Internal
Combustion Engine
An internal combustion engine is a type of engine -- the commonest
now used -- in which the fuel is burned inside the engine
and the expansion of the combustion gases is used to provide
the power. Because of their potential light weight, efficiency,
and convenience, internal combustion engines largely superseded
steam engines in the early 20th century. They are used industrially
and for all kinds of transport, notably to power automobiles.
There are three classes of internal combustion engine: reciprocal
engines, which include the gasoline engine, the diesel engine,
and the free-piston-engine; rotary engines, including the
gas turbine, the turbojet, and the Wankel engine; and rocket
engines and non-turbine jet engines, working by reaction.
Although originally coal gas and even powdered coal were used
as fuel, now almost all fuels used are hydrocarbon products.
The first working (though not usable) internal combustion
engine was a piston engine made by the Dutch physicist Christiaan
Huygens (1680) that burned gunpowder. In 1794 Robin Street
patented a practicable though inefficient engine into which
the air had to be pumped by hand. In 1876, A. N. Otto built
the first four-stroke engine, using the principles stated
earlier by Alphonse Beau de Rochas.
The cycle is (1) intake of fuel/air mixture; (2) compression
of mixture; (3) ignition (see also ignition system) and expansion
of burned gases; (4) expulsion of gases as exhaust. Only the
third stroke is powered, but the engine is highly efficient,
and modern gasoline engines are basically the same.
Generally four, six, or eight cylinders are linked to provide
balanced power. The engine is cooled by water circulating
through pipes or by air from a fan. The fuel/air mixture is
produced in the carburetor; greater power is given by supercharging
(see supercharger), by which the proportion of air and the
initial pressure of the mixture are increased.
The efficiency of an internal combustion engine increases
with the compression ratio; if this is too high, however,
“knocking” occurs due to irregular burning and detonations.
It is avoided by using fuel of high octane number (and in
the past by using antiknock additives). The 1973 energy crisis
sent engine designers in search of high efficiency. The results
were the fast-burn and lean-burn engines, with high compression
ratios but, more important, high air-fuel ratios.
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