Thermodynamics

Question 1

EQs: What do you use to find maximum efficiencies of everything? (heat engines, heat pumps and refrigerators)

Answer 1

Use Temperatures, (remember to swap Q with T for all equations)
eff max heat engine: (Tc-Th)/Th
CoP max hp: Th/(Th-Tc)
CoP max ref: Tc/(Th-Tc)

Question 2

EQs: What ratios can we use to to relate temperatures (IN KELVIN) and heat energies?

Answer 2

Qh/Qc = Th/Tc

Question 3

What equation allows us to relate heat energies and the work done by the engine/heat pump/refrigerator?

Answer 3

|Qh| = |W| + |Qc|

Question 4

What’s the difference between a heat engine and a refrigerator?

Answer 4

Heat Engine: Heat energy is transferred from the hot reservoir to the cold sink and mechanical work is done.
Refrigerator: We put in mechanical work in order to move heat energy from the cold sink to the hot reservoir.

Question 5

What factors affect the efficiency of a heat engine?

Answer 5

Eff = 1 - Tc/Th
Therefore:
- reducing temperature of the cool sink will increase the efficiency.
- increasing the temperature of the hot reservoir will increase efficiency.

Question 6

State the first law of thermodynamics?

Answer 6

Q = ΔU + W
Q is heat energy given to gas.
W is work done by the gas (-W is work done by the surroundings)
ΔU is change in temperature of the gas (kinda)

Question 7

What is the properties of an isothermal interaction?

Answer 7

iso - same
thermal - temperature
Therefore no change in temperature. ΔU = 0, Q = W

Question 8

What is the properties of an adiabatic interaction?

Answer 8

No thermal energy is transferred. Q =0, -W = ΔU.

internal energy increases as work is done ON the system.

Question 9

EQ: How do you find mechanical efficiency?

Answer 9

Mech eff = (Break power)/(Indicated power)

Question 10

EQ: How do you find thermal efficiency?

Answer 10

Thermal eff = (indicated power)/(input power)

Question 11

EQ: How do you find overall efficiency?

Answer 11

Either:

Overall eff = Mech*Thermal = Break/Input

Question 12

What is the second law of thermodynamics?

Answer 12

(relates to heat engines)
States Engines can never be 100% efficient.
This is because Qh = W + Qc:
- for engine to be 100% eff Qh = W however this would require Tc to be 0k which is impossible.
- Some of the energy from Qc ends up being transferred to heat sink (Qc)
W = useful work out.
Qh = Heat energy from heat source.
Qc = energy wasted/transferred to heat sink.

Question 13

What is the work done at a constant volume?

Answer 13

Zero.

Question 14

Why can work not be done if volume is constant?

Answer 14

Work = Force * Distance.

If change in volume = 0 then there with be no change in distance, therefore no work will be done.

Question 15

How do Isothermal and adiabatic graphs differ from each other?

Answer 15

Adiabatic is steeper. (you can remind yourself using the formula for each in the FB)

Question 16

What do constant pressure and constant volume graphs look like?

Answer 16

CP: Horizontal straight line
CV: Vertical straight line

Question 17

What is a four stroke engine?

Answer 17

An engine that burns fuel once (one power stroke) every four strokes of the piston.

Question 18

What is the difference between a diesel and a petrol engine?

Answer 18

Petrol engines: have spark plugs and taking in both fuel and air during induction stroke.
Diesel engines: use fuel injection at peak temperature inside the engine (gas at max U) and only taking in air during induction stroke.

Question 19

What happens in the Induction stroke? (1)

Answer 19

• Inlet value opened and exhaust valve closed.
• Piston goes down, which increases the volume of fuel and air in the engine (just air for diesel)
  During this step: PRESSURE remains CONSTANT.

Question 20

What happens in the Compression stroke? (2)

Answer 20

• Inlet value and Outlet value both close.
• Fuel injection/Spark plug process happens (for diesel air is compressed to a high temp which ignites fuel on entry)
• Dramatic increase in pressure and temperature at a constant volume. (which is about to drive piston down)

Question 21

What happens in the Expansion stroke? (3)

Answer 21

• Air-fuel mixture expands which does work on the piston. (W expansion > W compression)
  (Both diesel and petrol are similar this stroke)

Question 22

What happens in the Exhaust stroke? (4)

Answer 22

• Exhaust valve opens.
• Used up (burnt) air-fuel mixture is released as piston moves back up.
  (Both diesel and petrol are similar this stroke)

Question 23

Types of reverse heat engines?

Answer 23

Refrigerators = (Air conditioning), Heat pump.