1. Piston Rings

1.4. Compression Rings

LO 7-05: Identify and explain the purpose and functions of piston rings, their construction features, and their ring shapes according to ring function.

 

Compression Rings

Compression rings are located nearest the piston crown. Their primary job is to prevent gases from leaking past the piston during the compression and power strokes. Gas leakage past these rings, or blowby, robs the engine of power because it reduces compression pressures. It may even allow combustion gases to leak into the crankcase.

Compression rings perform a sealing function in a number of ways. One way is through the static force of ring tension against the cylinder wall. Because rings are manufactured to have a larger relaxed diameter than the diameter of the cylinder they are to be installed in, sealing is accomplished by the ring’s spring tension expanding out against the cylinder wall. In older rings, static ring force could easily be over 20 lb (9.1 kilogram [kg]). However, to minimize friction losses in an engine, high ring tension is not desirable. Thinner rings can seal better by using their knifelike edge. A thinner ring directs more pressure against a small surface area. Today, ring tension may be as low as 4–8 lb (1.8–3.6 kg). These low-tension rings significantly improve fuel economy by decreasing internal friction and cylinder wear.

Another way compression rings seal is through the use of gas pressure during the compression and power strokes, which pushes the ring out with more force against the cylinder wall. To accomplish this, the thickness of the ring is made slightly less than the height of the ring groove. With this arrangement, gas pressure is able to get behind the ring and push it out against the cylinder wall more forcefully than ring static tension Figure 31. Gas pressure also pushes the ring down against the bottom of the groove for better sealing. This means that the higher the cylinder pressure, the greater the sealing capability of the ring. During the power stroke, the compression and intermediate rings seal more effectively (this is also the reason why most blowby occurs during the compression stroke).

Piston speed and size are factors in determining how many compression rings are used on a piston. Engines operating at slower speeds require more rings to minimize the loss of compression pressure because more time (during the compression stroke) is available for gases to leak past the rings. Compression rings, like all piston rings, are split for installation and to accommodate thermal expansion. Because gases can escape through the ring end gap, two compression rings are used on some pistons. Even more compression rings are added to slow-speed diesels, such as those found in locomotive and ships. Again, the additional rings help minimize the loss of compression pressure due to increased blowby that can occur at low piston speeds. <.p>

   Fig07_31

FIGURE 07-31: Gas pressure enhances ring sealing and twists the intermediate ring. The ring notch can be either on the top or on the bottom to enhance the compression function oil control.

 

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