The piston head is exposed to over 500 psi when the engine is operating under load. The force differentials caused by the expanding combustion gases and the flame front crossing the piston head can reach two to three times this force. The piston is also exposed to relatively high temperature fluctuations during operation. The temperature of the initial flame front during combustion exceeds 3000°F. Although the piston is subjected to this temperature for a very short amount of time, the thermal stress and expansion of the piston head are significant.
In addition to the forces and the thermal fluctuations incurred by the piston, the piston changes direction in the cylinder bore 120 times per second at 3600 revolutions per minute (RPM). The changing of direction, with its inherent acceleration of mass from a static state, causes variable forces at the piston pin connection. The design, material selection, and manufacturing of the piston considers these operating conditions.
The piston is cooled by the contact of crankcase oil to the underside of the piston head and by contact with the piston rings and cylinder wall. The underside of the piston head is designed to remain open, allowing crankcase oil to contact the piston to remove combustion heat. Piston ring contact with the cylinder wall also transfers heat from the piston. The heat is then dissipated from the engine through cooling fins on the outside of the engine block.