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1. Physical Matters > Unit 5: Applied 2 > Flashcards

Unit 5: Applied 2 Flashcards

1
Q

How do engines typically work on the most fundamental level

A

Converting heat energy into movement energy

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2
Q

What is a system in the context of thermodynamics

A

An object of interest whose behaviour we are monitoring in relation to its surroundings

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3
Q

In words, what is the first law of thermodynamics and how is that portrayed mathematically

A

energy cannot be created or destroyed therefore: The internal energy of a system is the sum of energy inputs and outputs. expressed mathematically as ΔU = Q – W,
Q is heat added W is work done by system

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4
Q

What is meant by internal energy

A

It is the sum of kinetic energy f the molecules and vibration of the bonds

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5
Q

How is the first law typically wrote in maths

A 
ΔQ = ΔU + ΔW
Q = heat entering the system, U = Internal energy, W = Work done by system
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6
Q

work done for a cylinder at constant pressure

A

ΔW=pΔV

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7
Q

What is an isothermal change?

A

Temperature stays the same

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8
Q

How can isothermal changes happen (very nearly)

A

Compressing, expanding very slowly such that heat generated can flow out very slowly

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9
Q

Work done by a gas is called:

A

expansion

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10
Q

Work done on a gas is called:

A

compression

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11
Q

List all isothermal processes

A
  • pV = constant and p1V1 = p2V2;
  • ΔU = 0 (becasue the internal energy is dependant on temperautre)
  • ΔQ = ΔW (if a gas is to work W, Q amount should be supplied)
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12
Q

What is an adiabatic change

A

A change where there is no heat flow in/out of a system

ΔQ = 0

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13
Q

How can one calculate temperature change in an adiabatic change

A

p1V1/T1 = p2V2/T2

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14
Q

What is the equation for the adiabatic line?

A

see google

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15
Q

state the variables in the adiabatic line equation

A
  • Cv = the energy needed to give unit temperature rise in 1 mole of gas where the VOLUME is kept constant
  • Cp = energy needed to give unit temp rise in 1 mole of gas where pressure is kept constant
  • y is a ratio which is different for certain gases
  • k is constant
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16
Q

What is an isovolumetric process

A

a process occurring at a constant volume.

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17
Q

Key characterics and equation of isovolumetric change

A
  • p1/T1 = p2/T2

no change in volume means no work is done therefore all heat entering becomes internal energy

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18
Q

What is an isobaric process

A

Happens at a constant pressure

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19
Q

Key characterics and equation of isobaric process

A
  • v1/T1 = v2/T2

- Some heat is used to increase the internal energy and the rest to do work

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20
Q

what do P-V diagrams represent

A

A cycle of processes undergone by an ideal gas

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21
Q

What are the two equations for thermal efficiency

A
  • (work done per second)/(energy supplied per second)

- (indicated power)/(input power from fuel combustion)

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22
Q

What is the practical efficiency

A

Similiar to thermal but accounts for frictional losses.

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23
Q

What must an engineer take into account when designing an engine to be efficient

A
  • Considering the theory of how the gases behave as they expand and contact
  • designing the engine so friction is low, valves gas tight
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24
Q

what are the 4 stages of the otto cycle

A
  • Induction
  • compression
  • ignition
  • exhaust
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25
Q

What is the initial step of teh otto cycle

A

Begins at A after the induction stroke which draws the petrol air mixture at a constant pressure.

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26
Q

why is in practice the thermodynamics of a petrol engine are more complex

A
  • fuel burns during the cycle so number of moles is not constant
  • cycle happens v.quick so there is swirling of gas, this Ke is not taken into account
  • Ignition takes a finitie time and takes time to propagate through the fuel air mix therefore pressures will vary within gases
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27
Q

What happens A->B in otto cycle

A

Compression: The air in the cylinder is now trapped and compressed adiabatically by the incoming piston. No heat enters or leaves

28
Q

What happens B->C in otto cycle

A

Ignition: a spark ignites the mixture raising the pressure and temp v.rapidally

29
Q

What happens C->D in otto cycle

A

Power: High pressure in cylinder forces the piston outward. Work done adiabatically

30
Q

What happens D->A in otto cycle

A

Cooling: the gas is cooled rapidly by contact with the internal walls of the cylinder, reducing pressure to initial pressure. assuming constant volume.

31
Q

What happens just before the next cycle?

