Download Heat Engines

Heat Engines
Heat Pumps
Physics
Montwood High School
R. Casao
Heat Engine Cycle
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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.
The first law and second law of
thermodynamics constrain the operation of
a heat engine.
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The first law is the application of conservation of
energy to the system, and
the second sets limits on the possible efficiency
of the machine and determines the direction of
energy flow.
First Law of Thermodynamics
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The first law of thermodynamics is the
application of the conservation of
energy principle to heat and
thermodynamic processes: the change
in internal energy (U) of a system is
equal to the heat (Q) added to the
system minus the work (W) done by
the system.
Mathematically: U = Q - W
Internal Energy
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
Internal energy is defined as the energy
associated with the random, disordered
motion of molecules.
It is separated in scale from the
macroscopic ordered energy associated with
moving objects; it refers to the invisible
microscopic energy on the atomic and
molecular scale. For example, a room
temperature glass of water sitting on a table
has no apparent energy, either potential or
kinetic. But on the microscopic scale it is a
seething mass of high speed molecules
traveling at hundreds of meters per second.
Internal Energy

In the context of physics, the common scenario
is one of adding heat to a volume of gas and
using the expansion of that gas to do work, as in
the pushing down of a piston in an internal
combustion engine.
First Law of Thermodynamics

Heat engines such as
automobile engines
operate in a cyclic
manner, adding energy in
the form of heat in one
part of the cycle and
using that energy to do
useful work in another
part of the cycle.
PV Diagrams

Pressure-Volume (PV) diagrams are a primary
visualization tool for the study of heat engines.
Since the engines usually involve a gas as a
working substance, the ideal gas law relates the
PV diagram to the temperature so that the three
essential state variables for the gas can be
tracked through the engine cycle.
PV Diagrams

For a cyclic heat engine
process, the PV diagram will
be closed loop. The area
inside the loop is a
representation of the amount
of work done during a cycle.
Some idea of the relative
efficiency of an engine cycle
can be obtained by
comparing its PV diagram
with that of a Carnot cycle,
the most efficient kind of
heat engine cycle.
Heat Engines


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.
Thermodynamics is the study of the
relationships between heat and work.
The first law is the application of conservation
of energy to the system, and the second sets
limits on the possible efficiency of the machine
and determines the direction of energy flow.
Energy Reservoir Model

One of the general ways to illustrate a heat engine is the energy
reservoir model. The engine takes energy from a hot reservoir
and uses part of it to do work, but is constrained by the second
law of thermodynamics to exhaust part of the energy to a cold
reservoir. In the case of the automobile engine, the hot reservoir
is the burning fuel and the cold reservoir is the environment to
which the combustion products are exhausted.
Second Law of Thermodynamics


Second Law of Thermodynamics: It is
impossible to extract an amount of
heat QH from a hot reservoir and use
it all to do work W . Some amount of
heat QC must be exhausted to a cold
reservoir.
The maximum efficiency which can be
achieved is the Carnot efficiency.
Second Law of Thermodynamics
Carnot Cycle


The most efficient
heat engine cycle is
the Carnot cycle,
consisting of two
isothermal processes
and two adiabatic
processes.
The Carnot cycle can
be thought of as the
most efficient heat
engine cycle allowed
by physical laws.
Carnot Cycle

In order to approach the Carnot
efficiency, the processes involved in
the heat engine cycle must be
reversible and involve no change in
entropy. This means that the Carnot
cycle is an idealization, since no real
engine processes are reversible and all
real physical processes involve some
increase in entropy.
Carnot Cycle

The conceptual
value of the Carnot
cycle is that it
establishes the
maximum possible
efficiency for an
engine cycle
operating between
TH and TC .
e max
Thot  Tcold

Thot
Combustion Engines

Combustion engines: burn fuel to produce
the heat input for a thermodynamic cycle.
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Burning fuel turns chemical energy into heat
energy.
By-products of combustion have a very high
temperature and produce a very high pressure.
Results: piston pushed downward and a
fraction of the heat energy is converted to
mechanical work.
Some heat energy is carried away by the high
temperature exhaust gases, and some is lost to
the cylinder walls.
First law of thermodynamics
for combustion engine:
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Mathematically: QH = QC + W
QH = heat input due to fuel
combustion
QC = heat energy lost
W = work
First law of thermodynamics
for combustion engine:
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
Work output for combustion engine:
W = QH - Q C
Efficiency for combustion engine:
QH  QC
W
e

