One of the most important
inventions of the mid- to late 1800s, the internal-combustion engine generated
mechanical energy by burning fuel in a combustion chamber. The introduction of
the new engine led almost immediately to the development of the automobile,
which had been largely unfeasible with the unwieldy steam engine. Shown here is
a 1925 Morris engine, the basic unit for a family car. It features four in-line
cylinders with aluminum pistons. The valves are opened by push rods operated by
a camshaft and closed by springs. Power is transmitted by means of the
crankshaft to the gearbox.
Internal-Combustion Engine, 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.
II
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COMPONENTS OF
ENGINES
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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.
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OTTO-CYCLE ENGINES
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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.
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DIESEL ENGINES
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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.
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TWO-STROKE ENGINES
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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 power of a two-stroke engine is usually double 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.
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ROTARY ENGINE
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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. Mazda, a Japanese
car manufacturer, has continued to design and innovate the rotary engine,
improving performance and fuel efficiency.
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THE STRATIFIED
CHARGE ENGINE
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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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