The piston of a reciprocating engine is a
cylindrical member which moves back and forth within a steel cylinder. The piston acts as a moving wall within the
combustion chamber. As the piston moves
down in the cylinder, it draws in the fuel/air mixture. As it moves upward, it compresses the charge,
ignition occurs, and the expanding gases force the piston downward. This force is transmitted to the crankshaft
through the connecting rod. On the
return upward stroke, the piston forces the exhaust gases from the cylinder.
The majority of aircraft engine pistons are
machined from aluminium alloy forgings.
Grooves are machined in the outside surface of the piston to receive the
piston rings, and cooling fins are provided on the inside of the piston for
greater heat transfer to the engine oil.
Pistons may be either the trunk type or the
slipper type. Slipper type pistons are
not used in modern, high-powered engines. All the rings will be fitted
above the gudgeon pin(Piston pin). The top face of
the piston, or head, may be either flat, convex, or concave. Recesses may be machined in the piston head
to prevent interference with the valves.
As many as six grooves may be machined
around the piston to accommodate the compression rings and oil rings. The compression rings are installed in the
three uppermost grooves; the oil control
rings are installed immediately above the piston pin. The piston is usually drilled at the oil
control ring grooves to allow surplus oil scraped from the cylinder walls by
the oil control rings to pass back into the crankcase. An oil scraper ring is installed at the base
of the piston wall or skirt to prevent excessive oil consumption. The portions of the piston walls that lie
between each pair of ring grooves are called the ring lands.
In addition to acting as a guide for the
piston head, the piston skirt incorporates the piston-pin bosses. The piston-pin bosses are of heavy
construction to enable the heavy load on the piston head to be transferred to
the piston pin.
