Transcript Heat Engines

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.
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
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.
How well does one work?
• Your car often carries off waste heat via its
cooling system. But your car recycles some of
that heat—how?
• 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.
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.
Diagram of a heat engine
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.
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%
Carnot Cycle
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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
So how can we make this work for us:
The 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.
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.
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.
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
Newcomen’s atmospheric engine
Watt’s Steam Engine
• Improvement upon
Newomen’s
• 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.
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.
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.
4 stroke internal combustion engine
cycle.
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.
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: CxH2x+2 + 2xO2 → xCO2 + 2xH2O
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.
Pictures
• Metal core
• Ceramic core
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
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.
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.
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
Diesel engines-disadvantages
• Hard to start in cold weather-compression stroke
can’t 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.
Diesel engine disadvantages
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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).