The energy contained in the fuel is converted into heat and pressure in the cylinder during the power stroke. The heat and pressure values rise very sharply in a short 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 areas of piston head with ring section, piston boss and skirt. The piston head transmits 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.
Pistons must meet the following requirements:
- Adaptability to operating conditions
- Resistance against eating and at the same time very smooth running
- Low weight with sufficient strength
- The lowest possible friction losses
The sometimes conflicting requirements for both the design and the material require measures that can vary greatly from case to case.
Due to the operating requirements, aluminium-silicon alloys have generally prevailed as the piston material.
Functional areas of the piston are the piston head, the ring area with the top land, the piston boss and the piston skirt. The piston assembly also includes the piston rings, piston pin and piston clips.
In order to keep the masses low, the pistons must be carefully designed.
The piston expands and deforms under the influence of gas temperature and forces, particularly gas force. This change in shape must be taken into account when designing the piston to ensure jam-free operation at operating temperature.
Pistons usually have a slightly smaller diameter in the gudgeon pin direction than in the thrust/anti-thrust direction. The difference is the ovality.
The piston is pulled in a little at the upper and lower end in order to promote the formation of the supporting lubricating oil wedge. The greater indentation in the area of the piston ring section takes account of the strong thermal expansion due to high temperatures in this area and the deformation caused by the gas force. The piston therefore becomes crowned.
The final piston shape can only be verified by extensive simulations and engine tests.