Internal Combustion Engines


An internal-combustion engine is a heat engine that burns fuel and air
inside a combustion chamber located within the engine proper. Simply stated, a
heat engine is an engine that converts heat energy to mechanical energy. The
internal- combustion engine should be distinguished from the external-
combustion engine, for example, the steam engine and the Stirling engine, which
burns fuel outside the prime mover, that is, the device that actually produces
mechanical motion. Both basic types produce hot, expanding gases, which may then
be employed to move pistons, turn turbine rotors, or cause locomotion through
the reaction principle as they escape through the nozzle.
Most people are familiar with the internal-combustion reciprocating engine,
which is used to power most automobiles, boats, lawn mowers, and home generators.
Based on the means of ignition, two types of internal-combustion reciprocating
engines can be distinguished: spark-ignition engines and compression-ignition
engines. In the former, a spark ignites a combustible mixture of air and fuel;
in the latter, high compression raises the temperature of the air in the chamber
and ignites the injected fuel without a spark. The diesel engine is a
compression-ignition engine. This article emphasizes the spark-ignition engine.
The invention and early development of internal-combustion engines are
usually credited to three Germans. Nikolaus Otto patented and built (1876) the
first such engine; Karl Benz built the first automobile to be powered by such an
engine (1885); and Gottlieb Daimler designed the first high-speed internal-
combustion engine (1885) and carburetor. Rudolf Diesel invented a successful
compression-ignition engine (the diesel engine) in 1892.
The operation of the internal-combustion reciprocating engine employs
either a four-stroke cycle or a two-stroke cycle. A stroke is one continuous
movement of the piston within the cylinder.
In the four-stroke cycle, also known as the Otto cycle, the downward
movement of a piston located within a cylinder creates a partial vacuum. Valves
located inside the combustion chamber are controlled by the motion of a camshaft
connected to the crankshaft. The four strokes are called, in order of sequence,
intake, compression, power, and exhaust. On the first stroke the intake valve is
opened while the exhaust valve is closed; atmospheric pressure forces a mixture
of gas and air to fill the chamber. On the second stroke the intake and exhaust
valves are both closed as the piston starts upward. The mixture is compressed
from normal atmospheric pressure (1 kg/sq cm, or 14.7 lb/sq in) to between 4.9
and 8.8 kg/sq cm (70 and 125 lb/sq in). During the third stroke the compressed
mixture is ignited--either by compression ignition or by spark ignition. The
heat produced by the combustion causes the gases to expand within the cylinder,
thus forcing the piston downward. The piston\'s connecting rod transmits the
power from the piston to the crankshaft. This assembly changes reciprocating--in
other words, up-and-down or back-and-forth motion--to rotary motion. On the
fourth stroke the exhaust valve is opened so that the burned gases can escape as
the piston moves upward; this prepares the cylinder for another cycle.
Internal-combustion spark-ignition engines having a two-stroke cycle combine
intake and compression in a single first stroke and power and exhaust in a
second stroke.
The internal-combustion reciprocating engine contains several subsystems:
ignition, fuel, cooling, and exhaust systems.
The ignition system of a spark-ignition engine consists of the sparking
device (the spark plug); the connecting wire from the plug to the distributor;
and the distributor, which distributes the spark to the proper cylinder at the
proper time. The distributor receives a high-energy spark from a coil, or
magneto, that converts low-voltage energy to high-voltage energy. Some ignition
systems employ transistorized circuitry, which is generally more efficient and
less troublesome than the mechanical breaker-point system used in the past. Most
ignition systems require an external electrical energy source in the form of a
battery or a magneto.
Spark-ignition engines require a means for mixing fuel and air. This may be
either a carburetor or fuel injection. A carburetor atomizes the fuel into the
engine\'s incoming air supply. The mixture is then vaporized in the intake
manifold on its way to the combustion chamber. fuel injection sprays a
controlled mist of fuel into the airstream, either in the intake manifold or
just before the intake valve or valves of each cylinder. Both carburetors and
fuel injectors maintain the correct fuel- to-air ratio, about one part fuel to
fifteen parts air, over a wide range of air temperatures, engine speeds, and
loads. Fuel injection can compensate for changes in altitude as well.
Internal-combustion engines require some type of starting system. Small
engines are generally started by pulling a starting rope or kicking