Comparing Petrol & Diesel Engines (AQA A Level Physics): Revision Note

Comparing petrol & diesel engines

• Diesel engines are more efficient than petrol engines, and this can be seen from their indicator diagrams

• Efficiency is directly proportional to the compression ratio of an engine, which is defined as:

$\frac{\mathrm{volume}\mathrm{enclosed}\mathrm{at}\mathrm{the}\mathrm{beginning}\mathrm{of}\mathrm{compression}\mathrm{stroke}}{\mathrm{volume}\mathrm{enclosed}\mathrm{at}\mathrm{the}\mathrm{end}\mathrm{of}\mathrm{the}\mathrm{compression}\mathrm{stroke}}$

• The compression stroke of the engine is when the volume of the gas decreases, as the piston moves upwards

  • This is when work is done on the gas

• On an indicator diagram, this is the ratio $\frac{V_1}{V_2}$

• The area under this line is the work done on the gas

• A diesel engine can achieve a much higher compression ratio, typically 16, whilst a petrol engine is about 10

  • This is an indication that diesel engines are more efficient

• Diesel engines require this higher compression ratio to get the pressure and temperature of the air high enough for the diesel fuel to self-ignite

  • The petrol-air mixture in a petrol engine is ignited by a spark at lower pressures (and temperatures)

• Petrol engines are limited in their compression ratio

  • If the compression ratio is too high, the petrol-air mixture could self-ignite before the spark (pre-ignition), due to the higher temperatures and pressures

  • This can also happen if there has been a build-up of carbon in the cylinder (from burnt oil accidentally inside)

    • This can also ignite the petrol-air mixture before the spark

• The disadvantage of diesel engines is that they operate at higher working pressures, which makes them more expensive to produce as they have to be more robust and stable

• They have a lower power-to-weight ratio

• Petrol engines produce more carbon monoxide, hydrocarbons, and carbon dioxide than diesel engines

  • This can be improved using a catalytic converter, which oxidises the pollutants and reduces the harmful emissions

  • However, they are still not reduced below the levels of a diesel engine