Both the piston's design and the wristpin configuration employed to transfer the combustion gas forces to the connecting rod are largely determined by the combustion chamber's shape, including the geometry of the piston crown, while other variables include the selected combustion process and the associated pressure maxima. The priority is to produce the lightest possible piston in a unit capable of withstanding intense forces during operation in an environment with temperatures that can approach the physical limits of its materials. Precise definition of the dimensions for pistons, wrist pin and wrist pin bushings are essential for achieving this goal.
The most frequently used materials for cylinder liners and pistons are gray cast iron and aluminum. Pistons and cylinder liner have different coefficients of expansion. At the same time, variations in piston clearance within the cylinder must be minimized to reduce noise (piston slap) and improve sealing. To this end, steel strips or similar retainers are sometimes cast into pistons to limit its expansion. Piston rings from the sealing element between the combustion chamber and the crankcase. The upper two -the compression rings- serve as gas seals. At least one additional ring (generally of a different design), the oil control ring, is also present. This scraper ring determines the type of oil film against which pistons and compression rings will operate. Owing to the rings' extreme tension and the corresponding force that they exert against the cylinder walls, they are a major source of friction within the reciprocating piston engine.
The connecting rod is the joining element between engine pistons and crankshaft. The rod is subject to extreme tensile, compression and flex stresses, while it also houses the wrist pin bushings and crankshaft bearings. The length of the connecting rod is determined by the piston stroke and the counterweight radius; here engine height can be an important factor.
Car crankshaft with its rod extensions, or throws, converts the reciprocating motion of the pistons - conveyed to it by the connecting rods - into rotary motion, making effective torque available at the crankshaft end. The forces acting upon the crankshaft are characterized by highly variable periodicities and vary according the location. These torques and flex forces and the secondary vibrations which they generate all represent intense and highly complex stress factors for the crankshaft itself. As a result, its structural properties and vibrational response patterns rely upon precise calculations and carefully defined dimensions. Crankshaft design is further complicated by the indeterminate static conditions stemming from the virtually ubiquitous presence of multiple journal bearings.
The number of crankshaft bearings is primarily determined by overall load factor and maximum engine speed. To accommodate their intense operating pressures, all diesel crankshafts incorporate a main bearing journal before and after every cylinder. This arrangement is also found in high-speed spark-ignition engines designed for high specific outputs.
Crankshaft types in some smaller SI-engines designed for operation at lower load factors sometimes extend the interval between main bearings to 2 cylinders to reduce expense. The number of counter weights also depends upon the criteria above.
Stresses and load factors are also a primary consideration in the selection of both materials and manufacturing processes. A highly stressed crankshaft is usually drop-forged. In smaller and less highly stressed engines cast crankshafts, incorporating the dual advantages of lower weight and less expense, are becoming increasingly popular.