function
The energy contained in the fuel is converted into heat and pressure in the cylinder during the power stroke. The heat and pressure values increase rapidly in a short space of time. The piston, as a moving component of the combustion chamber, has the task of converting part of this energy into mechanical work.
In its basic form, the piston is a hollow cylinder closed on one side with the piston crown with ring section, piston boss and skirt. The piston crown transfers the pressure forces generated during the combustion of the fuel-air mixture to the crankshaft via the piston boss, the piston pin and the connecting rod.
Requirements
Pistons must meet the following requirements:
- adaptability to operating conditions
- seizure safety combined with high running smoothness
- Low weight with sufficient strength
- Lowest possible friction losses
The partly conflicting requirements both for the construction and the material require measures that can vary greatly from case to case.
design
Due to the operating requirements, aluminum-silicon alloys have generally become the preferred piston material.
The functional areas of the piston are the piston crown, the ring section with the top land, the piston hub and the piston skirt. The piston assembly also includes the piston rings, the piston pin and the piston clips.
In order to keep the masses low, a careful design of the pistons is necessary.
piston shape
The piston expands and deforms under the influence of the gas temperature and the forces, especially the gas force. This change in shape must be taken into account when designing the piston in order to ensure jam-free operation at operating temperature.
Pistons usually have a slightly smaller diameter in the piston pin direction than in the pressure-counterpressure direction. The difference is the ovality.
The piston is slightly drawn in at the top and bottom ends to encourage the formation of the supporting lubricating oil wedge. The greater drawing in in the area of the piston ring section takes into account the strong thermal expansion due to high temperatures in this area and the deformation caused by the gas force. The piston is therefore crowned.
The final piston shape can only be verified through extensive simulations and engine tests.
