Monday, September 3, 2012

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.