Principles of Engine Operation

Engines operate on cycles. There are four strokes of the piston in one cycle of engine operation. There are two outward strokes toward the crankshaft and two inward strokes away from the crankshaft.

When the pistons is at the end of the stroke away from the crankshaft (inward stroke) this is top dead centre (TDC). When the piston is at the end of the out ward stroke (toward the crankshaft) this is bottom dead centre (BDC). The piston movement from TDC to BDC is an engine stroke.

The four strokes in a cycle of the internal combustion engine are: intake, compression, power and exhaust.

Intake. During the intake stroke the piston moves to BDC and the intake valve opens. This movement of the piston draws a mixture of air and fuel into the cylinder (in a diesel this movement of the piston draws in air only).

Compression. When the piston reaches BDC it moves toward the cylinder head (inward motion). The valves do not open and the piston compresses the fuel mixture between the piston and the cylinder head (in a diesel the piston compresses air only).

Power. When the piston reaches TDC, an electric spark ignites the fuel mixture in the combustion chamber of the gasoline engine (in a diesel engine the heat of the highly compressed air ignites the fuel).

When the air-fuel mixture burns it moves the piston with great force.

There are higher pressures in the diesel engines and because of these pressures the diesel engines have heavier piston pins, connecting rods and crankshafts than the gasoline engines.

Exhaust. The exhaust stroke takes place when the piston moves up. The exhaust valve opens and the piston forces out the gases. The new cycle will begin in the cylinder.

Because of the four strokes we call this engine a four stroke-cycle engine. The four-stroke-cycle engine with spark ignition is the most common type of the internal combustion engine.