1. FIRST LAW OF THERMODYNAMICS
Q = ΔU + W
Q = heat entering the system
ΔU = increase in internal energy of the system
W = work done by the system
NB if the system expands, we refer to ‘work done by the system’, ie. it does positive work on its surroundings.
If the system contracts, we refer to ‘work done on the system’, ie. It does negative work on its surroundings.
NB the syllabus definitions above all refer to increases, but clearly ΔQ could be heat lost, ΔU a decrease in internal energy.
2. Non-flow processes (constant mass processes)
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Constant pressure (ISOBARIC) |
Constant volume (ISOVOLUMETRIC) |
Constant temperature (ISOTHERMAL) |
No Heat in or out (ADIABATIC) |
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P = const. |
V = const. |
pV = const. pV = nRT |
pVg = const. (g = 1.4 for air) |
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Work done |
W = pΔV |
Zero |
Area under P-V graph line |
Area under P-V graph line |
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!st Law of thermodynamics |
Q=ΔU+pΔV Energy supplied goes to raise internal energy AND to do work |
Q = ΔU + 0 All energy supplied goes to raise the internal energy |
Q = 0 + W All energy supplied goes to do work |
0 = ΔU + W Work is done at the expense of the internal energy |
You should be able to apply any of the above 4 processes to a p / V diagram.
3. p / V DIAGRAMS
Work Done = area below the graph line.
NB. In a cyclic process the WORK DONE PER CYCLE = THE AREA OF THE LOOP
Also, universal gas law applies to all of these changes of state : p1V1 / T1 = p2V2 / T2
4 ENGINE CYCLES
4 stroke petrol (Otto) cycle:
INDUCTION - COMPRESSION - POWER - EXHAUST
4 stroke diesel cycle:
Remember in a 4 stroke engine cycle ONE COMPLETE CYCLE = 2 REVOLUTIONS
INPUT POWER = calorific fuel value (J/kg) x fuel flow rate (kg/s)
INDICATED POWER = (area of P-V loop) x (no. of cycles per sec.) x (no. of cylinders)
OUTPUT OR BRAKE POWER, P = T ω
FRICTION POWER = (indicated power) – (brake power)
5 ENGINE EFFICIENCIES
THERMAL EFFICIENCY = work done by engine / heat supplied to engine
= (indicated power) / (input power)
OVERALL ENGINE EFFICIENCY = Pout / Pin
MECHANICAL EFFICIENCY = Pout / indicated power
6. SECOND LAW AND ENGINES
THERMAL EFFICIENCY = work done by engine / heat supplied to engine
= (Qin - Qout) / Qin
MAXIMUM THEORETICAL EFFICIENCY = (TH - TC) / TH (these are Kelvin temperatures!)
7. REVERSED HEAT ENGINES
basic principles of heat pumps and refrigerators:
ie. COPref = coefficient of performance for refrigerator.
Tony Mead 16.5.10