3. Kelvin-Planck Statement:
“It is impossible for any device
that operates on a cycle to receive
heat from a single reservoir and
produce a net amount of work.”
-No heat engine can convert all the
heat it receives to useful work.
6. • A heat engine is a device for extracting work
from a hot fluid. For example
• A car engine extracts power from the
combustion of fuel with air
• A steam steam turbine extracts power from
steam
• Both of these function by allowing a hot fluid
to expand so as to cause motion in a critical
component of the engine.
• In the process, high grade energy is said to be
degraded to lower grade energy.
7. Heat engines differ considerably from one
another, but all can be characterized by
the following
1. They receive heat from a high-
temperature source (solar energy, oil
furnace, nuclear reactor, etc.).
2. They convert part of this heat to work
(usually in the form of a rotating shaft).
8. 3. They reject the remaining waste
heat to a low-temperature sink (the
atmosphere, rivers, etc.).
4. They operate on a cycle.
9.
10. The diagram on the above represents
an ideal heat engine
• Heat is added at constant
temperature to the fluid at the high
temperature source
• The fluid flows through an
expansion device where work is
done, and the temperature of the
fluid falls from TH to TL
• Heat is then rejected at constant
temperature at the low temperature
source.
11. The cycle in the
previous slide is known
as an open cycle.
The closed cycle here
has four stages
Isothermal heat
addition
Adiabatic expansion
Isothermal heat
removal
Adiabatic compression
Isothermal = const.
Temp
Adiabatic = perfectly
insulated
12.
13. The fraction of the heat input that is converted
to net work output is a measure of the
performance of a heat engine and is called the
thermal efficiency, ηth
14. The cycles above are examples of the Carnot engine.
In the Carnot cycle all processes are reversible.
In a Carnot engine, the maximum work that can be
done, and hence the efficiency of the ideal engine
depends on the temperatures TH and TL
The efficiency of a Carnot engine is given by
H
L
H
LH
T
T
T
TT
1
The temperature is in the Kelvin or absolute scale
This efficiency is called the Carnot efficiency
15.
16.
17. Temperature
Entropy
Efficiency, = 1 - (Tlower / Thigher)
Isothermal Heat
Addition
1. Work done by gas
2. Heat added
Adiabatic expansion
1. No work done
2. Heat extracted by
gas
Isothermal Heat
rejection
1. Work done on gas
2. Heat extracted
Adiabatic
compression
1. No work done
2. Heat added
A B
C
D
The Carnot Cycle
18. The Carnot engine represents the theoretical
limit and is not a practical engine.
The main limitations of the Carnot engine are:
The processes in all four stages are reversible. For
this to be the case they must all take place infinitely
slowly
The work extracted on expansion is equal to the
work required for compression, so no net work is
extracted.
A practical heat engine has a lower efficiency
than a Carnot engine, but can make more
effective use of the energy in the hot fluid.
19. Practical Heat Engines include:
The Rankine cycle – basis of steam engines in power
stations
Otto and Diesel cycles – internal combustion engines
Gas turbine
These have lower efficiencies than the Carnot
cycle but are permit useful work to be
extracted.
22. This has two differences to the Carnot cycle
There must be reasonable temperature differences in
the boiler and condenser to ensure that heat addition
and rejection occurs at an acceptable rate
The turbine exhaust is completely condensed and
returned to the boiler by a pump. This uses very
much less energy than a compressor.
These result in lower efficiencies than the
Carnot cycle but permit useful work to be
done.
26. Otto, Diesel and Gas turbines all involve an
initial compression stage, but are otherwise
open cycle processes.
Combined cycle gas turbine:
This combines a gas turbine with a Rankine steam
cycle to maximise the work extracted from the fuel.
Efficiencies are much closer to Carnot efficiencies
than in other practical cycle used to date.
29. 1. Heat is transferred to a heat engine
from a furnace at a rate of 80 MW. If the
rate of waste heat rejection to a nearby
river is 50 MW, determine the net
power output and the thermal efficiency
for this heat engine.
30. 2. A car engine with a power output of
65 hp has a thermal efficiency of 24
percent. Determine the fuel
consumption rate of this car if the fuel
has a heating value of 19,000 Btu/lbm
(that is, 19,000 Btu of energy is released
for each lbm of fuel burned).
31. 3. The thermal efficiency of a particular
engine operating on an ideal cycle is
35%. Calculate the heat in KJ supplied
to the engine if the engine develops
1200 W-hr.
