Real heat engines are always less efficient than their theoretical equivalents

Why real engines fall short

• Practical engines have a much lower efficiency than their theoretical equivalent. There are six main reasons for this.

1. Friction

• An engine is made up of multiple moving parts (such as crankshafts and pistons) all in contact with each other, which naturally causes friction.
• There is also a transfer of energy out of the system by heating of the cylinder walls that make up the engine.

2. Incomplete combustion

• The fuel is not completely burnt in the process, so the temperature rise is not as high as expected.
• The higher the difference in temperature between the source and sink, the higher the efficiency. Incomplete combustion reduces this difference.

3. Power used to drive internal components

• Some of the power produced is used to drive internal components, such as pumps and motors.
• This power is therefore not available for useful external work.

4. Non-ideal gas behaviour

• The petrol-air mixture is not an ideal gas. It is a mixture of polyatomic molecules, which will sometimes be under high temperatures and pressures where ideal gas assumptions break down.

5. Imperfect heat transfer

• The heat energy in the compression stroke is not taken in entirely at the single temperature $T_H$, and not entirely rejected at the single temperature $T_C$.
• In reality, the heat is usually taken in over a range of temperatures and rejected over a range of temperatures.
• The maximum temperature is therefore not always obtained.

6. Irreversibility

• The processes that form the engine cycle are irreversible:
  • Energy is dissipated out of the system
  • There is no equilibrium with the surroundings as the processes are too rapid
  • The inlet and exhaust valves take a finite time to open and close (this gives the "rounded" edges in the actual p-V diagram)
  • The pistons are always moving, so the heating is not always at constant volume
  • The compression and expansion strokes are not truly adiabatic, as heat energy is lost from the system

Combined heat and power (CHP) schemes

• In heat engines, the useful work output ($W$) is usually less than the heat energy transferred to the sink ($Q_C$). This "waste" heat is normally lost to the surroundings through cooling towers or local water sources.
• Combined heat and power (CHP) schemes use this waste heat to heat homes and businesses nearby, making the overall system much more energy-efficient.
• Conventional power stations that use heat engines are in reality about 35% efficient (with a maximum theoretical efficiency of around 61%).
• In the UK, most power plants are positioned far away from homes and businesses, so the heat would have cooled down by the time it reached them. This limits the adoption of CHP schemes.