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An
internal combustion engine is any engine that operates by burning its fuel
inside the engine. In contrast a steam engine
burns its fuel outside the engine. The most common internal combustion
engine type is gasoline powered. Others include those fueled by diesel,
hydrogen, methane, propane, etc. Engines typically can only run on one
type of fuel and require adaptations to adjust the air/fuel ratio or mix
to use other fuels.
In a gasoline
engine, a mixture of gasoline and air is sprayed into a cylinder. This
is compressed by a piston and at optimal point in the compression stroke,
a spark plug creates an electrical spark that ignites the fuel. The combustion
of the fuel results in the generation of heat, and the hot gases that are
in the cylinder are then at a higher pressure than the fuel-air mixture
and so drive the piston back down. These combustion gases are vented and
the fuel-air mixture reintroduced to run a second stroke. The outward linear
motion of the piston is ordinarily harnessed by a crankshaft to produce
circular motion. Valves control the intake of air-fuel mixture and allow
exhaust gasses to exit at the appropriate times.
Two-Stroke
The two-stroke
type of internal combustion engine is typically used in utility or recreational
applications which require relatively small, inexpensive, and mechanically
simple motors (chainsaws, jetskis, small motorcycles, etc).
The two-stroke
engine is simple in construction, but complex dynamics are employed in
its operation. There are several features unique to a two-stroke engine.
First, there is a reed valve between the air-fuel intake and the crankcase.
Air-fuel mixture enters the crankcase and is trapped there by the one-way
reed valve. Next, the cylinder has no valves as in a conventional four
stroke engine. Intake and exhaust are accomplished by means of ports -
special holes cut into the cylinder wall which allow fuel-air mixture to
enter from the crankcase, and exhaust to exit the engine. These ports are
uncovered when the piston is in the down position.
Air-fuel mixture
is drawn into the crankcase from the carburetor or fuel injection system
through the reed valve. When the piston is forced down, the exhaust port
is uncovered first, and hot exhaust gases begin to leave the cylinder.
As the piston is now in the down position, the crankcase becomes pressurized,
and when the intake port into the cylinder is uncovered, pressurized air-fuel
mixture enters the chamber. Both the intake and exhaust ports are open
at the same time, which means the timing and air flow dynamics are critical
to proper operation. As the piston begins to move up, the ports are closed
off, and the air-fuel mixture compresses and is ignited; the hot gases
increase in pressure, pushing the piston down with great force and creating
work for the engine.
The major components
of two-stroke engines are tuned so that optimum airflow results. Intake
and exhaust tubes are tuned so that resonances in airflow give better flow
than a straight tube. The cylinder ports and piston top are shaped so that
the intake and exhaust flows do not mix.
Four-Stroke
The four-stroke
internal combustion engine is the type most commonly used for automotive
and industrial purposes today (cars and trucks, generators, etc). On the
first (downward) stroke of the piston, fuel/air is drawn into the cylinder.
The following (upward) stroke compresses the fuel-air mixture, which is
then ignited - expanding exhaust gases then force the piston downward for
the third stroke, and the fourth and final (upward) stroke evacuates the
spent exhaust gasses from the cylinder.
The four-stroke
cycle is more efficient than the two-stroke cycle, but requires considerably
more moving parts and manufacturing expertise.
Suggested Reading
Internal-Combustion
Engine Technical information and history.
(Reprinted
From Wikipedia, the free encyclopedia)
*licensed under the GNU
Free Documentation License
It uses material from the Wikipedia article "Internal-combustion
engine"
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