CYLINDER BLOCKS
All the major engine components, are installed on or in the engine block. These components including the cylinder bores, are machined very precisely. They must be thick enough to contain the pressure of the burning fuel mixture. A tight fit must be ensured between the cylinder base and the piston rings to enable the piston rings to seal the combustible gas. If the cylinder becomes oval due to wear some of the gas escapes through the piston rings. The gas which leaks through the piston rings is called blow-by (Fig. 3.2). Blow-by reduces the efficiency of an engine. The finishing on the cylinder walls also affects the ring seal. The cylinder walls are machined to provide a very
smooth finish. Special grinding stones produce small groves in the cylinder walls, which collect oil. These grooves help to lubricate the piston rings and piston skirts.
Previously, most cylinder blocks were made of cast iron or grey iron as the material was easy to machine. Aluminium pistons wear very well against cast iron cylinder walls. The main disadvan- tage of iron being is its weight, engine blocks are now being cast from lightweight aluminium. An aluminium block weighs much less than a cast iron block. An aluminium piston skirt rubbing against an aluminium cylinder wall wears very quickly. Most aluminium cylinder blocks are fitted with steel or ductile iron cylinder bore liners.
