Introduction
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Any type of machine
that obtains mechanical energy directly from the expenditure of
the chemical energy of fuel burned in a combustion chamber that
is an integral part of the engine. Four principal types of
internal-combustion engines are in general use: the Otto-cycle
engine, the diesel engine, the rotary engine, and the gas
turbine. For the various types of engines employing the
principle of jet propulsion, see Jet Propulsion; Rocket.
The Otto-cycle engine, named after its inventor, the German
technician Nikolaus August Otto, is the familiar gasoline engine
used in automobiles and airplanes; the diesel engine, named
after the French-born German engineer Rudolf Christian Karl
Diesel, operates on a different principle and usually uses oil
as a fuel. It is employed in electric-generating and
marine-power plants, in trucks and buses, and in some
automobiles. Both Otto-cycle and diesel engines are manufactured
in two-stroke and four-stroke cycle models.
Components of
Engines
The essential parts
of Otto-cycle and diesel engines are the same. The combustion
chamber consists of a cylinder, usually fixed, that is closed at
one end and in which a close-fitting piston slides. The
in-and-out motion of the piston varies the volume of the chamber
between the inner face of the piston and the closed end of the
cylinder. The outer face of the piston is attached to a
crankshaft by a connecting rod. The crankshaft transforms the
reciprocating motion of the piston into rotary motion. In
multicylindered engines the crankshaft has one offset portion,
called a crankpin, for each connecting rod, so that the power
from each cylinder is applied to the crankshaft at the
appropriate point in its rotation. Crankshafts have heavy
flywheels and counterweights, which by their inertia minimize
irregularity in the motion of the shaft. An engine may have from
1 to as many as 28 cylinders.
The fuel supply
system of an internal-combustion engine consists of a tank, a
fuel pump, and a device for vaporizing or atomizing the liquid
fuel. In Otto-cycle engines this device is either a carburetor
or, more recently, a fuel-injection system. In most engines with
a carburetor, vaporized fuel is conveyed to the cylinders
through a branched pipe called the intake manifold and, in many
engines, a similar exhaust manifold is provided to carry off the
gases produced by combustion. The fuel is admitted to each
cylinder and the waste gases exhausted through mechanically
operated poppet valves or sleeve valves. The valves are normally
held closed by the pressure of springs and are opened at the
proper time during the operating cycle by cams on a rotating
camshaft that is geared to the crankshaft. By the 1980s more
sophisticated fuel-injection systems, also used in diesel
engines, had largely replaced this traditional method of
supplying the proper mix of air and fuel. In engines with fuel
injection, a mechanically or electronically controlled
monitoring system injects the appropriate amount of gas directly
into the cylinder or inlet valve at the appropriate time. The
gas vaporizes as it enters the cylinder. This system is more
fuel efficient than the carburetor and produces less pollution.
In all engines some
means of igniting the fuel in the cylinder must be provided. For
example, the ignition system of Otto-cycle engines described
below consists of a source of low-voltage, direct-current
electricity that is connected to the primary of a transformer
called an ignition coil. The current is interrupted many times a
second by an automatic switch called the timer. The pulsations
of the current in the primary induce a pulsating, high-voltage
current in the secondary. The high-voltage current is led to
each cylinder in turn by a rotary switch called the distributor.
The actual ignition device is the spark plug, an insulated
conductor set in the wall or top of each cylinder. At the inner
end of the spark plug is a small gap between two wires. The
high-voltage current arcs across this gap, yielding the spark
that ignites the fuel mixture in the cylinder.
Because of the heat
of combustion, all engines must be equipped with some type of
cooling system. Some aircraft and automobile engines, small
stationary engines, and outboard motors for boats are cooled by
air. In this system the outside surfaces of the cylinder are
shaped in a series of radiating fins with a large area of metal
to radiate heat from the cylinder. Other engines are
water-cooled and have their cylinders enclosed in an external
water jacket. In automobiles, water is circulated through the
jacket by means of a water pump and cooled by passing through
the finned coils of a radiator. Some automobile engines are also
air-cooled, and in marine engines sea water is used for cooling.
Unlike steam engines
and turbines, internal-combustion engines develop no torque when
starting, and therefore provision must be made for turning the
crankshaft so that the cycle of operation can begin. Automobile
engines are normally started by means of an electric motor or
starter that is geared to the crankshaft with a clutch that
automatically disengages the motor after the engine has started.
Small engines are sometimes started manually by turning the
crankshaft with a crank or by pulling a rope wound several times
around the flywheel. Methods of starting large engines include
the inertia starter, which consists of a flywheel that is
rotated by hand or by means of an electric motor until its
kinetic energy is sufficient to turn the crankshaft, and the
explosive starter, which employs the explosion of a blank
cartridge to drive a turbine wheel that is coupled to the
engine. The inertia and explosive starters are chiefly used to
start airplane engines.
Otto-Cycle Engines
The ordinary
Otto-cycle engine is a four-stroke engine; that is, in a
complete power cycle, its pistons make four strokes, two toward
the head (closed head) of the cylinder and two away from the
head. During the first stroke of the cycle, the piston moves
away from the cylinder head while simultaneously the intake
valve is opened. The motion of the piston during this stroke
sucks a quantity of a fuel and air mixture into the combustion
chamber. During the next stroke, the piston moves toward the
cylinder head and compresses the fuel mixture in the combustion
chamber. At the moment when the piston reaches the end of this
stroke and the volume of the combustion chamber is at a minimum,
the fuel mixture is ignited by the spark plug and burns,
expanding and exerting a pressure on the piston, which is then
driven away from the cylinder head in the third stroke. During
the final stroke, the exhaust valve is opened and the piston
moves toward the cylinder head, driving the exhaust gases out of
the combustion chamber and leaving the cylinder ready to repeat
the cycle.
