Transcript Diesel Engines

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 chamber. 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).
Gas turbines
• Newer type of internal combustion engine.
• Used in jets and some electric power plants
• Air pulled in the front and compressed in a compressor.
(The rotating fan-like structure you see when you look
into a jet engine).
• Air is mixed with fuel and ignited, this heated mixture
expands.
• Expanding gas moves through the turbine, which is
connected to the compressor by a rotating shaft.
• Hot gases are expelled with a greater velocity than the
intake air, giving the engine is thrust.
Gas Turbines
• For electricity
generation, the power
output turbine turns
the shaft.
• For aircraft, the gas is
expelled out the jet
nozzle.
Gas Turbines
• 20-30% efficiency converting thermal energy
to mechanical energy
• Lightweight
• Respond quickly to changing power demands
• Relatively cheap for public utilities
• Limitations are the need for materials to
withstand T~ 1000 C and the high rotation
speeds
Generating Electricity
• 1831 Michael Faraday discovers that by
moving a magnetic bar near a loop of wire, an
electric current can be induced in the wire.
• Known as electromagnetic induction
• This allowed the generation and transmission
of electricity possible, along with electric
motors and modern communications and
computer systems
• Electromagnetic induction animation
Electromagnetism
• It was already known that the opposite was
true, that a metal placed inside a current loop
could become magnetized.
Generators
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Coil of copper wire mounted on a rotating armature
Coils are rotated through a magnetic field
This induces a current in the coils.
But, the induced current resists the rotation of the
coils, so we need an external energy source to rotate
the coils.
• The current exits the rotating coil via slip rings that are
in contact with carbon brushes.
• The direction of current flow changes as the coil
rotates in the magnetic field. This produces an
alternating current.
Generator
Before Faraday
• Electricity was generated via electrostatic means
• used moving electrically charged belts, plates and
disks to carry charge to a high potential
electrode.
• Charge was generated using either of two
mechanisms:
– Electrostatic induction or
– The triboelectric effect, where the contact between
two insulators leaves them charged.
• Generated high voltage but low current, not good
for commercial use
Wimshurst Machine
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two large contra-rotating discs mounted in a
vertical plane, two cross bars with metallic
brushes, and a spark gap formed by two
metal spheres.
two insulated disks and their metal sectors
rotate in opposite directions passing the
crossed metal neutralizer bars and their
brushes.
imbalance of charges is induced, amplified,
and collected by two pairs of metal combs
with points placed near the surfaces of each
disk.
The positive feedback increases the
accumulating charges exponentially until a
spark jumps across the gap.
The accumulated spark energy can be
increased by adding a pair of Leyden jars, an
early type of capacitor suitable for high
voltages
Van de graf generator
• an electrostatic
machine which uses a
moving belt to
accumulate very high
electrostatically stable
voltages on a hollow
metal globe.
Van de graaff generator
• Video:
http://www.youtube.com/watch?v=sy05B32X
TYY
Faraday’s Disk
• A copper disc rotating between
the poles of a horseshoe magnet.
produced a small DC voltage,
and large amounts of current.
• First electromagnetic generator
Dynamos
• First generator able to produce electricity for
industrial purposes
• First dynamo was built by Hippolyte Pixii in 1832.
• a stationary structure, which provides a constant
magnetic field, and a set of rotating windings
which turn within that field.
• Magnetic field may be provided by one or more
permanent magnets or by one or more
electromagnets, which are usually called field
coils.
Pixii's dynamo
Dynamos
• Produce a direct current
• Basis for later devices such as the electric
motor, the alternating-current alternator, and
the rotary converter.
• Developed as a replacement for batteries
Modern electrical power plants
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Boiler Unit: Almost all of power
plants operate by heating water in a
boiler unit into super heated steam
at very high pressures. The source of
heat from combustion reactions may
vary in fossil fuel plants from the
source of fuels such as coal, oil, or
natural gas. Biomass, waste plant
parts, solid waste incinerators are
also used as a source of heat. All of
these sources of fuels result in
varying amounts of air pollution, as
well as carbon
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In a nuclear power plant, the fission
chain reaction of splitting nuclei
provides the source of heat.
Modern electrical power plants
• The super heated steam
is used to spin the
blades of a turbine,
which turns a coil of
wires within a circular
arrangements of
magnets.
Modern Electric power plants
• Cooling Water: After the
steam travels through the
turbine, it must be cooled
and condensed back into
liquid water to start the
cycle over again. Cooling
water can be obtained from
a nearby river or lake. An
alternate method is to use a
very tall cooling tower,
where the evaporation of
water falling through the
tower provides the cooling
effect.