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Operating Principle of Four Stroke Petrol Engine (S.I. Engine)

The four stroke S.l. engine operates in a otto cycle which is air standard cycle. The four stages are explained below : (Ref Fig. 10.5 (a))

Stage 1 (Suction Stroke)

Suppose the piston is at TDC. Now it starts moving downward creating vacuum above it. The air fuel mixture from carburettor enters through inlet manifold to the cylinder. The inlet valve remains open till the piston reaches BDC. The outlet valve remains closed during this suction stroke. (Ref Fig. 10.5 (b))


Stage 2 (Compression Stroke) : (1 – 2) Process

The piston starts moving upward from BDC. Inlet cu<d outlet valves remain closed. The air fuel mixture is compressed with a compression ratio of 5 to 10.5 generally. The piston moves upward until it reaches TDC. The high pressure and high temperature (250°C to 500°C) are attained during this compression stroke. (Ref. Fig. 10.5 (c))


Stage 3 (Expansion Stroke or Working Stroke or Power Stroke) : (3-4 Process)

The charge is ignited by spark plug shortly before the piston reaches the TDC ·in the compression stroke (2-3 process). The product of combustion expands and give thrust to piston causing it to move downward. Both the valves are closed. We get power during this expansion stroke. The flywheel (not shown here) gets energy and provides other three strokes. At the end of expansion stroke heat is rejected at constant volume condition (4-1 process). (Ref. Fig. 10.5 (d))


Stage 4 (Exhaust Stroke) : (1-A) Process

Now the exhaust valve opens and the burnt gases are removed from the cylinder. The piston moves upto TDC. The inlet valve remains closed. The process of removing gases is also called scavenging. (Ref. Fig. 10.5 (e))