Engine pistons serve several purposes

(1) Transmit force of expansion to crankshaft through connecting rod.

(2) Act as a guide for upper end of connecting rod.

(3) Serve as a carrier for piston rings use to seal piston in cylinder.

(4) Aid in the burning of the fuel mixture by introducing a swirling action to the air/fuel mixture. (This is particularly true in the case of diesel engine pistons). The swirling is accomplished by altering the contour of the piston head.

Pistons operate under exceedingly difficult mechanical and thermal (heat) conditions, so they must be made and installed with the utmost care. Piston must be strong enough to stand the force of the expansion, yet light enough to avoid excessive inertia forces when their direction of travel is reversed twice each revolution.

Pistons must be able to withstand the heat from the burning air/fuel mixture, plus the heat generated by friction. They must slide freely in the cylinder. If fitted too tightly, the engine will over-heat and/or seize. If fitted with too much clearance in the cylinder, the piston will knock and rattle.

The piston head or “crown” is the top surface against which the explosive force is exerted. It may be flat, concave, convex or any one of a great variety of shapes to promote turbulence or help control combustion. In some applications, a narrow groove is cut into the piston above the top ring to serve as a “heat dam” to reduce the amount of heat reaching the top ring.

Piston rings carried in the ring groove are of two basic types: “compression” rings and “oil control” rings. Both types are made in a wide variety of designs.

The upper ring or rings are to prevent compression leakage; the lower ring or rings control the amount of oil being deposited on the cylinder wall. The lower groove or grooves often have holes or slots in the bottom of the grooves to permit oil drainage from behind the rings.

The piston ring lands are the parts of the piston between the ring grooves. The lands provide a seating surface for the sides of the piston rings.

The main section of the piston is known as skirt. It forms a bearing area in contact with the cylinder wall which takes the thrust caused by the crankshaft.

Some thrust is created on both sides of the piston. “Major” thrust is to the side opposite to the crank throw as it is driven down on the power stroke. “Minor” thrust is the side opposite to the crank throw as the piston moves up on the compression stroke. Pistons are internally braced to make them as strong as possible.

The piston pin (wrist pin) in the piston bosses may also serve as a bearing for the piston, and it may not be located exactly in the middle of the piston. It may be placed as much as 1/16 inch to one side to lessen side thrust of the piston on the cylinder wall.

In some designs, the piston skirt is extended downward on th thrust sides to form what is known as a “slipper” piston. This design feature increases the area of piston contact with the cylinder walls at the thrust faces.

Some pistons are also cut away, or partially away, around the piston pih holes. This “relief” is intended to provide additional clearance to avoid “seizing” if the piston should become overheated and expand excessively.

High compression for starting and lower compression under load is highly desirable for diesel engines. One method of attaining this goal is the variable compression ratio (VCR) piston.

One form of VCR piston has been developed that provides improved starting, better idling and improved output per cubic inch of displacement. Comparisons were made of a VCR piston engine and a conventional engine of the same displacement and same general design. Tests indicated a gross bake horsepower (bhp) for the conventional engine of 550. The engine with VCR pistons produced 1475 bhp.

Other variable compression ratio components include the connecting rod, which is modified to allow engine oil to transfer from the connecting rod bearing through a slipper to the inner piston. The outer piston corresponds to the normal piston in that it contains the combustion chamber and oil rings. The inner piston is an aluminum forging and contains the valves and hydraulic sealing rings. It is connected to its rod by the piston pin in the conventional manner.

The entire assembly of inner and outer pistons is limited n travel by the piston retaining ring, which provides a mechanical stop for upward motion of the piston.

As engine power and combustion pressures are increased, the pressure on the oil in the upper chamber is also increased.

When this pressure exceeds the setting of the spring-loaded discharge valve, a small amount of oil passes to the crankcase, and the outer piston moves downward. This movement results in an increase in clearance volume between the outer piston and the cylinder combustion dome. Which reduces the compression ratio. This process continues until the upper piston contacts the inner piston, preventing any further reduction in the compression ratio.