The Wright brothers used a
gasoline powered,
four-stroke,
internal combustion engine to power their
aircraft.
In an internal combustion engine, fuel and air are
ignited
inside a
cylinder.
The hot exhaust gas pushes a piston in the cylinder which is connected to a
crankshaft
to produce power. The burning of fuel is not a continuous process, but occurs
very quickly at regular time intervals. Between ignitions, the engine parts
move in a repeated sequence called a cycle.
The engine is called a four-stroke engine because there are four movements
(strokes)
of the piston during one cycle.
The brothers' design was based on early automobile engine designs which used the
Otto cycle, developed by the German, Dr. N. A. Otto, in 1876.
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. There are two main parts to engine operation: the
mechanical operation
of the engine parts, and the thermodynamics through which the engine produces
work
and
power.
On this page we will discuss the basic thermodynamic principles and on a
separate page we present the
thermodynamic analysis
that allows you to design and predict engine performance.
Thermodynamics
is a branch of physics which deals with the energy
and work of a system. It was born in the 19th century as scientists
were first discovering how to build and operate steam engines.
Thermodynamics deals only with the large scale response of a system
which we can observe and measure in experiments.
The basic ideas of thermodynamics are taught in high school physics classes,
so the Wright brothers knew and used these concepts, particularly in their
engine design.
We have broken the Otto cycle
into six numbered stages based on the
mechanical operation
of the engine.
At each stage, we show a cut through the cylinder to reveal the movement of
the piston and the amount of the gas volume created by the head of the
piston and the cylinder to the right of the piston head.
On the figure we show a plot of
pressure
versus gas volume throughout one cycle.
The cycle begins at the lower left with Stage 1 being the beginning of the
intake stroke of the engine. The pressure is near
atmospheric pressure and the gas volume is at a minimum with the piston far
to the right in the cylinder. Between Stage 1 and Stage 2 the piston
is moved to the left, the pressure remains constant, and the gas volume increases
as fuel/air mixture is drawn into the cylinder through the intake valve (red).
Stage 2 begins the
compression stroke of the engine with the
closing of the intake valve. Between Stage 2
and Stage 3, the piston moves back to the right, the gas volume decreases,
and the pressure increases because
work is done
on the gas by the piston. Stage 3 is the beginning of the
combustion
of the fuel/air mixture. The combustion occurs very quickly and the volume
remains constant.
Heat
is released during combustion which increases both the
temperature
and the pressure, according to the
equation of state.
Stage 4 begins the
power stroke of the engine. Between Stage 4 and Stage 5,
the piston moves back to the left, the volume in increased, and the pressure
falls as
work is done
by the gas on the piston. At Stage 5 the exhaust valve (blue) is opened
and the residual heat in the gas is
exchanged
with the surroundings. The volume
remains constant and the pressure adjusts back to atmospheric conditions.
Stage 6 begins the
exhaust stroke of the engine during which the
piston moves back to the right, the volume decreases and the pressure
remains constant. At the end of the exhaust stroke, conditions have returned
to Stage 1 and the process repeats itself.
During the cycle,
work
is done on the gas by the piston between stages 2 and 3. Work is done by
the gas on the piston between stages 4 and 5. The difference between the work done by the
gas and the work done on the gas is shown in yellow and is the work produced
by the cycle. The work times the rate of the cycle (cycles per second) is
equal to the
power
produced by the engine. The area enclosed by the cycle on a p-V diagram
is proportional to the work produced by the cycle. On this page we have
shown an ideal Otto cycle in which there is no heat entering (or
leaving) the gas during the compression and power strokes, no friction
losses, and instantaneous burning occurring at constant volume. In reality,
the ideal cycle does not occur and there are many losses associated with
each process. These losses are normally accounted for by efficiency factors
which multiply and modify the ideal result. For a real cycle, the shape
of the p-V diagram is similar to the ideal, but the area (work) is
always less than the ideal value.
Activities:
Navigation..
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page
- http://www.nasa.gov
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