8) Engines Flashcards
With reference to a refrigerator, explain the terms Qin and Qout.
- A refrigerator operates between a cold space and a hot space.
- Qout is the energy removed from the fridge contents.
- Qin is the energy given to the surroundings.
What does the horizontal line across the bottom of a four-stroke diesel engine indicator diagram represent.
Represents the induction and exhaust strokes which take place at nearly atmospheric pressure.
Explain why the compression stroke of a diesel engine is considered to be an adiabatic change.
- Adiabatic change requires no heat transfer to the surroundings.
- Compression stroke occurs in a short time so no time for heat transfer.
- Therefore change can be considered to be adiabatic.
Explain why, in a diesel engine, the fuel starts to be injected into the cylinder slightly before the piston reaches its highest point in the cylinder.
So that the fuel has partially evaporated when the piston is at top of stroke so max pressure obtained when the piston is at top of the stroke.
Explain what is meant by a reversed heat engine.
A device in which an input of work causes heat to transfer from a cold space to a hot space.
Explain why the coefficient of performance of a reversed heat engine when operating as a heat pump is always greater than the coefficient of performance of the same reversed heat engine when operating as a refrigerator.
Heat transfer to hot space equals work done plus heat transfer from cold space.
Explain what is meant by the coefficient of performance of a heat pump.
The ratio of: (energy given to the hot space) / (work input).
Explain the basic differences in operation between a petrol engine and a diesel engine.
- In a diesel, only air is drawn into the cylinder. In a petrol air and fuel is drawn into the cylinder.
- In a diesel, ignition is by temperature rise during
compression. In a petrol, ignition is by means of a spark plug.
