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Comparision of Petrol and Diesel Engine

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.

2 thoughts on “Comparision of Petrol and Diesel Engine

  1. Kashif says:

    * The Carburettor is employed to mix air and petrol in the required proportion and to supply it to the engine during suction stroke
    CORRECTION: Most of the engines have Injectors which don’t need a carburettor. They use throttle body to control air.
    * A petrol Engine draws a mixture of petrol and air during suction stroke.
    CORRECTION: Only air is drawn, petrol is injected by the injectors
    *The starting is little difficult due to high compression ratio.
    CORRECTION: It is not difficult at all, because of bigger flywheels as compared to petrol engines
    *Over heating trouble is less due to high thermal efficiency.
    CORRECTION: no practical evidence, depends more on the cooling systems
    *The maintenance cost is more
    CORRECTION: No practical evidence, depends on preventive maintenance plans

    Written by: Some one who is handling a fleet of 20 trailers, 15 trucks, 30 Pickups, 50 light cars, 15 lifting equipment, 35 earth moving equipment, 50 diesel generator sets and a lot of other construction related equipment.

  2. afaque says:

    Dear Kashif

    Thank you for your interest in the article.
    i want to more clear here that, as you are saying some engines using injectors, yes you are rite EFI (Electronic Fuel Injection) new model or some old model cars using injectors but not all. so you can not say it is a wrong, because carburetors are also used.

    * A petrol Engine draws a mixture of petrol and air during suction stroke.
    as i said im talking about carburetor engines so it will draws air and petrol in suction stroke.

    *The starting is little difficult due to high compression ratio.
    it is practically observed in our labs that diesel engines are difficult to start as compared to petrol engines.

    *The maintenance cost is more
    it is more because spare parts of petrol engines are expensive, simply you can compare spark plug and fuel injector, you will feel the difference in price.

    this article is written from the book of “THERMAL ENGINEERING” by ” R.S KHURMI AND J.K GUPTA on page no: 592, chapter no: 26 title: internal combustion Engines.

    im not saying you are wrong, but this may be our knowledge is low, and your correction is appreciated, we will definitely research on your comments.

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