Efficiency: Diesel vs Petrol engines

Petrol vs Diesel Engines. Image credit carblogindia.com

Efficiency refers to the fuel economy of an engine
It measures the amount of chemical energy in the fuel that is converted to kinetic energy
The efficiency of Petrol engines is around 25%
Diesel engines have an efficiency of around 40%
This means diesel engines have a higher fuel efficiency when compared to Petrol engines
That is a car with a diesel engine will in theory travel longer per each litre of fuel when compared to a Petrol car
Efficiency can thus also be measured in fuel economy terms
Both diesel and petrol engines however cause pollution due to incomplete combustion of their respective fuels
This is because the air supply in the engines is limited
Carbon (soot) and carbon monoxide are produced
Diesel engines produce less carbon monoxide as they are more efficient

How to improve the efficiency of diesel and petrol engines?

Cleaning/Replacing blocked and old air filters regularly to ensure an adequate supply of air
Ensuring the choke is functioning properly
Cleaning/Replacing blocked and worn out fuel jets
Ensure that the piston rings fit well.
This improves compress
Also if air seeps past the rings it burns in the cylinders producing black smoke(soot/carbon)
Poorly fitting rings results in an inefficient engine

Differences between Diesel and Petrol engines

Diesel Engine	Petrol Engine
Diesel fuel is used	Petrol is used as a fuel
Fuel is injected by the fuel injector after the air has been compressed	Petrol/Air mixture is introduced into the cylinder through jets
Fuel is ignited by the heat of compressed air	Fuel is ignited by sparks from spark plugs
High compression	The compression is half that of the diesel engine
The engines are heavier	The engines are lighter than diesel engines
40% with less carbon monoxide are thus more economical	25% efficient more carbon monoxide thus less efficient

To access more topics go to the Combined Science Notes page.

Efficiency: Diesel vs Petrol engines- Updated 2025