What happens when we add petrol in a diesel car or vice-versa?

His is a very common doubt among people of all ages.
Firstly, we should understand the common differences between petrol and diesel.

Petrol has higher self-ignition temperature (246 °C), compared to diesel (210 °C)
Petrol engines use spark plugs to ignite petrol, whereas diesel self ignites due to the temperature and pressure created during the compression ratio.
Petrol is highly volatile compared to diesel.
If petrol is ignited in a cylinder having a compression ratio exceeding 11:1, knocking becomes a common phenomenon.
Diesel needs to be injected at very high pressure (1000 bar to 2000 bar) so that it is atomized and readily burns when injected inside the combustion chamber. Petrol is injected or mixed with air at a very low pressure of 2 to 4 bar.

Also, read – Difference Between SI engine and CI engine

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When petrol is added in a diesel engine:

The temperature and pressure created during compression stroke will be very high in diesel engines. This will result in the self-ignition of petrol. It will lead to knocking creating loud thumping noises. It will put a lot of stress on piston and cylinder walls leading to damage.

When diesel is added in a petrol engine:

The engine won’t start simply because diesel is less volatile and
It will not mix with air to form a combustible mixture. Diesel has to be atomized to make sure it burns readily when injected. But the fuel injectors in petrol engines don’t develop
Enough pressure to atomize diesel. Hence, combustion doesn’t take place.