Limitations of Real Heat Engines (AQA A Level Physics)

Limitations of Real Heat Engines

• Practical engines have a much lower efficiency than their theoretical equivalent

Limitations of Real Heat Engines

1. Work done to overcome frictional forces within the engine

   • An engine is made up of multiple parts (such as crankshafts and pistons) all in contact with each other which will naturally cause friction

   • There is also a transfer of energy out of the system by the heating of the cylinder walls that make up the engine

2. The fuel is not completely burnt in the process, so the temperature rise isn't as high as expected

   • The higher the difference in the temperature between the source and sink, the higher the efficiency

3. The power is used to drive internal components, such as pumps and motors

   • This power is therefore not used for useful work

4. The petrol-air mixture is not an ideal gas

   • It is actually a mixture of polyatomic molecules, which will sometimes be under high temperatures and pressures

5. Imperfect combustion

   • The heat energy in the compression stroke is taken not 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 also over a range of temperatures

   • The maximum temperature is therefore not always obtained

6. 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 quick

   • The inlet and exhaust values take a finite time to open and close (this gave the 'curved' edges in the actual p-V diagram for the petrol engine)

   • The pistons are always moving, so the heating is not always at a constant volume

   • The compression and expansion strokes are not truly adiabatic, as heat energy is lost from the system

Combined Heat & Power (CHP) Schemes

• In heat engines, the useful work output (W) is usually less than the heat energy transferred to the sink (Q_C)

• Combined heat and power (CHP) schemes are used to maximise the useful work output (and hence, power output) and the energy transferred to the source (Q_H)

• Conventional power stations that use heat engines are in reality, about 35% efficient

  • Their maximum theoretical efficiency is around 61%

• They transfer large amounts of energy to their surroundings through cooling towers or a local river or sea

• Instead of removing this heat through cooling, this heat could then be used to heat homes and businesses which are close by

  • This is used in CHP power stations which are much more heat and energy-efficient

• In the UK, most power plants are naturally positioned far away from homes and businesses so the heat would have cooled down by the time it has reached them, so they are not as popular

A CHP system in a power plant can generate some electricity, but the huge amount of wasted heat energy is useful for other supplying heat to water or buildings