Petrol Engines

Diesel Engines

  • A petrol Engine draws a mixture of petrol and air during suction stroke.
  • A diesel Engine draws only air during suction stroke.
  • The Carburettor is employed to mix air and petrol in the required proportion and to supply it to the engine during suction stroke.
  • The injector or atomiser is employed to inject the fuel at the end of compression stroke.
  • The Pressure at the end of compression is about 10bar.
  • The pressure at the end of compression is 35bar.
  • The charge (i.e. Petrol and air mixture) is ignited with the help of spark plug.
  • The Fuel is injected in the form of fine spray. The temperature of the compressed air (about 600C at pressure of about 35 bar) is sufficiently high to ignite the fuel.

(1 bar = 10^5 N/m^2)

  • The combustion of fuel takes place approximately at constant volume. In other words, it works on Otto cycle.
  • The combustion of fuel takes place approximately at constant pressure. In other words, in other words it works on diesel cycle.
  • A petrol Engine has compression ratio approximately from 6 to 10.
  • A diesel engine has compression ratio approximately from 15 to 25.
  • The Starting is easy due to low compression ratio.
  • The starting is little difficult due to high compression ratio.
  • As the compression ratio is low, the petrol engine are lighter and cheaper.
  • As the compression ratio is high the diesel enginesĀ  are heavier and costillier.
  • The maintenance cost is less
  • The maintenance cost is more.
  • The thermal efficiency is upto about 26%.
  • The thermal efficiency is upto about 40%.
  • Over heating trouble is more due to low thermal efficiency.
  • Over heating trouble is less due to high thermal efficiency.
  • These are high speed engines
  • These are relatively low speed engines
  • The petrol engines are generally employed in light duty vehicles, such as scooters, motorcycles, cars, these are also used in aeroplanes.
  • The diesel engines are generally employed in heavy duty vehicles such as buses, truck, and earth moving machines.