Engineering Physics 3 Flashcards
Define a heat engine
A device or system that extracts energy from its environment in the form of heat and converts it into useful work is called a heat engine
Define a working substance
The substance (usually fluid) on which the thermodynamic processes are performed in the engine, by changes of temperature, pressure and volume.
What is the Otto Cycle
The 4 stroke cycle used by a 4 stroke petrol engine
The most common engine in everyday cars
With reference to an indicator diagram, explain how the Otto cycle works
- Inlet valve opens and exhaust valve closes, piston moves down letting in working substance A’ - A :- Induction Stroke
- Inlet Valve closes at A
- Piston moves up, compressing the substance Adiabatically, increasing its temperature from 50 to 300oC. A - B : Compression Stroke
- B to C a spark plug ignites the substance, increasing its temperature to 2000oC, increasing its pressure but not its volume
- C to D, increased pressure pushes the piston down as the gas expands adiabatically decreasing the pressure and temperature - Power stroke
- D to A, Exhaust valve opens, releasing most of the burnt gas mixture removing an amount of heat Qout, pressure/volume of remaining substance decrease
- A to A’ - Exhaust stroke : Piston moves up and remaining exhaust is removed
- At A’, Exhaust valve closes and inlet valve opens to repeat process
What does 1 thermodynamic cycle (A’ABCD) on an indicator diagram for the otto cycle represent
2 up and down motions of a piston
2 revolutions of the engine
What is the thermal efficiency of 4 stroke petrol engine that uses the otto cycle
Ratio of the maximum useful work that can be done by the engine to the energy supplied by the fuel
Name the 4 Strokes that make up the Otto cycle in order
Induction Stroke
Compression Stroke
Power Stroke
Exhaust Stroke
With reference to an indicator diagram, explain how the Diesel cycle works
- At A’: Exhaust valve closes and inlet valve opens
- A’-A: Induction Stroke, Air is drawn into each cylinder as the piston moves down
- At A: Inlet valve closes
- A-B: Compression stroke, Piston moves up, compressing air adiabatically so its temperature increases to 700oC
- B-C: Power Stroke Pt1, Diesel fuel is sprayed into the cylinder and ignited by hot air, supplying heat Qin so pressure stays constant
- C-D: Power Stroke Pt2, Fuel supply cut, so burnt gas expands adiabatically. Forcing piston down and decreasing temperature
- D-A: Exhaust valve opens at D and releases exhaust gas, removing heat Qout, pressure and temperature of remaining gas decrease
- A-A’: Exhaust Stroke, Piston moves up, expelling the remaining gas
- At A’, Exhaust valve closes, inlet valve opens, cycle repeats
Define Compression ratio in an engine
How is it shown on a p-V diagram
The ratio of the volume enclosed in the cylinder at the beginning of the compression stroke to the volume enclosed at the end of the stroke.
The ratio of V2:V1
Name 2 advantages of a diesel engine over a petrol engine
- Greater Torque
- Higher compression ratio, higher efficiency
Name 2 disadvantages of a diesel engine
Have higher working pressures, so need to be more robust
Lower power to weight ratio
Define Input Power
The power that is derived from the burning of the fuel
Define Indicated Power
The theoretical power that the engine can deliver, based on the indicator diagram
The area enclosed is the work done
Assumes frictionless motion so is the max theoretical power
What is brake/output power
It is the power delivered to the engine’s crankshaft (Flywheel)
A measure of the engine’s power without the loss in power caused other auxillary components
Define the calorific value of a fuel
Amount of energy released or produced when 1 kg of fuel burns
What is the unit of angular velocity usually used in engines
rpm
How to convert from brake horsepower to power
1bhp = 746W
What does brake horsepower measure
Engine power output
Define frictional power
Loss of power due to frictional processes in the engine
Name the 3 key power indicators
Input Power
Output/brake power
Indicated power
Name the 3 efficiency indicators
Mechanical efficiency
Thermal efficiency
Overall efficiency
How to calculate mechanical efficiency (η)
Pout / Pind
Output power / Indicated power
How to calculate thermal efficiency
Pind / Pin
Indicated power / Input Power
How to calculate overall efficiency
Pout / Pin
OR
Mechanical efficiency x Thermal efficiency
Explain how a general heat engine works
A working substance is enclosed in a cylinder
The thermodynamic processes experienced by the fluid cause a piston in the cylinder to move up and down (back and forth), and this motion is then converted, via a crankshaft, into a rotational motion to drive a vehicle or machinery.
What is the maximum theoretical/real world efficiency of a heat engine
Less than 1 or 100%
What is the ‘source’ in a heat engine
A high temperature ‘resevoir’ where the energy supplied by heating is said to come from
Energy = QH
What is a ‘sink’ in a heat engine
A low temperature ‘resevoir’ that is just the surroundings
Energy = QC
Why does W = QH-QC
If you think about a sink and source for an idealised heat engine, out of all the energy being supplied by heat, the amount that isn’t transferred to the surroundings will be transferred as work done to be useful
So the work done is the difference between energy supplied as heat and energy transferred to the surroundings
Since the working substance hasn’t changed state, change in U = 0
What is the main measure of success of an idealised heat engine
Thermal efficiency
Indicated power/Input power = work done per cycle/ Energy taken in as heat per cycle
=W/QH
=QH-QC / QH
How to calculate maximum theoretical efficiency of a general heat engine
=TH-TC / TH
=1 - TC / TH
Give 4 reasons for why real world engines are not even close to the efficiency idealised engines
- The petrol/air mixture (Working substance) doesn’t work as an ideal gas
- Processes that form the heat engine cycle are irreversible
- Heat is usually taken in over a range of temperatures and released also over a range of temperatures
- Frictional effects in the moving parts/heating up parts
What is the second law of Thermodynamics
Explain it
It is not possible to convert heat continuously into work without at the same time transferring some heat from a warmer to a colder body.
Heat cannot be transferred totally into work without transferring some energy to surroundings. The first law tells us however that work can be transferred completely to heat
How would one apply the 2nd Law of Thermodynamics to Heat engines
A heat engine needs to operate between a hot source and a cold sink.
How does refridgerator work
Reversed heat engines
Work is done on a working substance to transfer energy from the low temperature reservoir to the higher temperature resevoir
The purpose is to remove energy, cool, the room/fridge
ie. from inside a room to outside
How does a heat pump work
Reversed heat engine used to heat a building
Work is done to a working substance transfer energy from a low temp. resevoir to a high temp resevoir.
But the purpose is to transfer heat to the high temp. resevoir
Define the coefficient of performance (COPhp) of a heat pump
Ratio of energy given to hot space against work input
What is the first law of thermodynamics for an idealised heat engine working in reverse
QH = W + QC
Change in U is 0, no change in state
Define the coefficient of performance of a refrigerator (COPref)
Ratio of energy removed from cold space (inside fridge) to work input
What is a reversed heat engine
A device in which an input of work causes heat to transfer from a cold space / reservoir / sink to a hot space / source
Why is the coefficient of performance of a reversed heat engine when operating as a heat pump always greater than the coefficient of performance of the same reversed heat engine when operating as a refridgerator
Heat transfer to hot space equals work done plus heat transfer from cold space
So QH must be greater than QC
If COPref = QC / W
and COPhp = QH / W
And W is the same for both, then COPref < COPhp