QH
QH
First law of thermodynamics
for combustion engine:
Gasoline Engine

Five successive processes occur in
each cycle within a conventional
four-stroke gasoline engine.
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During the intake stroke of the piston,
air that has been mixed with gasoline
vapor in the carburetor is drawn into the
cylinder.
During the compression stroke, the
intake valve is closed and the air-fuel
mixture is compressed approximately
adiabatically.
Gasoline Engine
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At this point, the spark plug ignites the air-fuel
mixture, causing a rapid increase in pressure and
temperature at nearly constant volume.
The burning gases expand and force the piston back,
which produces the power stroke.
During the exhaust stroke, the exhaust valve is
opened and the rising piston forces most of the
remaining gas out of the cylinder.
The cycle is repeated after the exhaust valve is closed
and the intake valve is opened.
How Stuff Works Gasoline Engine Animation
How Stuff Works Gasoline Engine Animation
Otto Cycle
Otto Cycle
Otto Cycle
Otto Cycle
Otto Cycle
Otto Cycle
Heat Engines
Diesel Engines

The main differences between the gasoline
engine and the diesel engine are:
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
A gasoline engine intakes a mixture of gas and
air, compresses it and ignites the mixture with a
spark. A diesel engine takes in just air,
compresses it and then injects fuel into the
compressed air. The heat of the compressed air
lights the fuel spontaneously.
A gasoline engine compresses at a ratio of 8:1 to
12:1, while a diesel engine compresses at a ratio
of 14:1 to as high as 25:1. The higher
compression ratio of the diesel engine leads to
better efficiency.
Diesel Engines
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Gasoline engines generally use either
carburetion, in which the air and fuel is
mixed long before the air enters the
cylinder, or port fuel injection, in which
the fuel is injected just prior to the intake
stroke (outside the cylinder). Diesel
engines use direct fuel injection -- the
diesel fuel is injected directly into the
cylinder.
How Stuff Work Diesel Animation
Diesel Engines

Note that the diesel engine has no spark plug,
that it intakes air and compresses it, and that it
then injects the fuel directly into the combustion
chamber (direct injection). It is the heat of the
compressed air that lights the fuel in a diesel
engine.
Dodge Hemi
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Hemi: (HEM -e) adj. Mopar in type, V8, hot
tempered, native to the United States,
carnivorous, eats primarily Mustangs,
Camaros, and Corvettes. Also enjoys
smoking a good import now and then to
relax.
The hemispherically shaped combustion
chamber is designed to accommodate large
valves and put the spark plugs close to the
center of the combustion chamber.
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In a HEMI engine, the
top of the combustion
chamber is hemispherical, as seen in
the image. The
combustion area in
the head is shaped
like half of a sphere.
An engine like this is
said to have "hemispherical heads."
In a HEMI head, the
spark plug is normally
located at the top of
the combustion
chamber, and the
valves open on
opposite sides of the
combustion chamber.
Advantage: Horsepower

The engine produces 345 horsepower, and compares very
favorably with other gasoline engines in its class. For example
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Dodge 5.7 liter V-8 - 345 hp @ 5400 rpm
Ford 5.4 liter V-8 - 260 hp @ 4500 rpm
GMC 6.0 liter V-8 - 300 hp @ 4400 rpm
GMC 8.1 liter V-8 - 340 hp @ 4200 rpm
Dodge 8.0 liter V-10 - 305 hp @4000 rpm
Ford 6.8 liter V-10 - 310 hp @ 4250 rpm
The HEMI Magnum engine has two valves per cylinder as well
as two spark plugs per cylinder. The two spark plugs help to
solve the emission problems that plagued Chrysler's earlier
HEMI engines. The two plugs initiate two flame fronts and
guarantee complete combustion.
Disadvantage:
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If HEMI engines have all these advantages, why
aren't all engines using hemispherical heads? It's
because there are even better configurations
available today.
One thing that a hemispherical head will never
have is four valves per cylinder. The valve angles
would be so crazy that the head would be nearly
impossible to design. Having only two valves per
cylinder is not an issue in drag racing or NASCAR
because racing engines are limited to two valves
per cylinder in these categories. But on the street,
four slightly smaller valves let an engine breath
easier than two large valves. Modern engines use a
pentroof design to accommodate four valves.
Disadvantage:

