from the main bearings into the crank, so big-end bearings starve and there may be failures. The usual remedy is to feed the oil into one or both ends of the crank, and thence through internal drillings to the crankpins to lube the rod big-ends.
As engines are given larger bores and shorter strokes to make them breathe at very high revs, pistons must be made larger in diameter. To carry away the heat of combustion, they need thick crowns to act as heat conduits. The inertia of such heavier pistons would overload rod bearings and limit rpm. Therefore even more cooled oil is called into play in the form of piston-cooling oil jets, directed up at the undersides of piston crowns. Such oil jets make much thinner, lighter pistons practicable – a key to the success of modern high-rpm engines.