Using petrol in diesel engine and vice versa.

Why I can’t use petrol in diesel engine and vice versa?

This question arises in everyone’s mind once in a while but the answer is quite hard to find even today.

If you are here just for the answer then here it is.

1      1) If we add diesel in petrol engine, it won’t work at all or may run certain distance with huge smoke and less power. As the pressure inside the cylinder will not be sufficient to keep it running.

2      2) If we add petrol in diesel engine, it will run with extreme power and speed but the knocking in engine and pressure generated in the cylinder will end up in a blast of engine soon. As the engine is not designed to operate at such pressure.

Now for those who really want to explore this topic in more depth and have some crazy ideas to think about then,

Let’s start with understanding, what is the basic difference in these two engines, also the effects of all the relative parameters in shortest way possible.

Petrol Engine/Spark Ignition Engine:

Air fuel mixture (approx. 15:1) enters the engine at suction stroke and starts to get compressed upto the temperature and pressure enough to be ignited by the spark of about 20000 volts generated by the spark plug, which increases the temperature of charge around 700 degrees.

The important factor here is compression ratio of the engine, which is from 7 to 12 in case of SI engine.

Diesel Engine/ Compression ignition:

Only air is sucked in the engine at the suction stroke and compressed upto the high pressure upto 30 bars–50 bars and temperature around 600 degree Celsius. Which is sufficient for diesel to autoignite and hence no spark plug is needed.

To get such high pressure and temperature the compression ratio of the CI engine is very high around 15 to 22 in some cases.

Crazy imaginations:

1)   What if we go on increasing the compression ratio of petrol engine and use the diesel in it?

è High compression ratio will cause knocking and which will lead to high mechanical and thermal stresses on the engine cylinder and piston head.

       2) What if we blend some additives in petrol so that it can autoignite at lower temperature without the spark plug?

è Well it seems like a possible way but we should consider the factor of pre-ignition and blends may lead to change in calorific value and exhaust emission may get affected by it.