Pistons
Pistons and rings as they relate to an internal combustion engine
The burning of air and fuel inside an engine creates both heat and pressure. It is the job of pistons and piston rings to contain this pressure so that the maximum amount of work is transmitted to the engine’s crankshaft and vehicle drivetrain. There is also a measure of oil control that must be provided by an engine’s oil rings, rounding out the piston ring requirement. Just how all this is accomplished, in addition to what constitutes basic piston design, is the subject of this month’s Series. First, let’s define some basic terms.
Spend a few minutes studying the illustrations. These will introduce you to the fundamental terminology of pistons and rings. Note that cylinder pressure can be used to improve ring-to-wall seal. This is a typical method in the building of race or high-performance street engines where it is beneficial to have low piston ring tension (against the cylinder wall) yet maintain good combustion pressure ring seal.
Such an approach has also been used in engines built for fuel economy whereby the top compression ring has been located very near the piston’s top, thus reducing so-called “crevasse volume” or the air space standing between the cylinder wall and piston just above the top compression ring. There’s a photo showing one experimental example of crevasse volume reduction for purposes of improved fuel economy and lowered exhaust emissions. You might know of this approach as a Dykes ring design, but it’s different in that the top ring is shaped like a laid-over letter U: with the open end of the U facing the piston. Just a little food for thought.
Compression rings help hold combustion compression; thus the name. Oil rings are designed to help lubricate cylinder walls while preventing the passage of oil into the combustion chamber. Oil in this area can encourage detonation, reduce fuel economy, and generally diminish overall combustion efficiency.
Piston domes (or cavities) are used to establish specific mechanical compression ratios, and piston skirts are intended to provide support within a cylinder while aiding lubrication to remainder of the piston. The barrel shape of a piston skirt assists piston wear (and engine silencing) during times when a piston is “cocked” as a function of connecting rod angle and load. Piston skirt “cam” is a feature affecting the amount of skirt material in contact with the cylinder wall. As shown in the illustrations, the greater the cam angle, the greater will be the amount of thrust loading absorbed by a given piston’s skirt. And the more a piston can be designed to absorb thrust loadings, the less force will be transmitted into the cylinder walls. This reduces friction, which decreases power loss, which increases power and fuel economy, which . . . lived in the house that Jack built. Other design features are included in the illustrations, along with explanations.
A. Here you can see basic piston terminology, including major and minor thrust locations. Of particular note is the fact that piston pin offset can be used to counteract combustion pressures on the piston, resulting in the reduction of piston-to-wall frictional losses. Connecting rod length is a consideration in determining the amount of pin axis offset. For example, the longer the rod the less the tendency for a piston to become “cocked” in the cylinder, thus affecting the amount of offset that can be used. Piston skirt “barrel” (dotted line) helps maintain skirt clearance and aids wear life. B. Piston skirt “cam” dimension relates to the amount of skirt material in contact with cylinder wall (shaded area). As this contact area is increased, thrust loads are distributed over a greater amount of piston skirt material.