TABLE OF CONTENTS The basic function of an engine is to convert chemical energy (the fuel) into mechanical energy and to produce usable power and torque (this is the ability to turn the driving wheels and move the vehicle). The spark ignition (SI) engine operates on the principle of the Otto four-stroke cycle or the Clerk two-stroke cycle of operations.
To complete the cycle of operations, four strokes of the piston are used. This involves two complete revolutions of the crankshaft, the inlet and exhaust valves being mechanically opened and closed at the correct times. Starting with the piston at TDC and the crankshaft rotating clockwise (looking from the front of the engine), the strokes operate as follows.
With the inlet valve open and the exhaust valve closed the piston moves in a downwards direction drawing in a mixture of petrol vapour and air. This is called the induction stroke.
The piston moves up with both valves closed, thus compressing the mixture into the combustion chamber at the top of the cylinder. This is the compression stroke.
At the end of the compression stroke a spark occurs at the sparking plug. This ignites the mixture which burns very rapidly heating the gas to a very high temperature which also raises its pressure. This forces the piston down the cylinder and is called the power stroke.
As the piston begins to rise the exhaust valve...
