Diesel engines are more efficient than petrol engines due to higher compression ratios

Engineering Physics | AQA A-Level Physics

Key Definition

Compression ratio: The ratio of the volume enclosed at the beginning of the compression stroke to the volume enclosed at the end of the compression stroke, i.e. $\frac{V_1}{V_2}$.

Compression ratio and efficiency

Efficiency is directly proportional to the compression ratio of an engine.
The compression stroke is when the volume of gas decreases as the piston moves upwards, doing work on the gas.
On an indicator diagram, this is the ratio $\frac{V_1}{V_2}$, where $V_1$ is the maximum volume and $V_2$ is the minimum volume.
A diesel engine achieves a much higher compression ratio, typically about 16, whilst a petrol engine is about 10.
Diesel engines require this higher compression ratio to get the air temperature high enough for the fuel to self-ignite.

Why petrol engines have lower compression ratios

Petrol engines are limited in their compression ratio because if the ratio is too high, the petrol-air mixture could self-ignite before the spark. This is called pre-ignitionUnwanted early ignition of the fuel-air mixture before the spark plug fires, caused by excessive temperatures and pressures from too-high compression ratios..
Pre-ignition can also occur if there has been a build-up of carbon deposits in the cylinder from burnt oil.

Advantages and disadvantages

Diesel advantages: more efficient (higher compression ratio), produce fewer carbon monoxide and hydrocarbon emissions.
Diesel disadvantages: operate at higher working pressures, so they must be more robust and are more expensive to manufacture. They also have a lower power-to-weight ratio.
Petrol disadvantages: produce more carbon monoxide, hydrocarbons and carbon dioxide. This can be partly addressed using a catalytic converter, which oxidises the pollutants, but emissions are still not reduced below diesel levels.

Common Mistake

Students sometimes write that diesel engines are "better in every way." They are more efficient, but they are heavier, more expensive, and operate at higher pressures. Always discuss both advantages and disadvantages when comparing the two types.