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 exhaust has been expanded into the cylinder
to a moderate pressure and temperature
by the motion of the piston to the left.
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 open.
The exhaust has done
on the piston but there is some residual
in the exhaust gas. As the piston comes to a halt near the crankshaft,
the residual heat is quickly transferred to the water in the
surrounding the cylinder. In theory, the
transfer proceeds so quickly that we can consider the
piston to be motionless and the volume of the combustion chamber and cylinder
to be a constant. The end of
the heat rejection process is designated Stage 6 of the engine cycle
and is the beginning of the
Because the intake and exhaust valves are closed, the heat transfer from
the exhaust gas takes place in a nearly constant volume vessel. The
heat transfer decreases the
of the exhaust gas.
From considerations of the
of thermodynamics, the temperature decrease is given by:
T6 = T5 - Q /cv
where Q is the heat rejected, T is the temperature,
and cv is the specific heat at constant volume, From the
equation of state, we know that:
p6 = p5 * (T6 /T5)
where p is the pressure.
The numbers indicate the two stages of the cycle.
Since Q is a positive number, T6 is less than T5 and p6 is less than
p5. Temperature and pressure in the cylinder both decrease during the
cooling process. The final value of the pressure is atmospheric pressure
and this determines the amount of heat that is rejected.
In theory, the heat transfer takes place instantaneously when the piston
is motionless. In reality, the heat is transferred throughout the exhaust
stroke. The effect is the same, but reality is so much harder to
analyze that we make the assumption of instantaneous heat release
to obtain an initial estimate of the heat transferred.
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