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

• 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• 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• 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=sy05B32XTYY

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• 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

• 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.