Engineering - Heat Engines 2 Flashcards
The first law of thermodynamics applied to heat engines:
In a heat engine, the energy supplied as heat from the burning fuel does work. Even in an ideal engine, even if the output power could be equal to the indicated power (mechanical efficiency =1), there’ll be a max value of the thermal efficiency, and hence the overall efficiency, that is less than 1. Reason?
A heat CANNOT turn heat energy totally into work.
The first law of thermodynamics applied to heat engines:
Draw the engine diagram showing Th Tc Qh Qc W
Done!
The first law of thermodynamics applied to heat engines:
For an idealised heat engine, ΔU = ?
ΔU = 0 for a complete cycle of the working substance, as it’s in the same state at the end as it was initially. Within the system, there’s been no loss of energy.
The first law of thermodynamics applied to heat engines:
∴ W = ???
W = Qh - Qc for work done by the system.
The first law of thermodynamics applied to heat engines:
The measure of the success of an idealised heat engine is through its thermal efficiency, ε. Derive this eq in terms of Qh and Qc.
ε =indicated power/input power = work done per cycle/energy taken in as heat per cycle
∴ ε = W/Qh = Qh-Qc/Qh = 1 - Qc/Qh
The first law of thermodynamics applied to heat engines:
It can be shown that the max theoretical efficiency, εmax = ?
εmax = Th-Tc / Th = 1 - Tc/Th
Limitations of real heat engines:
3 assumptions?
-ideal gas
-single temp Th and Tc
-reversible processes
Limitations of real heat engines:
Assumption 1: The petrol-air mixture behaves as an ideal gas .
Polyatomic molecules which are sometimes under high T and P ∴ kinetic theory assumptions break down.
Limitations of real heat engines:
Assumption 2: The heat energy (in the compression stroke) is taken in entirely at the single temp Th and rejected at the single temp Tc.
Heat taken in and rejected over a range of temps. Max temp not attained by of imperfect combustion.
Limitations of real heat engines:
Assumption 3: The processes that form the engine cycle are reversible.
(4 many)
-Energy dissipated out of the system.
-There’s no eqm with the surroundings as the processes are too quick.
-Inlet and exhaust valves take a finite time to open and close, and combustion isn’t instantaneous, so the ‘sharp edged’ p-V diagrams would never occur for a real engine.
-In the petrol engine, heating isn’t achieved at a constant volume, bc the pistons are always moving, and expansion and compression strokes aren’t truly adiabatic, because heat energy is lost out of the system.
Limitations of real heat engines:
Alsoooo fr—?
friction = decreases efficiency, turbulence moves combustion rate from optimum = decreased efficiency.
Limitations of real heat engines:
Improve efficiency by decreasing?
Qc to reduce Qc/Qh ratio.
The second law of thermodynamics:
Essentially saying..
its not possible for a process to decrease the total entropy of the universe (S).
MS : engine must operate between Th and Tc reservoir, and must reject some energy to Tc reservoir (meaning W≠Q)
The second law of thermodynamics:
Objects emit and absorb thermal energy. Emission from hotter object greater until eqm reached, with rate of emission = ?? when object and surroundings at same final T.
rate of emission = rate of absorption when object and surroundings at same final T.
The second law of thermodynamics:
“Heat always flows from the hot body to the cold body, when they’re brought into contact.”
Relate to heat engine?
“No heat engine can completely convert heat into work.”