Another reason most
high-performance
engines no longer use
a HEMI design is the
desire to create a
smaller combustion
chamber. Small
chambers further
reduce the heat lost
during combustion,
and also shorten the
distance the flame
front must travel
during combustion.
The compact pentroof
design is helpful here,
as well.
Gas Turbine Engines
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In a gas turbine, a pressurized gas spins a
turbine.
In all modern gas turbine engines, the
engine produces its own pressurized gas,
and it does this by burning something like
propane, natural gas, kerosene or jet fuel.
The heat that comes from burning the fuel
expands air, and the high-speed rush of
this hot air spins the turbine.
Gas Turbine Engines
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Two big advantages of the turbine over
the diesel:
Gas turbine engines have a great powerto-weight ratio compared to gasoline or
diesel engines. That is, the amount of
power you get out of the engine compared
to the weight of the engine itself is very
good.
 Gas turbine engines are smaller than
their reciprocating counterparts of the
same power.
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Gas Turbine Engines
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The main disadvantage of gas turbines is
that, compared to gasoline and diesel
engines of the same size, they are
expensive.
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Because they spin at such high speeds and
because of the high operating temperatures,
designing and manufacturing gas turbines is a
tough problem from both the engineering and
materials standpoint.
Gas turbines also tend to use more fuel when
they are idling, and they prefer a constant rather
than a fluctuating load. That makes gas
turbines great for things like transcontinental jet
aircraft and power plants, but explains why you
don't have one under the hood of your car.
Gas Turbine Engines
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Three parts of the gas turbine engine:
Compressor - Compresses the incoming
air to high pressure
 Combustion area - Burns the fuel and
produces high-pressure, high-velocity gas
 Turbine - Extracts the energy from the
high-pressure, high-velocity gas flowing
from the combustion chamber.
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Gas Turbine Operation Animation
Heat Pumps
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Heat pumps: a mechanical device
that moves energy from a region at a
lower temperature to a region at
higher temperature.
Heat pump can be described by a
thermodynamic cycle just like that of
an engine. System absorbs heat at a
low temperature and rejects it at a
higher temperature.
Heat Pumps
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Heat pumps have long been used to cool
homes and buildings, and are now
becoming increasingly popular for heating
them as well.
Heat pump contains two sets of metal coils
that can exchange energy by heat with the
surroundings: one set is on the outside of
the building in contact with the air or the
ground; and the other set in the interior of
the building.
Heat Pumps
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In the heating mode, a circulating fluid
flowing through the coils absorbs energy
from the outside and releases it to the
interior of the building from the interior
coils.
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The fluid is cold and at low pressure when it is
in the external coils, where it absorbs energy by
heat from either the air or the ground.
The resulting warm fluid is then compressed
and enters the interior coils as a hot, highpressure fluid, where it releases its stored
energy to the interior air.
Heat Pumps

First law of thermodynamics for
heat pump: QH = QC + Win
 QC
= heat removed from low
temperature reservoir
 QH = heat pumped into high
temperature reservoir
 Win = work input
Coefficient of Performance
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Effectiveness of a heat pump is
described in terms of a ratio called the
coefficient of performance (COP). In
the heating mode, the COP is defined
as the ratio of the heat QH moved to a
higher temperature region divided by
the work input required to transfer
that energy.
COP (heating mode) = Q H
Win
Coefficient of Performance


The COP is similar to the thermal efficiency
for a heat pump in that it is a ratio of what
you get (energy delivered to the interior of
the building) to what you give (work input).
Because QH is generally greater than Win,
typical values for the COP are greater than
1.
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It is desirable for the COP to be as high as
possible.
Example: if the COP for a heat pump is 4, the
amount of energy transferred to the building is 4
times greater than the work done by the motor
in the heat pump.
Coefficient of Performance
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Maximum possible COP is called the
Carnot COP and is never achieved by
a real heat pump and depends on the
high and low temperature between
which the pump operates.
Carnot COP (heating mode) =
Qh
Qh
Th


