Diesel Engine Cycle (AQA A Level Physics): Revision Note

Diesel Engine Cycle

The Diesel Engine Cycle

• A diesel engine cycle is not that different from a petrol engine cycle

• It still consists of 4 strokes, but they work differently

The four 'strokes' of the diesel engine cycle

• In the induction (or intake) stroke, only air is drawn into the cylinder (compared to a petrol-air mixture in the petrol engine)

  • This means there is no fuel in the cylinder during compression

• During the compression stroke, the air is compressed at a high temperature which vapourises and ignites the diesel fuel (as a fine spray) pumped directly into the cylinder through an injector

• The expansion and exhaust stages are similar

Indicator Diagrams

• This theoretical diagram would look like this:

Theoretical indicator diagram for a four-stroke diesel engine

• From A to B:

  • The gas is compressed adiabatically 

• From B to C:

  • Heat is supplied and the pressure is kept constant

• From C to D:

  • The gas expands adiabatically (cooling)

• From D to A:

  • The system is cooled at a constant volume

• The actual indicator diagram looks like:

• The biggest difference is from B to C - there is no sharp peak at the start of the expansion stroke