The efficiency of a
modern Otto-cycle engine is limited by a number of factors,
including losses by cooling and by friction. In general, the
efficiency of such engines is determined by the compression
ratio of the engine. The compression ratio (the ratio between
the maximum and minimum volumes of the combustion chamber) is
usually about 8 to 1 or 10 to 1 in most modern Otto-cycle
engines. Higher compression ratios, up to about 15 to 1, with a
resulting increase of efficiency, are possible with the use of
high-octane antiknock fuels. The efficiencies of good modern
Otto-cycle engines range between 20 and 25 percent—in other
words, only this percentage of the heat energy of the fuel is
transformed into mechanical energy.
Diesel Engines
Theoretically, the
diesel cycle differs from the Otto cycle in that combustion
takes place at constant volume rather than at constant pressure.
Most diesels are also four-stroke engines but they operate
differently than the four-stroke Otto-cycle engines. The first,
or suction, stroke draws air, but no fuel, into the combustion
chamber through an intake valve. On the second, or compression,
stroke the air is compressed to a small fraction of its former
volume and is heated to approximately 440° C (approximately
820° F) by this compression. At the end of the compression
stroke, vaporized fuel is injected into the combustion chamber
and burns instantly because of the high temperature of the air
in the chamber. Some diesels have auxiliary electrical ignition
systems to ignite the fuel when the engine starts and until it
warms up. This combustion drives the piston back on the third,
or power, stroke of the cycle. The fourth stroke, as in the
Otto-cycle engine, is an exhaust stroke.
The efficiency of
the diesel engine, which is in general governed by the same
factors that control the efficiency of Otto-cycle engines, is
inherently greater than that of any Otto-cycle engine and in
actual engines today is slightly more than 40 percent. Diesels
are, in general, slow-speed engines with crankshaft speeds of
100 to 750 revolutions per minute (rpm) as compared to 2500 to
5000 rpm for typical Otto-cycle engines. Some types of diesel,
however, have speeds up to 2000 rpm. Because diesels use
compression ratios of 14 or more to 1, they are generally more
heavily built than Otto-cycle engines, but this disadvantage is
counterbalanced by their greater efficiency and the fact that
they can be operated on less expensive fuel oils.
Two-Stroke Engines
By suitable design
it is possible to operate an Otto-cycle or diesel as a
two-stroke or two-cycle engine with a power stroke every other
stroke of the piston instead of once every four strokes. The
efficiency of such engines is less than that of four-stroke
engines, and therefore the power of a two-stroke engine is
always less then half that of a four-stroke engine of comparable
size.
The general
principle of the two-stroke engine is to shorten the periods in
which fuel is introduced to the combustion chamber and in which
the spent gases are exhausted to a small fraction of the
duration of a stroke instead of allowing each of these
operations to occupy a full stroke. In the simplest type of
two-stroke engine, the poppet valves are replaced by sleeve
valves or ports (openings in the cylinder wall that are
uncovered by the piston at the end of its outward travel). In
the two-stroke cycle, the fuel mixture or air is introduced
through the intake port when the piston is fully withdrawn from
the cylinder. The compression stroke follows, and the charge is
ignited when the piston reaches the end of this stroke. The
piston then moves outward on the power stroke, uncovering the
exhaust port and permitting the gases to escape from the
combustion chamber.
Rotary Engine
In the 1950s the
German engineer Felix Wankel developed an internal-combustion
engine of a radically new design, in which the piston and
cylinder were replaced by a three-cornered rotor turning in a
roughly oval chamber. The fuel-air mixture is drawn in through
an intake port and trapped between one face of the turning rotor
and the wall of the oval chamber. The turning of the rotor
compresses the mixture, which is ignited by a spark plug. The
exhaust gases are then expelled through an exhaust port through
the action of the turning rotor. The cycle takes place
alternately at each face of the rotor, giving three power
strokes for each turn of the rotor. Because of the Wankel
engine's compact size and consequent lesser weight as compared
with the piston engine, it appeared to be an important option
for automobiles. In addition, its mechanical simplicity provided
low manufacturing costs, its cooling requirements were low, and
its low center of gravity made it safer to drive. A line of
Wankel-engine cars was produced in Japan in the early 1970s, and
several United States automobile manufacturers researched the
idea as well. However, production of the Wankel engine was
discontinued as a result of its poor fuel economy and its high
pollutant emissions.
The Stratified
Charge Engine
A modification of
the conventional spark-ignition piston engine, the stratified
charge engine is designed to reduce emissions without the need
for an exhaust-gas recirculation system or catalytic converter.
Its key feature is a dual combustion chamber for each cylinder,
with a prechamber that receives a rich fuel-air mixture while
the main chamber is charged with a very lean mixture. The spark
ignites the rich mixture that in turn ignites the lean main
mixture. The resulting peak temperature is low enough to inhibit
the formation of nitrogen oxides, and the mean temperature is
sufficiently high to limit emissions of carbon monoxide and
hydrocarbon.
Research on
modifications of conventional engines as well as alternatives to
conventional engines continues. Some of these options include a
modified version of the two-stroke engine, the twin engine (a
combination of an internal-combustion engine and an electric
engine), and the Stirling engine.
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