The piston of the diesel engine is made to be strong because the compression pressure, combustion temperature and combustion pressure are higher than that of the gasoline engine. In some models, a heat dam is built in above the No.1 piston ring groove or the piston head up to the No.1 piston ring groove is cast of FRM (Fiber-Reinforced Metal), which is a special alloy made of aluminum and ceramic fibers. Also, some pistons are made with a cooling channel inside the piston head in order to cool the No.1 piston ring groove. The oil injected from the oil nozzle passes through this cooling channel and cools the piston.
There are the following types of piston rings:
No.1 piston ring (No.1 compression ring)
A. Semi-keystone ring
No.2 piston ring (No.2 compression ring)
B. Tapered ring
C. Tapered under – cut ring
No.3 piston ring (Oil ring)
D. Solid ring with coil
E. Three piece type ring
Role of the semi-keystone ring
The top surface of this piston ring is tapered to prevent the piston ring from sticking due to carbon sludge. While the engine is running, the piston also moves slightly in the radial direction, causing the clearance between the piston ring groove and the piston ring to change. This causes the carbon sludge inside the piston ring groove to be rubbed off and pushed out of the piston ring groove together with the oil.