Question

If petrol is used in a diesel engine, then __________

(a) low power will be produced

(b) efficiency will be low

(c) higher knocking will occur

(d) black smoke will be produced

This question was addressed to me in an international level competition.

My question comes from Comparison of Two Stroke S.I. and C.I. Engines topic in portion Two Stroke Engines of IC Engine

Answer 1

The correct answer is:

(c) higher knocking will occur

Explanation:

If petrol is used in a diesel engine, several issues may arise:

1. Higher Knocking: Diesel engines rely on compression ignition (high pressure and temperature in the cylinder) to ignite fuel. Petrol has a much lower ignition temperature and a lower cetane rating compared to diesel. When petrol is used, it can ignite prematurely due to the higher compression, causing knocking (abnormal combustion) in the engine.

2. Low Power and Efficiency: Petrol doesn’t burn as efficiently in a diesel engine, resulting in low power output and reduced efficiency. Diesel engines are specifically designed to use the high-energy density of diesel fuel, and using petrol disrupts the combustion process.

3. Black Smoke: Black smoke is typically associated with diesel engines running rich (too much fuel or poor combustion). While it could potentially happen when using petrol, it’s not the primary outcome. The primary issue will be knocking rather than the production of black smoke. 

Thus, the most accurate answer is that higher knocking will occur when petrol is used in a diesel engine.