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Heat Engines

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Heat Engines How do we get the heat energy of the fuel and turn it into mechanical energy? Simply put we combine the carbon and hydrogen in the fuel with oxygen. – PowerPoint PPT presentation

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Title: Heat Engines


1
Heat Engines
  • How do we get the heat energy of the fuel and
    turn it into mechanical energy?
  • Simply put we combine the carbon and hydrogen in
    the fuel with oxygen.
  • 2 reactions that occur are
  • C O2 ? CO2 heat energy
  • H2 O ? H2O heat energy
  • This process is just the reverse of
    photosynthesis.

2
Just a little chemistry
  • For example, the the equation for burning heptane
    looks like
  • C7H16 11O2 ? 7CO2 8 H2O 1.15 X 106 calories
    per 100g of Heptane
  • 1.15 x106 is called the heat of combustion for
    heptane. Every hydrocarbon has such a number
  • It is the maximum amount of energy for a certain
    amount of mass of a substance you can extract.
  • It represents the energy from the sun stored in
    the fuel since ancient times

3
So what is a heat engine?
  • A heat engine is any device that can take energy
    from a warm source and convert it to mechanical
    energy
  • Efficiency not all of the energy from the
    burning of the fuel goes into the production of
    energy. Heat is lost as waste heat and needs to
    be disposed of.
  • For example, most energy generating plants are
    located near bodies of water or have cooling
    towers which are used to carry off waste heat.

4
How well does one work?
  • Your car often carries off waste heat via its
    cooling system. But your car recycles some of
    that heathow?
  • No heat engine will perfectly convert all the
    heat energy to mechanical energy.
  • We need to quantify the efficiency and designers
    of heat engines work to maximize this efficiency.

5
Carnot and his cycle
  • Sadi Carnot created an efficiencey
    measure for a heat engine, now
    named after him (Carnot Efficiency).
  • Always less than 100
  • Simply put it is the percentage of the energy
    taken from the heat source which is actually
    converted to mechanical work.

6
Diagram of a heat engine
7
Carnot Efficciency
  • Efficiency work done/energy put into the system
  • In terms of the flow of heat (Q) energy this
    becomes (Qhot - Qcold)/Qhot X 100
  • Now energy is not easy to quantify, but
    temperature is, and since we know the Kelvin T
    scale is true measure of energy, we can express
    the efficiency in terms of temperature.

8
Carnot Efficciency
  • So our efficiency, in terms of T becomes
  • Carnot Efficiency (Thot - Tcold)/Thot X 100
  • Or with some algebraic wizardry we get
  • Carnot efficiency 1- (Tcold/Thot) X 100
  • Example for a coal fired electric power plant,
    the boiler temperature 825K and the cooling
    tower temperature is 300k. So 1-(300/825) X
    100 64

9
Carnot Cycle
  • From an initial stat A, the gas is placed in
    contact with the hot temperature reservoir (Th)
    and expands isothermally (keeping T Th
    constant) to some state B. During this isothermal
    expansion heat Qh flows into the gas from the hot
    temperature Th.
  • From state B, the gas undergoes an adiabatic
    expansion to state C. No heat is exchanged during
    this expansion. Expanding an insulated gas means
    work is done at the "expense" of the internal
    energy. That means the gas will have a lower
    temperature. This is the cold temperature Tc.
  • At state C, we place the gas in contact with the
    cold temperature heat reservoir (like a large
    tank of water) and do an isothermal compression
    to state D. In compressing the gas, work is done
    on the gas by the outside. But the temperature
    remains constant -- meaning the internal energy U
    of the gas remains constant. For this to happen,
    heat Qc is given out to the cold temperature heat
    reservoir.
  • From state D we do an adiabatic compression back
    to state A. Remember, "adiabatic" means insulated
    so there is no heat exchange.

Figure 1
Figure 2
10
So how can we make this work for usThe Steam
Engine
  • Concept of a heat engine was revolutionary-if the
    heat energy could be turned into mechanical
    energy, human and labor could be replaced cheaply
    and more efficiently.

11
Simple steam engine
  • Water is heated in the boiler and steam forces
    piston up
  • At the valve, steam escapes into the cooling
    tower, where it cools and condenses.
  • Cool water is pumped back into boiler, T drops
    and piston drops, until sufficient steam is
    created to cause the process to repeat.

12
A little history
  • First writings on the power of steam are from
    Hero of Alexandria (10-70 CE).
  • The aeolipile (known as Hero's engine)
    was a rocket-like reaction engine
    and the first recorded
    steam engine.
  • He also created an engine that used air from a
    closed chamber heated by an altar fire to
    displace water from a sealed vessel the water
    was collected and its weight, pulling on a rope,
    opened temple doors.
  • Taqi al-Din in 1551 and Giovanni Branca in 1629
    both created experimental steam engines.

13
More History
  • Thomas Savery (1650-1715), in 1698, patented the
    first crude steam engine.
  • Based on Denis Papin's Digester or pressure
    cooker of 1679.
  • Savery had been working on solving the problem of
    pumping water out of coal mines
  • Thomas Newcomen created the atmospheric engine,
    which was relatively inefficient, and in most
    cases was only used for pumping water out of deep
    mines

14
Newcomens atmospheric engine
15
Watts Steam Engine
  • Improvement upon Newomens
  • Used 75 less coal than Newcomen's, and was hence
    much cheaper to run.
  • Watt developed his engine further, modifying it
    to provide a rotary motion suitable for driving
    factory machinery.
  • This enabled factories to be sited away from
    rivers, and further accelerated the pace of the
    Industrial Revolution.

