Free runner
A four-stroke engine is called a freewheel , in which the movement space of the pistons does not overlap with that of the valves .
This property is important in the event of an interruption or malfunction of the camshaft drive, when the crankshaft and camshaft are no longer in a fixed, structurally determined phase relationship to one another. In this case, a collision between pistons and valves is impossible with a freewheel. With other engines, a skipping or cracking of the timing belt (or a skipping or skipping of the timing chain ) can result in severe engine damage, which often results in irreparable damage to the valves, pistons and cylinder head. The freewheel property is particularly important for timing belt-controlled motors, since timing belts are generally more at risk of failure than other control systems. In addition, a toothed belt usually breaks suddenly - without any prior acoustic signals, as is the case with other control systems.
The structural prerequisite for being a freewheeler is a low compression ratio , so that there is still room for the valve lift in the comparatively large combustion chamber when the piston is at top dead center. Therefore, only low-compression gasoline engines are usually free-wheeling. Diesel engines are ruled out due to their particularly high compression. The individual shape of the combustion chamber of an engine can also prevent it from being a freewheeler, even if it is obvious due to other properties.
Even with free-wheel constructions, there can be mechanical contact between the components due to deposits of soot or carbon on pistons or valve disks.
In principle, all motors with vertical valves are free-wheeling.