A-LEVEL PHYSICS: 11.2.11: Limitations of Real Heat Engines (SaveMyExams)

Question 1

practical engines have a much ___ ___ than their theoretical equivalent

Answer 1

lower efficiency

Question 2

limitations of real heat engines: (6)

Answer 2

-work done to overcome frictional forces in the engine

-fuel is not completely burnt, so temperature is not as high as expected

-power is used to drive internal components, eg pumps/motors

-petrol-air mixture is not an ideal gas

-imperfect combustion

-the processes that form the engine cycle are irreversible

Question 3

explain the limitation of real heat engines ‘work done to overcome friction in the engine’: (2)

Answer 3

-engine is made up of multiple parts all in contact with each other. causes friction

-friction between piston & walls transfer heat energy out of system

Question 4

explain the limitation of real heat engines ‘fuel is not completely burnt in process, so temperature isn’t as high as expected’: (1)

Answer 4

the higher the temp. difference between source TH and sink TC, the higher the efficiency

Question 5

explain the limitation of real heat engines ‘power is used to drive internal components eg pumps/motors’: (1)

Answer 5

this power is ∴ not used for useful work

Question 6

explain the limitation of real heat engines ‘petrol-air mixture is not an ideal gas’: (1)

Answer 6

actually a mixture of polyatomic molecules, sometimes be under high temp. & pressure

Question 7

explain the limitation of real heat engines ‘imperfect combustion’: (2)

Answer 7

-actuality: heat usually taken in over range of temperatures & rejected over range of temperatures

-max. temp. is ∴ not always obtained

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Question 8

explain the limitation of real heat engines ‘processes that form the engine cycle are irreversible’: (3)

Answer 8

-energy is dissipated out of system

-no equilibrium with surroundings, processes are too quick

-compression & expansion are not truly adiabatic (Q lost from system)

Question 9

in heat engines, useful work output W is usually ___ than the heat energy transferred to the sink (QC)

Answer 9

less

Question 10

CHP stand for:

Answer 10

combined heat & power scheme

Question 11

CHP used to:

Answer 11

maximise the useful work output W (and ∴ power output) and the energy transferred to the source QH