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Extending Engine Life
Helpful tips to keep your power plant happy!
Extend Engine Life from Model Airplane News
Evolution
of the Internal-Combustion Engine
The
first person to experiment with an internal-combustion
engine was the Dutch physicist Christian Huygens,
about 1680. But no effective gasoline-powered
engine was developed until 1859, when the French
engineer J. J. Étienne Lenoir built a double-acting,
spark-ignition engine that could be operated
continuously. In 1862 Alphonse Beau de Rochas,
a French scientist, patented but did not build
a four-stroke engine; sixteen years later, when
Nikolaus A. Otto built a successful four-stroke
engine, it became known as the “Otto cycle.”
The first successful two-stroke engine was completed
in the same year by Sir Dougald Clerk, in a
form which (simplified somewhat by Joseph Day
in 1891) remains in use today. George Brayton,
an American engineer, had developed a two-stroke
kerosene engine in 1873, but it was too large
and too slow to be commercially successful.
In
1885 Gottlieb Daimler constructed what is generally
recognized as the prototype of the modern gas
engine: small and fast, with a vertical cylinder,
it used gasoline injected through a carburetor.
In 1889 Daimler introduced a four-stroke engine
with mushroom-shaped valves and two cylinders
arranged in a V, having a much higher power-to-weight
ratio; with the exception of electric starting,
which would not be introduced until 1924, all
modern gasoline engines are descended from Daimler's
engines.
| Internal-combustion engine |
internal-combustion engine, one in which combustion of the
fuel takes place in a confined space, producing
expanding gases that are used directly to provide
mechanical power. Such engines are classified
as reciprocating or rotary, spark ignition or
compression ignition, and two-stroke or four-stroke;
the most familiar combination, used from automobiles
to lawn mowers, is the reciprocating, spark-ignited,
four-stroke gasoline engine. Other types of
internal-combustion engines include the reaction
engine . Engines are rated by their maximum
horsepower, which is usually reached a little
below the speed at which undue mechanical stresses
are developed.
The
Four-Stroke Cycle
In
most engines a single cycle of operation
(intake, compression, power, and exhaust)
takes place over four strokes of a piston,
made in two engine revolutions. When
an engine has more than one cylinder
the cycles are evenly staggered for
smooth operation, but each cylinder
will go through a full cycle in any
two engine revolutions. When the piston
is at the top of the cylinder at the
beginning of the intake stroke, the
intake valve opens and the descending
piston draws in the air-fuel mixture.
At
the bottom of the stroke the intake
valve closes and the piston starts upward
on the compression stroke, during which
it squeezes the air-fuel mixture into
a small space at the top of the cylinder.
The ratio of the volume of the cylinder
when the piston is at the bottom to
the volume when the piston is at the
top is called the compression ratio.
The higher the compression ratio, the
more powerful the engine and the higher
its efficiency. However, in order to
accommodate air pollution control devices,
manufacturers have had to lower compression
ratios.
Just
before the piston reaches the top again,
the spark plug fires, igniting the air-fuel
mixture (alternatively, the heat of
compression ignites the mixture). The
mixture on burning becomes a hot, expanding
gas forcing the piston down on its power
stroke. Burning should be smooth and
controlled. Faster, uncontrolled burning
sometimes occurs when hot spots in the
cylinder preignite the mixture; these
explosions are called engine knock and
cause loss of power. As the piston reaches
the bottom, the exhaust valve opens,
allowing the piston to force the combustion
products—mainly carbon dioxide, carbon
monoxide, nitrogen oxides, and unburned
hydrocarbons—out of the cylinder during
the upward exhaust stroke.
The
Two-Stroke Cycle
The
two-stroke engine is simpler mechanically
than the four-stroke engine. The two-stroke
engine delivers one power stroke every
two strokes instead of one every four;
thus it develops more power with the
same displacement, or can be lighter
and yet deliver the same power. For
this reason it is used in lawn mowers,
chain saws, small automobiles, motorcycles,
and outboard marine engines.
However,
there are several disadvantages that
restrict its use. Since there are twice
as many power strokes during the operation
of a two-stroke engine as there are
during the operation of a four-stroke
engine, the engine tends to heat up
more, and thus is likely to have a shorter
life. Also, in the two-stroke engine
lubricating oil must be mixed with the
fuel. This causes a very high level
of hydrocarbons in its exhaust, unless
the fuel-air mixture is computer calculated
to maximize combustion. A highly efficient,
pollution-free two-stroke automobile
engine is currently being developed
by Orbital Engineering, under arrangements
with all the U.S. auto makers.
