Understanding Fuel Mix-up: Diesel in a Petrol Engine

When diesel fuel is accidentally put into a petrol engine, the effect is quite significant, leading to the engine not running correctly, if at all.

Let's understand why this happens by looking at the fundamental difference between how petrol and diesel engines operate and how their respective fuels ignite:

• Petrol Engine Ignition: Petrol engines are also known as spark-ignition engines. They mix petrol with air, compress the mixture, and then use a spark plug to ignite it. Petrol is a volatile fuel that ignites easily with a spark.
• Diesel Engine Ignition: Diesel engines are known as compression-ignition engines. They compress air to a very high pressure, which makes it very hot. Then, diesel fuel is injected into this hot, compressed air, and the heat of the air causes the diesel to ignite spontaneously without a spark plug.

Diesel fuel is much less volatile than petrol. It requires high compression and temperature to ignite, unlike petrol which ignites readily with a spark. When diesel fuel enters the combustion chamber of a petrol engine, the spark plug attempts to ignite it. However, the spark is not hot or powerful enough to ignite the diesel fuel under the conditions present in a petrol engine (lower compression ratio compared to a diesel engine).

Because the fuel-air mixture fails to ignite properly or consistently, the engine will not be able to complete its combustion cycle. The engine might crank, but it will not start or run. Trying to run it could potentially cause damage to the fuel system and catalytic converter as unburnt fuel passes through.

Therefore, if diesel fuel is supplied in a petrol engine, the engine will not run.