The diesel engine uses compression ignition rather than a spark

How the diesel cycle differs

• The diesel engine still operates on four strokes, but the key difference is in how the fuel is ignited.
• During the induction stroke, only air is drawn into the cylinder (not a petrol-air mixture). This means there is no fuel present during compression.
• During the compression stroke, the air alone is compressed adiabatically to a very high temperature.
• Crucially, diesel fuel is then injected as a fine spray directly into the hot, compressed air. The air is so hot that the fuel self-ignites without needing a spark.
• The expansion and exhaust stages are similar to the petrol engine.

Theoretical indicator diagram

• The theoretical diesel cycle has four stages:
  • A to B: Adiabatic compression of air
  • B to C: Heat supplied at constant pressure (fuel is injected and burns as the piston begins to move down)
  • C to D: Adiabatic expansion
  • D to A: Heat rejected at constant volume
• The key part is that heat input happens at constant pressure in the diesel cycle, whereas it happens at constant volume in the petrol (Otto) cycle.

Actual indicator diagram

• The actual diesel indicator diagram differs from the theoretical one in the same general ways as for the petrol engine (rounded corners, non-ideal compression, heat losses).
• The biggest visual difference is from B to C: there is no sharp peak at the start of the expansion stroke, because fuel is injected continuously during this phase and burns at roughly constant pressure.