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Rotary
Engines
The
most successful rotary engine is the Wankel
engine. Developed by the German engineer Felix
Wankel in 1956, it has a disk that looks like
a triangle with bulging sides rotating inside
a cylinder shaped like a figure eight with a
thick waist. Intake and exhaust are through
ports in the flat sides of the cylinder. The
spaces between the sides of the disk and the
walls of the cylinder form combustion pockets.
During a single rotation of the disk each pocket
alternately grows smaller, then larger, because
of the contoured outline of the cylinder. This
provides for compression and expansion. The
engine runs on a four-stroke cycle.
The
Wankel engine has 48% fewer parts and about
a third the bulk and weight of a reciprocating
engine. Its main advantage is that advanced
pollution control devices are easier to design
for it than for the conventional piston engine.
Another advantage is that higher engine speeds
are made possible by rotating instead of reciprocating
motion, but this advantage is partially offset
by the lack of torque at low speeds, leading
to greater fuel consumption.
Reciprocating
Engines
The
most common internal-combustion engine is the
piston-type gasoline engine used in most automobiles.
The confined space in which combustion occurs
is called a cylinder. The cylinders can be arranged
in one of three ways: a single row (in-line)
with the centerlines of the cylinders vertical;
a double row with the centerlines of opposite
cylinders converging in a V (V-engine); or two
horizontal, opposed rows (opposed or pancake
engine). In each cylinder a piston slides up
and down. One end of a connecting rod is attached
to the bottom of the piston by a joint; the
other end of the rod clamps around a bearing
on one of the throws, or convolutions, of a
crankshaft; the reciprocating (up-and-down)
motions of the piston rotate the crankshaft,
which is connected by suitable gearing to the
drive wheels of the automobile. The number of
crankshaft revolutions per minute is called
the engine speed. The top of the cylinder is
closed by a metal cover (called the head) bolted
onto it. Into a threaded aperture in the head
is screwed the spark plug, which provides ignition.
Two
other openings in the cylinder are called ports.
The intake port admits the air-gasoline mixture;
the exhaust port lets out the products of combustion.
A mushroom-shaped valve is held tightly over
each port by a coil spring, and a camshaft rotating
at one-half engine speed opens the valves in
correct sequence. A pipe runs from each intake
port to a carburetor or injector, the pipes
from all the cylinders joining to form a manifold;
a similar manifold connects the exhaust ports
with an exhaust pipe and noise muffler. A carburetor
or fuel injector mixes air with gasoline in
proportions of weight varying from 11 to 1 at
the richest to a little over 16 to 1 at the
leanest. The composition of the mixture is regulated
by the throttle, an air valve in the intake
manifold that varies the flow of fuel to the
combustion chambers of the cylinders. The mixture
is rich at idling speed (closed throttle) and
at high speeds (wide-open throttle), and is
lean at medium and slow speeds (partly open
throttle).
The
other main type of reciprocating engine is the
diesel engine, invented by Rudolf
Diesel and patented in 1892. The diesel uses
the heat produced by compression rather than
the spark from a plug to ignite an injected
mixture of air and diesel fuel (a heavier petroleum
oil) instead of gasoline. Diesel engines are
heavier than gasoline engines because of the
extra strength required to contain the higher
temperatures and compression ratios. Diesel
engines are most widely used where large amounts
of power are required: heavy trucks, locomotives,
and ships.
Cooling
and Lubrication of Engines
Most
small two-stroke engines are air-cooled. Air
flows over cooling fins around the outside of
the cylinder and head, either by the natural
motion of the vehicle or from a fan. Many aircraft
four-stroke engines are also air-cooled; larger
engines have the cylinders arranged radially
so that all cylinders are directly in the airstream.
Most four-stroke engines, however, are water-cooled.
A water jacket encloses the cylinders; a water
pump forces water through the jacket, where
it draws heat from the engine. Next, the water
flows into a radiator where the heat is given
off to the air; it then moves back into the
jacket to repeat the cycle. During warm-up a
thermostatic valve keeps water from passing
to the radiator until optimum operating temperatures
are attained.
Four-stroke
engines are lubricated by oil from a separate
oil reservoir, either in the crankcase, which
is a pan attached to the underside of the engine,
or in an external tank. In an automobile engine
a gear pump delivers the oil at low pressure
to the bearings. Some bearings may depend on
oil splashed from the bottom of the crankcase
by the turning crankshaft. In a two-stroke engine
the lubricating oil is mixed with the fuel.
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