Valve operation

Function

The two main types of internal combustion engine are: spark ignition (SI) engines (petrol, gasoline, or gas engines), where the fuel ignition is caused by a spark; and compression ignition (CI) engines (diesel engines), where the rise in pressure and temperature is high enough to ignite the fuel. Valves are used in these engines to control the induction and exhaust processes.

Both types of engine can be designed to operate in either two strokes of the piston or four strokes of the piston. The four-stroke operating cycle can be explained by reference to Fig. 2.1. This details the position of the piston and valves during each of the four strokes.

1. The induction stroke The inlet valve is open. The piston moves down the cylinder drawing in a charge of air.

2. The compression stroke Inlet and exhaust valves are closed. The piston moves up the cylinder. As the piston approaches the top of the cylinder (top dead centre – tdc) ignition occurs. In engines utilizing direct injection (DI) the fuel is injected towards the end of the stroke.

3. The expansion stroke Combustion occurs causing a pressure and temperature rise which pushes the piston down. At the end of the stroke the exhaust valve opens.

4. The exhaust stroke The exhaust valve is still open. The piston moves up forcing exhaust gases out of the cylinder.

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N G I N E E R I N G R E S E A R C H S E R I E S

Automotive Engine Valve Recession

R Lewis and R S Dwyer-Joyce

Series Editor

Duncan Dowson

Professional Engineering Publishing Limited,

London and Bury St Edmunds, UK

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