Construction of a Piston

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.

Piston Engine Cylinders

The portion of the engine in which the power is developed is called the cylinder.  The cylinder provides a combustion chamber where the burning and expansion of gases takes place, and it houses the piston and the connecting rod.

There are four major factors that need to be considered in the design and construction of the cylinder assembly.  These are:

·         It must be strong enough to withstand the internal pressures developed during engine operation.

·         It must be constructed of a lightweight metal to keep down engine weight.

·         It must have good heat-conducting properties for efficient cooling.

·         It must be comparatively easy and inexpensive to manufacture, inspect, and maintain.

The head is either produced singly for each cylinder in air-cooled engines, or is cast ‘in-block’ (all cylinder heads in one block) for liquid-cooled engines.  The cylinder head of an air-cooled engine is generally made of aluminium alloy, because aluminium alloy is a good conductor of heat and its light weight reduces the overall engine weight.  Cylinder heads are forged or die-cast for greater strength.  the inner shape of a cylinder head may be flat, semispherical, or peaked, in the form of a house roof.  The semispherical type has proved most satisfactory because it is stronger and aids in a more rapid and thorough scavenging of the exhaust gases.

·         The Cylinder Head
·         The Cylinder Barrel

At assembly, the cylinder head is expanded by heating and then screwed down on the cylinder barrel which has been chilled, thus, when the head cools and contracts, and the barrel warms up and expands, a gastight joint results.  While the majority of the cylinders used are constructed in this manner, some are one-piece aluminium alloy sand castings.  The piston bore of a sand cast cylinder is fitted with a steel liner which extends the full length of the cylinder barrel section and projects below the cylinder flange of the casting.  This liner is easily removed, and a new one can be installed in the field.