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May 10 2013

Key Differences Between Petrol & Diesel Engines


The main difference is the ignition process. Both petrol and diesel engines are internal combustion engines, designed to convert the fuel added into mechanical power for the pistons inside the cylinders to move in a vertical manner. We wouldn’t go into the technical details of how an engine works. Basically, both petrol and diesel engines convert these fuels into energy by all the small scale multiple explosions that happen in each cylinder.


In a petrol engine, fuel is mixed with air ignited by sparks from a spark plug while in a diesel engine; first the air is compressed and then injected with fuel. A diesel engine doesn’t have a spark plug to ignite the fuel, it needs to compress it resulting in the air heating up and igniting the fuel injected directly into the chambers, hence called direct injection. Certain diesel engines use what is known as glow plugs, they heat the diesel well in advance before reaching the chamber, making it easier for diesel cars to start up in winters.


Both petrol and diesel engines use a four-stroke engine, the stroke being as follows:

  • Intake Stroke,
  • Compression Stroke,
  • Combustion Stroke,
  • Exhaust Stroke.


A petrol engine over a longer runtime allows a faster revolution rate churning out more power than a diesel engine, which is one of the main reasons why diesel engine is never used for racing cars. Petrol engines are capable of performing at higher RPMs while the diesel engines provide much more torque at lower speeds. Diesel engines also produce approx. 12-13% more CO2 per liter of diesel burned against petrol, meaning diesel pollutes more than petrol.

A diesel engine is always known to give out lag, why does this happen? The accelerator pedal doesn’t exactly respond to a quick throttle response, it has to consider other pressure conditions as well before being able to release more power to the driver, remember, this happens with diesel engines without turbo. However, with the advent of CRDi engines and turbo charged souped up diesel engines are close enough and sometimes even better than a petrol engine. If you drive a petrol engine car then you must have noticed this plenty, the moment you press the accelerator pedal the engine gives an immediate throttle response while releasing more petrol and air to burn for that extra power.


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