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:
- 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
- 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 oxidised the pollutants and reduces the harmful emissions
- However, they are still not reduced below the levels of a diesel engine
