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Wrong Fuel Recovery – Diesel/Petrol Car Engine

October 19, 2016

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Ever wondered what would happen if you would put petrol in a diesel car? Or diesel in a petrol car?

Well, before I tell you the science behind what happens, you should definitely be prepared for a shock!

To explain things simply, let me enlighten you with some basic properties of petrol and diesel before diving into the science of interchange.

Petrol and diesel are very different fuels. Petrol is much lighter than Diesel, evaporates easily and is less oily. While diesel, in contrast, is much heavier, evaporates very slowly and is much oily. Diesel is typically less volatile than Petrol. Diesel is also less flammable, making it tougher to burn. It also has a higher flash point (The lowest temperature at which fuel can vaporise to form an ignitable mixture with air). The flash point of Petrol is -43 degrees while that of Diesel is 52 degrees.

Now let’s talk about the science behind the interchange.

  1. A petrol engine is based on Otto cycle in which, the heat addition is done at almost constant volume. The ignition is a spark and the compression ratio of the engine is lower than that of diesel engines as the petrol tends to pre-ignite at higher pressures. The charge (petrol + air) is homogeneous and works very close to the stoichiometric ratio.
  1. A diesel engine is based on Diesel cycle in which, the heat addition is done at almost constant pressure. The ignition is through high temperature achieved using higher compression ratio as compared to petrol engines. The air is sometimes additionally heated using glow plugs. The charge (diesel + air) is heterogeneous and multiple pockets of charge having stoichiometric ratio ignite simultaneously. Overall, the diesel engine typically runs lean (much more air than that required as per Stoichiometry).

So, it is easier to burn Petrol, and that is achieved using a simple spark plug. However in case of Diesel, the air is first compressed to a high temperature, and then the fuel (diesel) is injected. Therefore if you put Diesel in the Petrol Engine, it just won’t burn.

Ultimately your engine would be damaged and more importantly burning Petrol in a highly compressed cylinder isn’t safe.


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