Similarly, in diesel engines, air is compressed just below TDC (top dead centre) and then fuel is injected into the cylinder to combust the atomized fuel with air. Even here the air is compressed to the Lowest volume (clearance volume).
If the piston clearance is too small, then on getting heated-
- piston will seize inside the cylinder on more expansion
- piston will get too tight in the cylinder, resulting in excessive friction loss
- piston can damage the cylinder wall
If the piston clearance is too large, then
- piston will move back and forth very freely, resulting in engine knock and may even damage the piston skirt.
- large clearance may also reduce the sealing property of compression rings to seal the compression chamber.