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 four strokes, but they work differently

• 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 vaporises 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:

• 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