Thermodynamics Flashcards
EQs: What do you use to find maximum efficiencies of everything? (heat engines, heat pumps and refrigerators)
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)
EQs: What ratios can we use to to relate temperatures (IN KELVIN) and heat energies?
Qh/Qc = Th/Tc
What equation allows us to relate heat energies and the work done by the engine/heat pump/refrigerator?
|Qh| = |W| + |Qc|
What’s the difference between a heat engine and a refrigerator?
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.
What factors affect the efficiency of a heat engine?
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.
State the first law of thermodynamics?
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)
What is the properties of an isothermal interaction?
iso - same
thermal - temperature
Therefore no change in temperature. ΔU = 0, Q = W
What is the properties of an adiabatic interaction?
No thermal energy is transferred. Q =0, -W = ΔU.
internal energy increases as work is done ON the system.
EQ: How do you find mechanical efficiency?
Mech eff = (Break power)/(Indicated power)
EQ: How do you find thermal efficiency?
Thermal eff = (indicated power)/(input power)
EQ: How do you find overall efficiency?
Either:
Overall eff = Mech*Thermal = Break/Input
What is the second law of thermodynamics?
(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.
What is the work done at a constant volume?
Zero.
Why can work not be done if volume is constant?
Work = Force * Distance.
If change in volume = 0 then there with be no change in distance, therefore no work will be done.
How do Isothermal and adiabatic graphs differ from each other?
Adiabatic is steeper. (you can remind yourself using the formula for each in the FB)
What do constant pressure and constant volume graphs look like?
CP: Horizontal straight line
CV: Vertical straight line
What is a four stroke engine?
An engine that burns fuel once (one power stroke) every four strokes of the piston.
What is the difference between a diesel and a petrol engine?
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.
What happens in the Induction stroke? (1)
- 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.
What happens in the Compression stroke? (2)
- 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)
What happens in the Expansion stroke? (3)
- Air-fuel mixture expands which does work on the piston. (W expansion > W compression)
(Both diesel and petrol are similar this stroke)
What happens in the Exhaust stroke? (4)
- Exhaust valve opens.
- Used up (burnt) air-fuel mixture is released as piston moves back up.
(Both diesel and petrol are similar this stroke)
Types of reverse heat engines?
Refrigerators = (Air conditioning), Heat pump.
