Chapter 5 Fundamentals of Engine Construction and Operation 75
Pistons and Rings
Pistons transfer the force of expanding combustion
gases to the connecting rods. They are made of aluminum
to reduce weight. Most automotive pistons have two com-
pression rings and one oil ring, as shown in Figure 5-3.
Compression rings seal in the pressure created dur-
ing the compression and power strokes. If this pressure is
allowed to leak out, the engine will not start or will have
severe power and driveability problems. The compres-
sion rings are installed at the top of the piston. A film of
oil between the compression ring and cylinder wall seals
pressure in the cylinder. This oil film is only about .001‡
(.0025 mm) thick, but if it is removed, the engine will not
develop enough compression to start.
The oil-control ring is installed below the compres-
sion rings to prevent excessive oil consumption. During the
piston’s intake stroke, vacuum in the cylinder tries to pull
oil from the cylinder wall. To reduce oil loss, the oil-control
ring scrapes most of the oil from the cylinder wall when the
piston is moving down in the cylinder. A small amount of
oil passes by the oil-control ring to seal the compression
rings against the cylinder wall.
Connecting Rods and Crankshaft
The connecting rods are forged steel rods that connect
the piston to the crankshaft. They transfer the force from the
piston to the crankshaft. Each connecting rod is connected
to a piston by a piston pin. The rod is attached to the crank-
Figure 5-2. A—The engine block starts out as molten metal poured into a mold. B—The casting is then machined to accept and
support the other engine parts. (BMW, Honda)
A B
Figure 5-3. The pistons and rings are the parts that most people associate with the compression system. A—This shows the
various parts of the piston. B—Compression and oil-control ring installation. Most engines with spark ignition use this type of ring
configuration.
Head
Skirt
Ring
grooves
Oil-control
ring
Compression
rings
{
A B