What factors reduce the efficiency of heat engine from its ideal value?

Answer 1

This inefficiency can be attributed to three causes. There is an overall theoretical limit to the efficiency of any heat engine due to temperature, called the Carnot efficiency. Second, specific types of engines have lower limits on their efficiency due to the inherent irreversibility of the engine cycle they use. Thirdly, the non ideal behaviour of real engines, such as mechanical friction and losses in the combustion process causes further efficiency losses.

OR

· friction of moving parts

· inefficient combustion

· heat loss from the combustion chamber

· departure of the working fluid from the thermodynamic properties of an ideal gas

· aerodynamic drag of air moving through the engine

· energy used by auxiliary equipment like oil and water pumps

· inefficient compressors and turbines

· imperfect valve timing

Answer 2

If we consider the "ideal value" to be a Carnot engine operating between the heat source and heat sink of the actual engine, there are several factors that reduce the efficiency of the actual engine - most of which are deviations from the Carnot cycle:

1. Reversible isothermal expansion of the gas at the "hot" temperature, TH (isothermal heat addition). In reality, no expansion can be completely isothermal because heat transfer occurs at a finite rate, so an isothermal expansion would have to occur over infinite time. Also expansion in real heat engines is never reversible - some energy is lost to friction.
2. Isentropic (reversible adiabatic) expansion of the gas (isentropic work output). This would require perfect insulation but perfect insulation does not exist. As in the previous step, the expansion cannot be reversible because of losses to friction.
3. Reversible isothermal compression of the gas at the "cold" temperature, TC. (isothermal heat rejection) In reality, no compression can be completely isothermal because heat transfer occurs at a finite rate, so an isothermal compression would have to occur over infinite time. Also compression in real heat engines is never reversible - some energy is lost to friction.
4. Isentropic compression of the gas (isentropic work input). This would require perfect insulation but perfect insulation does not exist. As in the previous step, the compression cannot be reversible because of losses to friction.

Adding to this is the fact that most heat engines do something closer to isobaric expansions (across a valve) or isochoric (constant volume) heating and cooling, which are less efficient than the corresponding steps in the Carnot Cycle.