A

Piston moves from A to E to exhaust the gases out and then returns to draw more fuel

32
Q

What is the indicated power of the engine (words and equation)

A

Rate of net work done by the engine

Area of indicator loop * number of cycles per second * number of cylinders

33
Q

What is the input power to the engine

A

Calorific value of fuel*fuel flow rate

34
Q

What is the calorific value

A

The energy released per unit mass of fuel when the fuel is burned

35
Q

What are the units for calorific value

A

JKg^-1

36
Q

What are the units for fuel flow rate

A

kgs^-1

37
Q

Why is no spark plug needed in the diesel engine

A

diesel oil is prayed into the cylinder at max compression. Because air is at a high temperature after comp, the fuel ignites on contact with the air, which pushes the piston down at constant pressure

38
Q

Why does fuel injection in a diesel engine happen at a constant temperature

A

The fuel ignites as it is injected and pushes the piston out

39
Q

What are the 4 stages of the indicator diagram for a diesel engine

A

1 - Compression
2 - Fuel injection
3 - Power
4 - Cooling and exhaust

40
Q

Why is the output power of an engine less than its indicated power

A

Some work done by engine is wasted due to friction between moving parts such as bearings.

41
Q

What meant by brake power

A

Output power of an engine

42
Q

What is mechanical efficiency and why is it needed

A

It is the fraction of power it delivers as brake power, needed to account for friction between moving parts

43
Q

What is the equation for mechanical efficiency

A

(Brake Power)/(Indicated Power)

44
Q

What is meant by friction power

A

Difference between the indicated power and brake power; power lost due to friction

45
Q

how does one calculate brake power

A

(Engine torque)*(angular speed of drive shaft)

46
Q

What is the overall efficiency of the engine and derive an equation from the definition.

A

The friction of the input power from fuel combustion that is delivered as power, therefore:

=(brake power)/(input power from fuel combustion)

47
Q

Derive another equation for overall efficiency considering mechanical and thermal efficiency

A

Because mechanical efficiency = (Brake power)/(indicated power)
and thermal = (indicated)/(power from combustion)

overall must = mechanical*thermal

48
Q

What is the Second law of thermodynamics

A

It is impossible for heat transfer from a high temp source to produce and equal amount of work

49
Q

The reversible engine is supposed to hold what key trait

A

The most efficient type of engine according to the laws of thermodynamics

50
Q

What is a reversible engine

A

One that can be reversed such that work done on the line causes max heat transfer from cold to hot as well as vice versa

51
Q

State the formula showing the principle of a reversible engine and its fundamental implication

A

Qin = W + Qout.

This means the reversible heat engine doesn’t waste energy

52
Q

In a reversible engine, what is the efficiency dependent on

A

The temperatures of the source temp (Th) and sink temp (Tc)

53
Q

What is an equation for efficiency in a reversible engine

A

Work/Qin
or
(Th - Tc)/Th

54
Q

What are the components of the reversible engine

A
  • temp source
  • temp sink
  • Engine
55
Q

Derive a formula for efficiency of reversible engine consisting of the energy transfers in out out the engine when it is in REVERSE

A

Since eff = W/Qin
and W = Qin - Qout
therefore eff = (Qin - Qout)/Qin

56
Q

What is the relationship between the ratios of energy transfer and the heat storages

A

Qout/Qin = Tc/Th

57
Q

Real engines operating at leess than max efficiency have what major implication

A

Energy becomes less and less useful every when it is transferred

58
Q

What is the main reason the reversible engine cannot produce 100% efficiency

A

For this Tc would have to be 0, which is impossible

59
Q

What are the main reasons thermal efficiency values are lower in practice (3 reasons)

A
  • The indicator loop does not have sharp corners, therefore less work is done than if it had sharp corners
  • The expansion and compression strokes are not perfectly adiabatic as assumed in calculating the theoretical thermal efficiency
  • Work is done by the engine to bring about the induction and exhaust strokes
60
Q

What are the reasons mechanical efficiency is less than 100%

A
  • Friction between moving parts cannot be eliminated
  • Oil used to lubricate the bearings in an engine are viscous which means they cause some resistance to motion of the moving parts
61
Q

What is a heat pump

A

A device used to transfer energy from a cold space to a hot one

62
Q

How is the coefficient of performance of a heat pump defined

A

(Heat transferred into hot space (Qin))/(work done to bring about the heat transfer)

63
Q

What happens at stage A->B in the refrigeration process

A

The refrigerant is in the vapor state and is compressed adiabatically to high pressure.

64
Q

What happens at stage B->C in the refrigeration process

A

The high pressure vapor condenses in tubes outside the refrigerator, releasing internal

energy to the surroundings.

65
Q

What happens at stage C->D in the refrigeration process

A

The condensed liquid passes through an expansion valve, causing its pressure and

temperature to fall.

66
Q

What happens at stage D->A in the refrigeration process

A

The liquid evaporates in tubes inside the ice box, gaining internal energy from objects

in the ice box. The low pressure vapor is then drawn into the compressor for the next cycle.

1. Physical Matters (12 decks)

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