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 fuel/air mixture has been compressed into the combustion chamber
to a moderate pressure and temperature
by the motion of the piston to the right.
From our considerations of the
we designate this condition as
of the Otto cycle.
The intake valve and exhaust valve are closed and the electrical
contact is closed.
The combustion process begins by opening the electrical contact through
the action of the ignition cam and springs. As the contact moves away from
the plug, a spark is produced which ignites the mixture.
Igniting the fuel/air mixture results in rapid
of the fuel, a release of
and the production of exhaust gases. The
combustion proceeds so quickly that we can consider the
piston to be motionless and the volume of the combustion chamber
to be a constant. The end of
the combustion process is designated Stage 4 of the engine cycle
and is the beginning of the
Because the intake and exhaust valves are closed, the combustion of the
fuel takes place in a nearly constant volume vessel. The
combustion increases the
of the exhaust gases, any residual air
in the combustion chamber, and the combustion chamber itself.
From considerations of the
of thermodynamics, the temperature increase is given by:
T4 = T3 + Q /cv
where Q is the heat released and depends on the fuel and fuel/air ratio,
cv is the specific heat at constant volume,
and T is the temperature. From the
equation of state, we know that:
p4 = p3 * (T4 /T3)
where p is the pressure.
The numbers indicate the two stages of the cycle.
Since Q is a positive number, T4 is greater than T3 and p4 is greater than
p3. Temperature and pressure in the combustion chamber both increase during the
combustion process. The final value of the pressure depends on a
temperature factor times the initial value of the pressure.
The more that we can compress the gas (the higher
the p3), the greater will be the final pressure, p4. The final
and power output of the engine depends on the value of p4. That is
why we compress the gas as much as possible before combustion.
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