There are two distinct types of pistons: the trunk type and the crosshead type. Variations in the design of trunk-type pistons can be seen in figures 4-9 and 4-10.

The CROWN, or head, of a piston acts as the moving surface that changes the volume of the content of the cylinder (compression), removes gases from the cylinder (exhaust), and transmits the energy of combustion (power). Generally, the crown end of a piston is slightly smaller in diameter than the skirt end. The resulting slight taper allows for expansion of the metal at the combustion end. Even though slight, the taper is sufficient so that, at normal operating temperatures, the diameter of the piston is the same throughout.

Manufacturers have produced a variety of crown designstruncated, cone, recessed, dome or convex, concave or cup, and flat. Piston crowns of concave design are common in marine engines used by the Navy; however, other types may be encountered. An advantage of the concave shape is that it assists in creating air turbulence, which mixes the fuel with air during the last part of compression in diesel engines.

Figure 4-10.Piston (Colt-Pielstick PC 2.5).

Some concave types of pistons have recesses in the crown to allow room for the parts that protrude into the combustion space. Examples of such parts are the exhaust and intake valves, the air starting valve, and the injection nozzle. In some 2-stroke cycle engines, piston crowns are shaped with irregular surfaces which deflect and direct the flow of gases.

The SKIRT of a trunk-type piston receives the side thrust created by the movement of the crank and connecting rod. In turn, the piston transmits the thrust to the cylinder wall. In addition to receiving thrust, the skirt aids in keeping the piston in proper alignment within the cylinder. Some pistons are plated with a protective coating of tin which permits close fitting, reduces scuffings, and prolongs piston life. Still other pistons may be given a phosphate treatment to aid skirt lubrication. This process etches the surface and provides a nonmetallic, oil-absorbent, antifriction coating that promotes rapid break-in and reduces subsequent wear.

Most trunk-type pistons are of one-piece construction. Some trunk pistons are made of two parts and two metals; the trunk or skirt is made of cast iron or an aluminum alloy, and the crown or head is made of steel. In some pistons of this type of construction, the crown is fitted to the trunk with a ground joint, while in others the parts are welded together.

Without GROOVES and LANDS, the piston rings cannot be properly spaced or held in position. The number of grooves and lands on a piston will vary considerably, depending on such factors as the size and the type of the piston. (See figs. 4-9 and 4-10.)

Some pistons have OIL DRAINS (small holes) in the bottom of some of the grooves (fig. 4-9); some pistons have oil drains in the skirt of the piston or in the land. These holes serve as oil returns, permitting lubricating oil from the cylinder wall to pass through the piston into the crankcase.

Generally, the BOSSES (hubs) of a piston are heavily reinforced openings in the piston skirt. (See fig. 4-10.) Some bosses are a part of an insert which is secured to the inside of the piston. The principal function of the bosses is to serve as mounting places for the bushings or bearings which support the piston pin. The bosses provide a means of attaching the connecting rod to the piston. Generally, the diameter of the piston at the bosses is slightly less than the diameter of the rest of the piston. This difference serves to compensate for the expansion of the extra metal in the bosses.

Because of the intense heat generated in the combustion chamber, adequate cooling must be provided. The heat transmitted through the rings (approximately 30 percent of the heat absorbed by the piston) to the cylinder wall is not sufficient in many engines to keep the unit cooled within operating limits. Most pistons have fins or ribs and struts as internal parts. (See fig. 4-9.) The additional surfaces of these parts help to dissipate heat; much of the heat is carried away by oil which may be pump-forced, sprayed, splashed, or thrown by centrifugal force onto the underside of the piston assembly. A different approach to cooling the piston head is with the use of drilled passages from the connecting rod through the piston pin to the piston bosses. Drilled passages in the piston direct the oil to cavities in the piston crown. Oil discharged from these cavities is controlled so a sizeable amount is retained at all times to cool the crown by cocktail shaker action as the piston moves up and down in the cylinder. (A cutaway view of this type of design is shown in figure 4-17.) Oil is the principal means of cooling for most piston assemblies. Intake air is also used in the cooling of hot engine parts. In order to exhaust or scavenge a cylinder of burned gases and cool the engine parts, the intake and exhaust valves or ports are so timed that both are open for a short time at the end of the exhaust stroke. This action allows the intake air to enter the cylinder, clean out the hot gases, and, at the same time, cool the parts.