Working of a Petrol Engine

Petrol Engine is an internal combustion engine in which a mixture of air and petrol (98% air and 2% petrol vapour is used as working substance. The working of petrol engine is divided into four steps as explained below and its PV diagram is shown in figure.

Petrol Engine and its PV diagram
Petrol Engine and its PV diagram
  1. Suction stroke: In this stroke, the inlet valve I is opened and air – petrol mixture is sucked into the cylinder by the outward motion of the piston. This process is represented by PA on the PV diagram.
  2. Compression stroke: In this stroke, both the inlet valve I and the outlet valve O are closed. The mixture undergoes adiabatic compression and its volume is reduced to about 1/5th of the original volume. As a result, the temperature of the mixture is raised to about 600°C. This process is represented by AB on the PV-diagram.
  3. Working stroke: During this stroke, both the inlet and outlet valves remain closed. A spark is produced through the spark plug S and the compression mixture of petrol vapour and air ignites. The temperature rises to about 2000°C and a pressure of about 15 atmospheres is developed inside the cylinder. This process is represented by BC on the PV diagram. Due to the ignition of the mixture, the piston is pushed out with a great force and the shaft rotates. The engine does work in this stroke. This process is represented by CD on the PV diagram.
  4. Exhaust stroke: The outlet valve opens and the burnt out gases are driven out until the pressure in the cylinder falls to the atmospheric pressure. This process is represented by DA on the PV diagram. Now, from position A, the piston moves inside and the rest of the combustion products are exhausted at constant pressure. It is called exhaust stroke and is represented by AP on the PV diagram.

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