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Heat Engines Heat Pumps

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Heat Engines Heat Pumps Physics Montwood High School R. Casao In a HEMI engine, the top of the combustion chamber is hemi-spherical, as seen in the image. – PowerPoint PPT presentation

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Title: Heat Engines Heat Pumps


1
Heat Engines Heat Pumps
  • Physics
  • Montwood High School
  • R. Casao

2
Heat Engine Cycle
  • 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.
  • 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.

3
First Law of Thermodynamics
  • 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

4
Internal Energy
  • 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.

5
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.

6
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.

7
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.

8
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.

9
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.

10
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.

11
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.

12
Second Law of Thermodynamics
13
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.

14
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.

15
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 .

16
Combustion Engines
  • Combustion engines burn fuel to produce the
    heat input for a thermodynamic cycle.
  • 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.

17
First law of thermodynamics for combustion
engine
  • Mathematically QH QC W
  • QH heat input due to fuel combustion
  • QC heat energy lost
  • W work
  • Net heat absorbed per cycle
  • QT QH QC

18
First law of thermodynamics for combustion engine
  • Work output for combustion engine
  • W QH - QC
  • Efficiency for combustion engine

19
First law of thermodynamics for combustion engine
20
Gasoline Engine
  • Five successive processes occur in each cycle
    within a conventional four-stroke gasoline
    engine.
  • 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.

21
Gasoline Engine
  • 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

22
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23
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24
Otto Cycle
25
Otto Cycle
26
Otto Cycle
27
Otto Cycle
28
Otto Cycle
29
Otto Cycle
30
Diesel Engines
  • The main differences between the gasoline engine
    and the diesel engine are
  • 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 81 to
    121, while a diesel engine compresses at a ratio
    of 141 to as high as 251. The higher
    compression ratio of the diesel engine leads to
    better efficiency.

31
Diesel Engines
  • 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

32
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.

33
Dodge Hemi
  • 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.

34
  • In a HEMI engine, the top of the combustion
    chamber is hemi-spherical, 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 "hemi-spherical 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.

35
Advantage Horsepower
  • The engine produces 345 horsepower, and compares
    very favorably with other gasoline engines in its
    class. For example
  • Dodge 5.7 liter V-8 - 345 hp _at_ 5400 rpm
  • Ford 5.4 liter V-8 - 260 hp _at_ 4500 rpm
  • GMC 6.0 liter V-8 - 300 hp _at_ 4400 rpm
  • GMC 8.1 liter V-8 - 340 hp _at_ 4200 rpm
  • Dodge 8.0 liter V-10 - 305 hp _at_4000 rpm
  • Ford 6.8 liter V-10 - 310 hp _at_ 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.

36
Disadvantage
  • 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.

37
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.

38
Gas Turbine Engines
  • 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.

39
Gas Turbine Engines
  • Two big advantages of the turbine over the
    diesel
  • Gas turbine engines have a great power-to-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.

40
Gas Turbine Engines
  • The main disadvantage of gas turbines is that,
    compared to gasoline and diesel engines of the
    same size, they are expensive.
  • 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.

41
Gas Turbine Engines
  • 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.
  • Gas Turbine Operation Animation

42
Heat Pumps
  • 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.

43
Heat Pumps
  • 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.

44
Heat Pumps
45
Heat Pumps
  • 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.
  • 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, high-pressure
    fluid, where it releases its stored energy to the
    interior air.

46
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47
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

48
Coefficient of Performance
  • 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)

49
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.
  • 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.

50
Coefficient of Performance
  • 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)

51
Heat Pumps
  • 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.

52
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53
Heat Pumps
  • The heat pump must have a way to release energy
    to the outside. Refrigerator as an example
  • 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.

54
Heat Pumps
  • 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)

55
Heat Pumps
  • 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)

56
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.

57
Second Law Refrigerator
58
Second Law Entropy
  • 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 DS
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