Transcript Gasoline Engines

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
• Compression stroke –
chamber only contains air
and the piston increases
the air pressure 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 back up.
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 crystalize 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 sulfur 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
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