Internal Combustion
Engines
Internal combustion engines
are most commonly used for mobile propulsion in automobiles, equipment,
and other portable machinery. In mobile equipment, internal combustion
is advantageous, since it can provide high power-to-weight ratios together
with excellent fuel energy-density.
These engines have appeared in transport in almost all automobiles,
trucks, motorcycles, boats, and in a wide variety of aircraft and locomotives,
generally using petroleum. Where very high power is required, such as
jet aircraft, helicopters and large ships, they appear mostly in the
form of turbines.
All internal combustion
engines depend on the exothermic chemical process of combustion: the
reaction of a fuel, typically with the oxygen from the air, although
other oxidizers such as nitrous oxide may be employed.
The most common modern
fuels are made up of hydrocarbons and are derived mostly from petroleum.
These include the fuels known as dieselfuel, gasoline and petroleum
gas, and the rarer use of propane gas. Most internal combustion engines
designed for gasoline can run on natural gas or liquefied petroleum
gases without major modifications except for the fuel delivery components.
Liquid and gaseous biofuels, such as ethanol and biodiesel (a form of
diesel fuel that is produced from crops that yield triglycerides such
as soybean oil) can also be used. Some can also run on hydrogen gas.
All internal combustion
engines must achieve ignition in their cylinders to create combustion.
Typically engines use either a spark ignition (SI) method or a compression
ignition (CI) system. In the past, other methods using hot tubes or
flames have been used.
The efficiency of various
types of internal combustion engines varies, but it is lower than electric
motor energy efficiency. Most gasoline-fueled internal combustion engines,
even when aided with turbochargers and stock efficiency aids, have a
mechanical efficiency of about 20%. The efficiency may be as high as
37% at the optimum operating point. Most internal combustion engines
waste about 36% of the energy in gasoline as heat lost to the cooling
system and another 38% through the exhaust. The rest, about 6%, is lost
to friction.
Hydrogen Fuel Injection,
or HFI, is an engine add-on system that improves the fuel economy of
internal combustion engines by injecting hydrogen as a combustion enhancement
into the intake manifold. Fuel economy gains of 15% to 50% have been
claimed. A small amount of hydrogen added to the intake air-fuel charge
increases the octane rating of the combined fuel charge and enhances
the flame velocity, thus permitting the engine to operate with more
advanced ignition timing, a higher compression ratio, and a leaner air-to-fuel
mixture than otherwise possible.
The result is lower pollution
with more power and increased efficiency. Some HFI systems use an on
board electrolyzer to generate the small amount of hydrogen needed in
the system, around 5% of total BTU. A small tank of pressurized hydrogen
can also be used, but this method necessitates refilling. Hydrogen in
liquid form is seldom used because it is difficult to store.
There has also been discussion
of other types of internal combustion engines, such as the Split Cycle
Engine, that utilize high compression pressures in excess of 2000 psi
and combust after top dead center (the highest & most compressed point
in an internal combustion piston stroke). The claimed efficiency of
this engine, by calculation, is 42%. This has yet to be demonstrated
as of March 2007.
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