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The Heat Engine
A heat engine typically uses energy provided in the form of heat to do work and then exhausts the heat which cannot be used to do work
Example of heat engines: steam engine (for example in trains),Gasoline, diesel engines, jet engines, steam turbines
What do these engines have I common?
They all are powered by the expansion of heated gases.
THE SECOND LAW OF THERMODYNAMICS (FIRST EXPRESSION)
Heat transfer occurs spontaneously from higher- to lower-temperature bodies but never spontaneously in the reverse direction.
OR
It is impossible for any process to have as its sole result heat transfer from a cooler to a hotter object
Qh – Heat the gas, gas expands, gas does work
Qc – heat output, into environment.
Heat transfer from the hot object (or hot reservoir ) Qh
Into
( cold reservoir,) Qc
Model of an Engine
The temperatures of the hot reservoir(Th) and the temperature cold reservoir (Tc)
The heat engines a cyclical process
Cyclical process - brings the system ( the cylinder) back to its original state at end of every cycle.
In a cyclical process ΔU=0
ΔU= Q – W
Q net heat transfer during the cycle
Q= Qh –Qc
W net work done by system
Since ΔU=0
0 = Q – W
W = Q
But not all Q is converted to Work, thus
W = Qh – Qc (cyclical process)
W = Qh – Qc (cyclical process)
When work is done by a heat engine running between two temperature, only some of the input heat (Qc) can be converted to work (W), and the rest is expelled (Qc)
THE SECOND LAW OF THERMODYNAMICS (SECOND EXPRESSION)
It is impossible in any system for heat transfer from a reservoir to completely convert to work in a cyclical process in which the system returns to its initial state.
Using the model 1)-Gain heat (Qh) from Th,
thus increases - internal energy
2) Convert some of the energy into mechanical work (W)
3) expel the remaining energy Qc as heat to Tc
Gasoline Engine 1) -Burning fuel I combustion
chamber provides high Temperature reservoir (Qh)
2)- Hot gases do mechanical work on the piston (W)
3) – Heat is expelled to the environment via the cooling system and the exhaust (Qc)
Paths AB and CD are adiabatic and correspond to the compression and power strokes of an internal combustion engine
Paths BC and DA are isochoric and accomplish similar results to the ignition and exhaust-intake portions
Work is done on the gas along path AB
Work is done on the gas along path AB
If the cycle is operated clockwise, the engine uses heat to do net work. Counterclockwise, it uses work to transport heat and is therefore acting as a refrigerator or a heat pump.
The Diesel Engine
Diesel internal combustion engine differs from the gasoline powered Otto cycle by u ("compression ignition" rather than "spark ignition")
Diesel Engine
-Air is compressed adiabatically,
-With compression ratio (15-20)
-Compression raises the temperature to the ignition temperature fuel mixture.
(a –b)Adiabatic compression,
(b –c)Constant pressure combustion
(c-d) Adiabatic expansion ( power stoke
(d-c) Isochoric exhaust