11: Engineering Physics Flashcards
1
Q
First Law of Thermodynamics (4)
A
- Q = ΔU + W
- Q is energy transferred to the system by heating
- ΔU is the increase in internal energy
- W is work done by the system
2
Q
Ideal Gas Equation
A
p V = n R T
3
Q
Isothermal Changes (4)
A
- Where the temperature of the system is constant
- The gas’s absolute temperature determines its internal energy
- So ΔU = 0, meaning Q = W
- Hence, supplying heat energy to the system results in an equivalent amount of work being done by the gas so its volume increasing
4
Q
Isothermal Change Equation
A
p V = constant
5
Q
Adiabatic Changes (3)
A
- Where no heat is transferred in or out of the system
- So Q = 0 meaning ΔU = -W
- Hence, any change in the internal energy of the system is caused by work done by/on the system
6
Q
Adiabatic Change Equation
A
p V^γ = constant
7
Q
Constant Pressure Change Equation
A
V / T = constant
8
Q
Constant Volume Changes (3)
A
- If volume is constant, no work is done by/on the system
- So W = 0 meaning Q = ΔU
- Hence, all heat energy transferred to the system goes into increasing its internal energy
9
Q
Constant Volume Change Equation
A
p / T = constant
10
Q
p-V Diagrams for Isothermal Changes (3)
A
- p-V diagram for a compression, where work is done on the system:
Textbook p539 Figure 2 - p-V diagram for an expansion, where work is done by the system:
Textbook p539 Figure 2 - The higher the temperature of the process, the further from the origin the p-V diagram is
11
Q
p-V Diagrams for Adiabatic Changes (3)
A
- The p-V curves for adiabatic processes have a steeper gradient than isothermal processes
- More work is done to compress gas adiabatically than isothermally:
Textbook p540 Figure 4 - The gas does less work if it expands adiabatically instead of isothermally
Textbook p541 Figure 5
12
Q
p-V Diagram for Constant Volume Changes (3)
A
- p-V diagrams for changes with constant volume are straight vertical lines
- No work is done as volume doesn’t change and there is no area under the line
- If a system is heated at constant volume, its pressure will increase:
Textbook p541 Figure 6
13
Q
p-V Diagram for Constant Pressure Changes (3)
A
- p-V diagrams for constant pressure changes are straight horizontal lines
- The work done is the area of the rectangle under the graph – W = p ΔV
- Textbook p541 Figure 7
14
Q
Work Done =
A
Area below the graph
15
Q
Work Done per Cycle =
A
Area of loop
16
Q
Four-Stroke Petrol Engine Cycle (4)
A
- Induction
- Compression
- Expansion
- Exhaust
17
Q
Induction (3)
A
- The piston moves down, increasing the volume of the gas (air-fuel mix) as the inlet valve is open
- The pressure of the gas remains constant just below atmospheric pressure
- Indication diagram: Textbook p544 Figure 1
18
Q
Compression (5)
A
- The inlet valve is closed and the piston moves up the cylinder
- This does work on the gas, increasing the pressure
- Just before the piston is at the end of this stroke, the spark plug creates a spark, igniting the gas
- The temperature and pressure increase at almost constant volume
- Indication diagram: Textbook p544 Figure 3
19
Q
Expansion (4)
A
- The hot gas expands and does work on the piston, pushing it down
- The work done by the gas is greater than the work done to compress it as it is now at a higher temperature
- Just before the piston is at the end of this stroke, the exhaust valve opens, reducing the pressure
- Indication diagram: Textbook p545 Figure 4
20
Q
Exhaust (3)
A
- The piston moves up the cylinder and the burnt gas leaves through the exhaust valve
- The pressure remains almost constant
- Indication diagram: Textbook p545 Figure 5
21
Q
Four-Stroke Diesel Engine Cycle (5)
A
- In the induction stroke, only air is pulled into the cylinder
- Diesel engines don’t have a spark plug, so in the compression stroke, the air is compressed until its temperature is high enough to ignite the fuel
- Just before the end of the stroke, diesel is sprayed into the cylinder through a fuel injector and ignites
- The expansion and exhaust strokes are the same as a petrol engine
- The indicator diagram has a flatter peak at the start of the expansion stroke, showing the point where fuel is injected and heats up to combustion temperature:
Textbook p545 Figure 7
22
Q
Assumption of Theoretical Cycles (5)
A
- The same gas is taken continuously around the cycle
- The gas is pure air with an adiabatic constant γ = 1.4
- Pressure and temperature changes can be instantaneous
- The heat source is external
- The engine is frictionless
23
Q
Petrol Engine Cycle Theoretical Diagram (5)
A
- A: The gas is compressed adiabatically
- B: Heat is supplied at constant volume
- C: The gas cools adiabatically
- D: The system cools at constant volume
- Textbook p546 Figure 8
24
Q
Diesel Engine Cycle Theoretical Diagram (5)
A
- A: The gas is compressed adiabatically
- B: Heat is supplied at constant pressure
- C: The gas cools adiabatically
- D: The system cools at constant volume
- Textbook p546 Figure 10
25
Q
Comparison of Theoretical and Real Diagrams (7)
A
- Curved corners: because valves take finite time to open and close
- No constant volume process: because piston would have to stop
- Compression and expansion not adiabatic curves: because energy is lost by heat transfer
- The cycle is open because engine needs to
draw in air and expel exhaust - Heating not at constant pressure: because fuel injection and combustion cannot be exactly controlled
- Area of diagram is less because energy is lost by heat
transfer and incomplete combustion - Pressure not as high because incomplete combustion
26
Q
Indicated Power =
A
(Area of p–V loop) x (no. of cycles per second) x (no. of cylinders)
