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Physics Yr2 > Engineering Physics 3 > Flashcards

Engineering Physics 3 Flashcards

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1
Q

Define a heat engine

A

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

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2
Q

Define a working substance

A

The substance (usually fluid) on which the thermodynamic processes are performed in the engine, by changes of temperature, pressure and volume.

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3
Q

What is the Otto Cycle

A

The 4 stroke cycle used by a 4 stroke petrol engine

The most common engine in everyday cars

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4
Q

With reference to an indicator diagram, explain how the Otto cycle works

A
  • 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
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5
Q

What does 1 thermodynamic cycle (A’ABCD) on an indicator diagram for the otto cycle represent

A

2 up and down motions of a piston

2 revolutions of the engine

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6
Q

What is the thermal efficiency of 4 stroke petrol engine that uses the otto cycle

A

Ratio of the maximum useful work that can be done by the engine to the energy supplied by the fuel

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7
Q

Name the 4 Strokes that make up the Otto cycle in order

A

Induction Stroke

Compression Stroke

Power Stroke

Exhaust Stroke

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8
Q

With reference to an indicator diagram, explain how the Diesel cycle works

A
  • 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
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9
Q

Define Compression ratio in an engine

How is it shown on a p-V diagram

A

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

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10
Q

Name 2 advantages of a diesel engine over a petrol engine

A
  • Greater Torque
  • Higher compression ratio, higher efficiency
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11
Q

Name 2 disadvantages of a diesel engine

A

Have higher working pressures, so need to be more robust

Lower power to weight ratio

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12
Q

Define Input Power

A

The power that is derived from the burning of the fuel

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13
Q

Define Indicated Power

A

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

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14
Q

What is brake/output power

A

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

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15
Q

Define the calorific value of a fuel

A

Amount of energy released or produced when 1 kg of fuel burns

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16
Q

What is the unit of angular velocity usually used in engines

A

rpm

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17
Q

How to convert from brake horsepower to power

A

1bhp = 746W

18
Q

What does brake horsepower measure

A

Engine power output

19
Q

Define frictional power

A

Loss of power due to frictional processes in the engine

20
Q

Name the 3 key power indicators

A

Input Power

Output/brake power

Indicated power

21
Q

Name the 3 efficiency indicators

A

Mechanical efficiency

Thermal efficiency

Overall efficiency

22
Q

How to calculate mechanical efficiency (η)

A

Pout / Pind

Output power / Indicated power

23
Q

How to calculate thermal efficiency

A

Pind / Pin

Indicated power / Input Power

24
Q

How to calculate overall efficiency

A

Pout / Pin

OR

Mechanical efficiency x Thermal efficiency

25
Q

Explain how a general heat engine works

A

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.

26
Q

What is the maximum theoretical/real world efficiency of a heat engine

A

Less than 1 or 100%

27
Q

What is the ‘source’ in a heat engine

A

A high temperature ‘resevoir’ where the energy supplied by heating is said to come from

Energy = QH

28
Q

What is a ‘sink’ in a heat engine

A

A low temperature ‘resevoir’ that is just the surroundings

Energy = QC

29
Q

Why does W = QH-QC

A

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

30
Q

What is the main measure of success of an idealised heat engine

A

Thermal efficiency

Indicated power/Input power = work done per cycle/ Energy taken in as heat per cycle

=W/QH

=QH-QC / QH

31
Q

How to calculate maximum theoretical efficiency of a general heat engine

A

=TH-TC / TH

=1 - TC / TH

32
Q

Give 4 reasons for why real world engines are not even close to the efficiency idealised engines

A
  • 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
33
Q

What is the second law of Thermodynamics

Explain it

A

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

34
Q

How would one apply the 2nd Law of Thermodynamics to Heat engines

A

A heat engine needs to operate between a hot source and a cold sink.

35
Q

How does refridgerator work

A

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

36
Q

How does a heat pump work

A

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

37
Q

Define the coefficient of performance (COPhp) of a heat pump

A

Ratio of energy given to hot space against work input

38
Q

What is the first law of thermodynamics for an idealised heat engine working in reverse

A

QH = W + QC

Change in U is 0, no change in state

39
Q

Define the coefficient of performance of a refrigerator (COPref)

A

Ratio of energy removed from cold space (inside fridge) to work input

40
Q

What is a reversed heat engine

A

A device in which an input of work causes heat to transfer from a cold space / reservoir / sink to a hot space / source

41
Q

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

A

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

Physics Yr2 (15 decks)

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