Apart from sealing the combustion chamber, the piston should mainly absorb pressure and heat. The pressure is, for the most part, transferred to the connecting rod and the heat to the cylinder block. In two-stroke engines the piston controls the outlet- and/or inlet ports. Because the piston operates with a high acceleration, its weight is of essential importance. Particularly in the last few years this has been considerably lowered through changes in the construction and in the choice of materials.
Maximum strain and precision
- In one minute 12.000 times from 0 to 80 km/h and back again, - up to 8 t of strain at a pressure of 75 bar, - maximum temperature of up to 350°C at the piston dome, - with pinch-points in the cylinder head less than 1 mm clearance, - machined with a tolerance of up to 1/1000 mm.
AlSi-cast or forged, surface protection
The piston should have a low specific weight and, therefore, is cast from an aluminium alloy or, e.g., in high performance engines, is pressed (forged) together with 10 - 25% silicon and otherwise very little other materials. Since its center, and particularly the lower part of the shank is in contact with the cylinder, its largest diameter is here. A 1/10 mm thick coating of graphite, tin, lead or anodise improves the surface slip qualities. An iron coating (Ferrocoat) is one of the possibilities for using the piston in aluminium cylinders.
Combustion temperatures of up to 2500°C
Temperatures on a liquid cooled four-stroke piston
up to 250°C
up to 200°C
up to 200°C
up to 120°C
Diesel engine Direct injection
up to 350°C
up to 300°C
up to 300°C
up to 180°C
The compensation and offset of the larger circumference
A piston made from Al-Si alloy has too great a thermal expansion, particularly in a cast-iron cylinder block. By casting in steel-stripping (see picture 2) the piston expands more strongly in the direction of the pin axle. Hence, a cold piston is flattened at the piston-bore-eyes and becomes perfectly round only at operating temperature. With a gudgeon pin exactly in the center, at TDC the piston would change sides in the cylinder. Therefore, one distinguishes between the more- and the less strain loaded cylinder-side. To bring forward the change to the lower strain area, the pin-axle is slightly offset to the higher strain side (offset compensation).
Oil-splash cooling and strengthened piston-ring groove
Charged engines are additionally cooled by splashing oil from below through an inlet passage in a ring channel. The piston travel provides for the circulation of the oil in the cooling channel. Pistons for especially high compression engines (Diesel engines) have a ring-carrier (see picture 2). This is a cast iron groove for the first piston ring. 10/10