16
Steam Engines
  • Efficiencies were only 1 for converting heat to
    mechanical energy.
  • Now they are above 30.
  • Class of engine known as external combustion
    engines. Fuel is burned outside the pressurized
    part of the engine
  • Results in low CO and NO emissions
  • Particulate and sulfur oxides emissions depend
    upon the fuel being burned.

17
Gasoline Engines
  • Use internal combustion fuel is vaporized and
    mixed with air inside a closed chamber
  • Mixture is compressed to 6-10 times atmospheric
    pressure and ignited with a spark
  • Fuel burns explosively forming a gas of CO2 and
    water vapor. Since the nitrogen in the air is not
    part of the reaction to burn hydrocarbons, it
    also heats up to over 1000 C.
  • Now when a gas heats it expands and exerts a
    force. The expanding gases exert the force on a
    piston, which pushes it downward and causes the
    crankshaft to rotate.

18
4 stroke internal combustion engine cycle.
19
Gasoline engines
  • Efficiency of converting chemical to mechanical
    energy of about 25.
  • Produces carbon monoxide (CO), nitrogen oxides
    and hydrocarbons. All are considered pollutants
  • Enter the catalytic converter.

20
Catalytic converter
  • Starting in 1975, catalytic converters were
    installed on all production vehicles via
    increasing government controls on pollutants from
    gasoline powered vehicles.
  • Catalytic converters have 3 tasks
  • 1. Reduction of nitrogen oxides to nitrogen and
    oxygen 2NOx ? xO2 N2
  • 2. Oxidation of carbon monoxide to carbon
    dioxide 2CO O2 ? 2CO2
  • 3. Oxidation of unburnt hydrocarbons (HC) to
    carbon dioxide and water CxH2x2 2xO2 ? xCO2
    2xH2O

21
Catalytic converters
  • The catalytic converter consists of several
    components
  • 1. The core, or substrate. In modern catalytic
    converters, this is most often a ceramic
    honeycomb however, stainless steel foil
    honeycombs are also used.
  • 2. The washcoat. In an effort to make
    converters more efficient, a washcoat is
    utilized, most often a mixture of silica and
    alumina. The washcoat, when added to the core,
    forms a rough, irregular surface which has a far
    greater surface area than the flat core surfaces,
    which then gives the converter core a larger
    surface area, and therefore more places for
    active precious metal sites.
  • 3. The catalyst itself is most often a
    precious metal. Platinum is the most active
    catalyst and is widely used. However, it is not
    suitable for all applications because of unwanted
    additional reactions and/or cost. Palladium and
    rhodium are two other precious metals that are
    used. Platinum and rhodium are used as a
    reduction catalyst, while platinum and palladium
    are used as an oxidization catalyst. Cerium,
    iron, manganese and nickel are also used, though
    each has its own limitations. Nickel is not legal
    for use in the European Union (due to reaction
    with carbon monoxide). While copper can be used,
    its use is illegal in North America due to the
    formation of dioxin.

22
Pictures
  • Metal core
  • Ceramic core

23
Limitations
  • Susceptable to lead build up, require use of lead
    free gasoline.
  • Require richer fuel mixture, burn more fossil
    fuels and emit more CO2
  • In fact most of emission is CO2 which is a
    greenhouse gas
  • The manufacturing of catalytic converters
    requires palladium and/or platinum for which
    there are environmental effects from the mining
    of these metals

24
Diesel Engines
  • Found mostly in large trucks, locomotives, farm
    tractors and occasionally cars.
  • An internal combustion engine
  • Does not mix the fuel and air before they enter
    the combustion chamber
  • Does not use a spark for emission
  • Heavier and bulkier than gasoline engine
  • Slower speed and slower response to driver
  • More efficient than gasoline engines,
    efficiencies of over 30 of converting fuel
    energy to mechanical energy.

25
Diesel Engines
  • Piston moves down, drawing air into the cylinder
  • Compression stroke chamber only contains air and
    the piston pushes up, increases the air pressure
    and temperature until ignition can occur when the
    fuel is introduced.
  • Short burst of fuel is sent into the chamber when
    this pressure is reached.
  • Explosion heats gases in chamber and causes them
    to expand, pushing the piston down.
  • Piston pushes up, expelling the exhaust gasses.

26
Diesel engines-advantages
  • Ignition occurs at a higher T, resulting in
    higher efficiency than gasoline engines (more
    than 30 efficient in converting chemical to
    mechanical energy).
  • Can run on low grade fuels and diesel fuels have
    10 more BTU per gallon.
  • CO emissions are lower more air in the chamber
    means more CO2 than CO is formed

27
Diesel engines-disadvantages
  • Hard to start in cold weather-compression stroke
    cant reach the ignition temperature. Solved with
    installation of a glow plug, a small heater.
  • Gelling-Diesel fuel can crystallize in cold
    weather clogging fuel filters and hindering fuel
    flow. Solved via electric heaters on fuel lines.
  • Fuel injection is critical, if timing is off,
    combustion is not complete and results in excess
    exhaust smoke with unburned particles and excess
    hydrocarbons.

28
Diesel engine disadvantages
  • Noisy
  • More expensive initially
  • Smell
  • Diesel fuel has become routinely more expensive
    than gasoline
  • Why?-rising demand, cheap gas due to decreased
    demand, environmental restrictions (need for
    lower sulfer emissions and higher taxes on diesel
    fuel than gasoline).
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