• Question: What is the difference between a diesel and a petrol engine?

    Asked by anon-74766 on 2 Jun 2020.
    • Photo: James Mitchell

      James Mitchell answered on 2 Jun 2020:

      A diesel and petrol engine work in a very similar way. They are both internal combustion engines that convert the chemical enenrgy of a fuel into mechanical energy. They both work on a four stroke cycle, but the fuel-air mixture is ignited slightly differently:

      PETROL ENGINE
      1. The engine intakes air and fuel at the same time, which moves the piston down.
      2. The valves close and the piston moves back up compressing the air-fuel mixture. The fuel-air mixture is not compressed too much and a spark plug (electrcity) is used to ignite the air-fuel mixture.
      3. The piston moves back down because of the combustion which is connected to something (crankshaft) that then turns the wheels
      4. The piston moves back up and the ignited air-fuel mixture is sent out of the engine to the exhaust.

      DIESEL ENGINE
      1. The engine ONLY intakes AIR, which moves the piston down
      2. The valves close and the piston moves back up compressing the air. The air is compressed much more than in a petrol engine, which makes the air much hotter. The fuel is fed into the engine when the air is compressed and it combusts automatically without having to supply electricity (there is no spark plug).
      3. The piston moves back down because of the combustion which is connected to something (crankshaft) that then turns the wheels.
      4. The piston moves back up and the ignited air-fuel mixture is sent out of the engine to the exhaust.

      I hope that makes sense and this video helps explaoin it too:

    • Photo: Juan Carlos Fallas-Chinchilla

      Juan Carlos Fallas-Chinchilla answered on 3 Jun 2020:

      In addition to James explanation, diesel engines produce more torque (hence their traditional use in lorries, tractors, buses and heavy machinery). Torque is defined as a “twisting force that causes rotation,” and in the case of diesel engines, that rotation leads straight to the wheels.

      Petrol engines accelerate faster and hence their traditional use in smaller vehicles. I remember there was an eager in the late 90s to replace petrol cars for diesel ones. Engines were adapted for that purpose and diesel cars were also accelerating faster but now the trend is reverting and diesel cars are being banned in cities due to higher pollution.

      Personally I have had both petrol and diesel cars. I preferred my diesel engine but I’m happier commuting by public transport or cycling and have clean air 🙂

    • Photo: Martin McKie

      Martin McKie answered on 3 Jun 2020:

      Great expiation by James.

      Just to add there are different variations on the petrol engine. So there is the conventional piston engine which is shown in the video by James and there is also engines such as the rotary engine.

    • Photo: anon

      anon answered on 14 Jun 2020:

      I second James, with the ‘Car Throttle’ video. It is very informative and explains well to laymen’s.

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