The two-piece pistons represent a major contribution to an improvement in the performance of internal combustion engines. The basic construction of the two-piece piston comprises a top portion, or head, or crown, generally of cast iron or steel, from whose lower part and integral with it depend a pair of legs for bearing a wrist pin which connects the head portion to another member, the skirt, generally of aluminum or an aluminum alloy, designed to guide the piston in the cylinder liner.
The two-piece piston offers several advantages. One important advantage is a lower top land height, thus decreasing the volume within the room defined by the top land and the cylinder liner, hereby reducing engine emissions. The piston head is made of materials having a low coefficient of thermal conductivity, which helps to maintain ideal temperatures for a complete combustion of the fuel-air mixture and, accordingly, a reduction of engine emissions.
Also, the use of iron or steel for the piston top makes this component compatible with other engine components such as the cylinder liner and the cylinder head, also of iron or steel, regarding coefficients of thermal expansion.
There are also many other advantages such as the high strength of the head even at high temperatures, as a result of the material of which the head is usually made. The lower portion, or skirt, generally of aluminum or an aluminum alloy, is extremely light, thus contributing to reduce the total weight of the assembly.
Another advantage is that their components are independent and do not make contact with each other with the piston in operation in the engine. Thus, the heat generated in the combustion region is not transmitted from the head to the skirt, whereby it is possible to mount the piston in the cylinder liner with extremely small clearances thereby attaining a remarkable reduction in engine noise levels even at cold starts.
Below are some examples of how the two pieces bolted together.
