For the forty years following the
of the Wright brothers, airplanes used
internal combustion engines
Today, most general aviation or private airplanes are still
powered by propellers and internal combustion engines, much like your
On this page we will discuss the fundamentals of the
internal combustion engine using the
Wright brothers' 1903 engine, shown in the figure, as an example.
The brothers' design is very simple by today's standards, so it is a good
engine for students to study to learn the fundamentals of
engine operation. This type of
internal combustion engine
is called a
engine because there are four movements
of the piston before the entire engine firing sequence is repeated.
In the figure, we have colored the
fuel/air intake system
green, and the
blue. We also represent the fuel/air mixture and the exhaust gases by small
colored balls to show how these gases move through the engine.
Since we will be referring to the movement of various engine parts, here is
a figure showing the names of the parts:
At the end of the
the combustion chamber is filled with exhaust gases at high pressure and
From our considerations of the
we designate this condition as
of the Otto cycle.
The intake and exhaust valves are closed and the electrical contact switch
With both valves closed, the combination of the cylinder and combustion chamber
form a completely closed vessel containing exhaust gases. The piston
is pushed to the left because of the high pressure on the face of the
piston. As the piston moves to the left, the volume is
increased as the exhaust gas expands.
When the piston has moved completely to the left, we designate
the conditions as Stage 5 of the cycle.
At the beginning of Stage 5, residual heat in the exhaust mixture is
During the power stroke, no
is transferred to the exhaust gases.
As the volume is increased because of the piston's motion,
the pressure in the gas is decreased.
In the figure, the mixture has been colored red at stage 4 and
yellow at stage 5 to denote a moderate decrease in pressure.
Unlike the compression stroke, the hot gas does work on the piston during the power stroke. The force
on the piston is transmitted by the piston rod to the crankshaft, where the linear
motion of the piston is converted to angular motion of the crankshaft. The work
done on the piston is then used to turn the shaft, and the propellers, and
to compress the gases in the neighboring cylinder's compression stroke.
There are thermodynamic
which relate the pressure decrease and temperature decrease to the
change in volume:
p5 / p4 = (V4 / V5) ^ gamma
T5 / T4 = (V4 / V5) ^ (gamma - 1)
where p is the pressure, T is the temperature,
V is the volume of the mixture,
and gamma is the ratio of
specific heats of the mixture.
The numbers indicate the two stages of the cycle.
Since V4 is less than V5 and gamma is greater than 1 (1.4 for pure air),
p5 is less than p4 and T5 is less than T4. Pressure and temperature
of the exhaust both decrease during the power stroke, and
the final value (p5 and T5) depends only on a geometric expansion ratio
(V4/V5) to some power multiplied by the intial value (p4 and T4).
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