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:
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
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