WORKING PRINCIPLE OF SPARK-IGNITION ENGINE
SI engine is a type of ICE consisting of a spark plug, for the purpose of igniting the mixture of air and fuel, mainly designed to run on petrol (gasoline). Usually, the air and fuel are mixed together in the intake, either by using a carburetor or fuel-injection system (electronically controlled). The ratio of mass flow of air to fuel must be approximately around 15 for reliable combustion.
CYCLE ON WHICH SI ENGINE IS BASED
OTTO CYCLE
Otto cycle; an idealized thermodynamic cycle, describes the theoretical working of an SI engine. It is a description of what happens to a mass of gas when it is subjected to a change of pressure, temperature, addition of heat, removal of heat and volume.
Fig: P-V and T-S Diagram of SI Engine
Process 0-1: Intake stroke
Mass of air is drawn into the cylinder, at constant pressure, from the atmosphere.
Process 1-2: Compression stroke
Piston moves from BDC to TDC as the working fluid is compressed isentropically. The compression ratio for SI engine should in the range of 6-12.
Process 2-3: Ignition or heat addition stroke
It is a constant volume heat addition process where the compressed mixture is ignited with the help of spark plug.
Process 3-4: Expansion stroke (power stroke)
Increased pressure exerts a force on the piston causing it to move downwards to the BDC. Expansion of the working fluid is isentropic, and work is done on the piston.
Process 4-1: Heat rejection stroke
Working fluid pressure decreases during a constant volume process as heat is rejected to an idealized sink.
Process 1-0: Exhaust stroke
The exhaust valve opens, as the piston moves from BDC to TDC, and the gaseous mixture is vented out into the atmosphere and the process starts anew.
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