The four-stroke petrol engine operates on the Otto cycle

The four strokes

• 1. Induction: The piston moves down the cylinder, increasing the volume. A petrol-air mixture is drawn into the cylinder through the inlet valve. The pressure remains roughly constant, just below atmospheric pressure.
• 2. Compression: The inlet valve closes and the piston moves back up, compressing the gas. The volume decreases and the pressure increases. This process is approximately adiabatic. Near the top of the stroke, a spark plug ignites the mixture, causing a rapid increase in temperature and pressure at almost constant volume.
• 3. Power: The high pressure forces the piston back down. Work is done by the expanding gas. The exhaust valve opens near the bottom of the stroke and the pressure drops to roughly atmospheric.
• 4. Exhaust: The piston moves back up, forcing the burnt gases out through the open exhaust valve. The pressure stays just above atmospheric pressure.

The theoretical indicator diagram (Otto cycle)

• The theoretical indicator diagram uses several simplifying assumptions: the same gas cycles repeatedly, pressure and temperature changes are instantaneous, expansion and compression are adiabatic, there is no friction, and the heat source is external.
• The theoretical cycle has four stages:
  • A to B: Adiabatic compression
  • B to C: Heat supplied at constant volume (spark ignition)
  • C to D: Adiabatic expansion (power stroke)
  • D to A (or E): Heat rejected at constant volume

Actual vs theoretical indicator diagrams

• The actual diagram looks quite different from the theoretical one. Crucially, the differences arise because real engines are not ideal:
  • The corners of the graph are rounded because valves take a finite time to open and close (combustion is not instantaneous).
  • Heating and cooling cannot occur at truly constant volume because the piston would have to stop instantaneously.
  • The expansion and compression are not truly adiabatic: some heat is always transferred through the cylinder walls.
  • Exhaust gas and fuel vapour may remain in the cylinder.
  • The fuel may not burn completely.
• The net work done per cycle equals the area enclosed by the loop on the p-V diagram. For a real engine, this area is always less than the theoretical loop.