The critical shape of the piston, its ovality and installation clearance are decisive factors affecting performance and long term satisfactory operation of the piston in the cylinder. Ring stability at the piston rockover point and subsequent pressure build is very critical and largely controlled by the piston skirt. Frictional forces on the skirt and the skirt lubrication characteristics are also critical for trouble-free operation. The gap or lubrication window between the skirt and the cylinder wall is established by the proper skirt profile, ovality and operating clearance. In this complex environment, the hydrodynamic lubrication toggles to mixed lubrication as the piston slows down approaching top and bottom dead center.
At the piston reversal points (top and bottom dead center) the mixed lubrication condition moves to a boundary lubrication condition. The piston’s skirt profile is tasked not only with distributing the thrust loading from combustion and maintaining optimum ring seal, but also maintaining the desired lubrication boundary to ensure good durability under high dynamic conditions.
Developing skirt profiles and cam shapes to deal with the expansion characteristics of aluminum piston alloys remains a fundamental challenge to piston engineers. Pistons do not expand evenly, primarily due to uneven piston heating caused by combustion heat acting directly on the piston crown, and the non-uniform shape of the piston itself. The piston by necessity incorporates sections of varying thickness (heavy in the pin tower area, thinner in the skirt area) that require the skirt profile and cam shape be appropriately and accurately matched to the thermal and mechanical loading conditions. Exposure of the piston crown to high combustion heating requires more clearance in the adjacent land area of the piston to allow for the greater heat expansion than in the lower extremities of the skirt.
The classic “Barrel Shape” skirt profile remains the most common profile because it addresses many of the unique problems of piston design. But technology is a moving art form and there are still advances to be realized from this basic piston shape. In addition, the expansion of the area adjacent to the thicker section of the pin boss region also warrants attention. Machining the piston to an elliptical shape (smaller diameter near the pin bore area of the skirt than on the major and minor thrust areas) provides the ovality which helps to distribute thrust loading with minimal friction. For many years, this shape was achieved by mounting the piston on a rotating, clocked spindle that was moved in and out of a rotating grinding wheel via an eccentric cam device. Ovality in “cam ground” pistons is adjusted to suit the application and anticipated loading.
With a conventional cam-grinding machines, the ovality is applied to the piston at the same value for the full length of the skirt top to bottom. This means that the area of largest expansion requires the entire length of the skirt to be ovalized to meet its maximum expansion requirement. It allows the piston to expand and run without seizure in the cylinder, but results in a less than desirable loose fit of the remaining skirt area, which can contribute to inaccurate guidance of the piston in the bore and reduced ring stability.
More modern diamond turning has replaced the older grinding method of finishing the O.D. to size, and variable camming of the skirt has allowed pistons to be designed with ovality to address the specific expansion characteristics of the entire piston skirt. to Compounding the complexity are other shape and ovality fine tuning strategies such as asymmetrical profiling of the major and minor thrust areas of the piston and reverse camming of certain areas of the piston.
Engine performance is significantly impacted by the friction force against the cylinder wall and piston rock over which can affect piston ring seal. The difficulty of determining the piston profile stems from the fact that piston primary and secondary motion and thermal deformation are not all that have to be taken into consideration. The piston contact area is also a function of actual cylinder bore geometry, which can be less than ideal at operating temperature. In addition, considerable skill is required in the development stage to critique piston and cylinder bore markings produced by the profile test in a running engine, and provide the proper corrective action.