Heat engine

it consists Concept of heat engine and carnot,rankine,otto,diesel cycles

1. Heat Engine
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Shivkumar Panjabi
1
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2. Heat Engine
A heat engine takes in energy by
heat and partially converts it to
other forms
In general, a heat engine carries
some working substance through a
cyclic process
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3. Heat Engine, cont.
Energy is
transferred from
a source at a high
temperature (Q
h)
Work is done by
the engine (W
eng)
Energy is expelled
to a source at a
lower
temperature (Q
c)
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4. Heat Engine, cont.
Since it is a cyclical
process, ΔU = 0
Its initial and final internal
energies are the same
Therefore, Q
net= W
eng
The work done by the
engine equals the net
energy absorbed by the
engine
The work is equal to the
area enclosed by the
curve of the PV diagram
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5. Thermal Efficiency of a
Heat Engine
Thermal efficiency is defined as the
ratio of the work done by the engine to
the energy absorbed at the higher
temperature
e = 1 (100% efficiency) only if Q
c= 0
No energy expelled to cold reservoir1
eng h c c
h h h
W Q Q Q
Q Q Q

   e
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6. Heat Pumps and
Refrigerators
Heat engines can run in reverse
Energy is injected
Energy is extracted from the cold reservoir
Energy is transferred to the hot reservoir
This process means the heat engine is
running as a heat pump
A refrigerator is a common type of heat
pump
An air conditioner is another example of a
heat pump
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7. Heat Pump, cont
The work is what
you pay for
The Q
cis the desired
benefit
The coefficient of
performance (COP)
measures the
performance of the
heat pump running
in cooling mode
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8. Second Law of
Thermodynamics
Constrains the First Law
Establishes which processes
actually occur
Heat engines are an important
application
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9. Second Law of
Thermodynamics
No heat engine operating in a
cycle can absorb energy from a
reservoir and use it entirely for the
performance of an equal amount
of work
Kelvin –Planck statement
Means that Q
ccannot equal 0
Some Q
cmust be expelled to the
environment
Means that e must be less than 100%
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10. Summary of the First and
Second Laws
First Law
We cannot get a greater amount of
energy out of a cyclic process than
we put in
Second Law
We can’t break even
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11. Reversible and Irreversible
Processes
A reversibleprocess is one in which every
state along some path is an equilibrium
state
And one for which the system can be returned
to its initial state along the same path
An irreversibleprocess does not meet
these requirements
Most natural processes are irreversible
Reversible process are an idealization, but
some real processes are good
approximations
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12. Sadi Carnot
1796 –1832
French Engineer
Founder of the
science of
thermodynamics
First to recognize
the relationship
between work
and heat
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13. Carnot Engine
A theoretical engine developed by Sadi Carnot
A heat engine operating in an ideal, reversible
cycle (now called a Carnot Cycle) between two
reservoirs is the most efficient engine possible
Carnot’s Theorem: No real engine operating
between two energy reservoirs can be more
efficient than a Carnot engine operating
between the same two reservoirs
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14. Carnot Cycle
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15. Carnot Cycle, A to B
A to B is an
isothermal expansion
at temperature T
h
The gas is placed in
contact with the high
temperature
reservoir
The gas absorbs
heat Q
h
The gas does work
W
ABin raising the
piston
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16. Carnot Cycle, B to C
B to C is an adiabatic
expansion
The base of the
cylinder is replaced
by a thermally
nonconducting wall
No heat enters or
leaves the system
The temperature
falls from T
hto T
c
The gas does work
W
BC
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17. Carnot Cycle, C to D
The gas is placed in
contact with the cold
temperature reservoir
at temperature T
c
C to D is an isothermal
compression
The gas expels energy
Q
C
Work W
CDis done on
the gas
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18. Carnot Cycle, D to A
D to A is an adiabatic
compression
The gas is again placed
against a thermally
nonconducting wall
So no heat is exchanged
with the surroundings
The temperature of the
gas increases from T
Cto
T
h
The work done on the
gas is W
CD
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19. Carnot Cycle, PV Diagram
The work done by
the engine is shown
by the area
enclosed by the
curve
The net work is
equal to Q
h-Q
c
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20. Efficiency of a Carnot
Engine
Carnot showed that the efficiency of the
engine depends on the temperatures of
the reservoirs
Temperatures must be in Kelvins
All Carnot engines operating between
the same two temperatures will have
the same efficiencyh
C
c
T
T
1e 
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21. Notes About Carnot
Efficiency
Efficiency is 0 if T
h= T
c
Efficiency is 100% only if T
c= 0 K
Such reservoirs are not available
The efficiency increases as T
cis
lowered and as T
his raised
In most practical cases, T
cis near
room temperature, 300 K
So generally T
his raised to increase
efficiency
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22. Real Engines Compared to
Carnot Engines
All real engines are less efficient
than the Carnot engine
Real engines are irreversible because
of friction
Real engines are irreversible because
they complete cycles in short
amounts of time
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23. Rankine Cycle:
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25. Process 1-2: Water from the condenser at low pressure is pumped into the
boiler at
high pressure. This process is reversible adiabatic.
Process 2-3: Water is converted into steam at constant pressure by the
addition of heat
in the boiler.
Process 3-4: Reversible adiabatic expansion of steam in the steam turbine.
Process 4-1: Constant pressure heat rejection in the condenser to convert
condensate
into water.
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28. Air Standard Otto Cycle:
The air-standard-Otto cycle is the idealized cycle for
the spark-ignition internal
combustion engines. This cycle is shown above on p-v
and T-s diagrams. The Otto
cycle 1-2-3-4 consists of following four process:
Process 1-2: Reversible adiabatic compression of air.
Process 2-3: Heat addition at constant volume.
Process 3-4: Reversible adiabatic expansion of air.
Process 4-1: Heat rejection at constant volume.
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32. Air Standard Diesel Cycle
Process 1-2: Reversible adiabatic Compression.
Process 2-3: Constant pressure heat addition.
Process 3-5: Reversible adiabatic Compression.
Process 4-1: Constant volume heat rejection.
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35. Thank
you for
your
attention