Win Qh  Qc Th  Tc
Heat Pumps
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Heat pumps can also operate in the
cooling mode. Air conditioners and
refrigerators are examples of heat
pumps operating in the cooling mode.
Energy is absorbed into the
circulating fluid in the interior coils;
then, after the fluid is compressed,
energy leaves the fluid through the
external coils.
Heat Pumps
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The heat pump must have a way to release
energy to the outside. Refrigerator as an
example:
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A refrigerator cannot cool the kitchen if the
refrigerator door is left open.
The among of energy leaving the external coils
behind or underneath the refrigerator is greater
than the amount of energy removed from the
food or from the air in the kitchen if the door is
left open.
The difference between the energy out and the
energy in is the work done by the electricity
supplied to the refrigerator. Energy, Win, allows
compressor to remove heat from inside the
refrigerator and transfer it to the kitchen.
Air Conditioner
Heat Pumps
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For a heat pump operating in the
cooling mode, “what you get” is energy
removed from the cold reservoir. The
most effective refrigerator or air
conditioner is one that removes the
greatest amount of energy from the
cold reservoir in exchange for the least
amount of work.
COP (cooling mode) = Q c
Win
Heat Pumps
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The greatest possible COP for a heat
pump in the cooling mode is that of a
heat pump whose working substance
is carried through a Carnot cycle in
reverse.
Carnot COP (cooling mode) =
Qc
Qc
Tc


Win Qh  Qc Th  Tc
Second Law: Refrigerator

Second Law of
Thermodynamics: It is
not possible for heat to
flow from a colder body
to a warmer body
without any work
having been done to
accomplish this flow.
Energy will not flow
spontaneously from a
low temperature object
to a higher
temperature object.
Second Law: Refrigerator
Second Law: Entropy
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Second Law of Thermodynamics: In
any cyclic process the entropy will
either increase or remain the same.
Entropy: a measure of the amount of
energy which is unavailable to do
work; a measure of the disorder of a
system.
Entropy S = ΔQ
TK
Internal Energy Example
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When 50 J of heat is absorbed by a
gas, the system performs 30 J of
work. What is the change in the
internal energy of the system?
U = Q – W = 50 J – 30 J = 20 J
Heat Engines Examples
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A heat engine absorbs 200 J of heat from a
hot reservoir, does work, and exhausts 160
J of heat to a cold reservoir. What is the
efficiency of the engine?
QH = 200 J; QC = 160 J
QH  QC 200 J  160 J
W
e


 0.2  20%
QH
QH
200 J
Heat Engines Examples

A heat engine has an efficiency of 35%.
 A. How much work does it perform in a cycle if it
extracts 150 J of heat energy from a hot reservoir
per cycle?
35%
e
 0.35
100
W
e
; W  e  QH  0.35  150 J  52.5 J
QH
B. How much heat energy is exhausted per cycle?
W = QH – QC; QC = QH – W = 150 J – 52.5 J = 97.5 J

Heat Pump (Heating Mode)
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A heat pump is used to pump heat from the outside air at -5º C
to the hot air supply for the heating fan in a house, which is at
40º C. How much work is required to pump 1000 J of heat
into the house?
Convert the temperatures to Kelvins:
TC = -5º C + 273 = 268 K; TH = 40º C + 273 = 313 K
TH
313 K
COP 

 6.9556
TH  TC 313 K  268 K
QH
COP 
W
QH
1000 J
W

 143.77 J
COP 6.9556
Refrigerator (Heat Pump – Cooling Mode)
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A refrigerator has a coefficient of performance
(COP) of 4. How much heat is exhausted to the hot
reservoir when 200 kJ of heat is removed from the
cold reservoir?
COP = 4; QC = 200 kJ = 200000 J; QH = ?
QC
QC
200000 J
COP 
W

 50000 J
W
COP
4
QH  W  QC  200000 J  50000 J  250000 J