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
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 and learn the
fundamentals of engines
On this page we present a
computer drawing of the electrical system of the Wright
brothers' 1903 aircraft engine.
The figure at the top shows the major components of the electrical systemon the Wright 1903 engine.
In any internal combustion engine,
fuel and oxygen are combined in a
to produce the power to turn the
crankshaft of the engine.
The job of the electrical system is to provide the spark which
Electrical power is generated by the
magneto at the rear of the engine. The magneto relies on the
physics principle of electrical inductance to produce electricity;
when a wire is moved through a magnetic field an electrical current
is induced in the wire. The magneto has a large, U - shaped, permanent
magnet at the top. Between the arms of the magnet, a wire is rotated on a
shaft which is turned by the friction drive wheel rubbing on the
engine flywheel. An electrical current is induced in the moving wire.
The power to turn the magneto is provided by the running
engine. The magneto is very similar to the alternator or generator on a
modern automobile. The Wright brothers purchased their magneto and it
provided a very modest 10 volts at 4 amps in operation.
Two wires connect the magneto to the engine; a ground wire to the
leg of the
and a power wire to the bus bar on the outside
of the four combustion chambers of the engine.
On each combustion chamber, the bus bar conducts electricity to an
ignition plug which is screwed through the wall of the chamber. The
plug is insulated from the wall of the chamber. Inside the chamber, there
is a contact switch which is movable.
When the switch is closed, a circuit is created and electricity flows through
the wires, bus bar and plug. When the switch is opened quickly, a spark is
generated. You can see this effect if you pull the plug on an operating
appliance at home.
Spring levers mounted on the outside of the chamber are
used to open and close the contact switch by an insulated shaft which passes
through the wall of the combustion chamber.
The spring levers are attached to the crankcase of the engine which is
grounded to the magneto.
The levers are activated by cams
which turn on a cam shaft under the
engine. The cam shaft is linked by gears to the exhaust valve cam shaft
which is turned by the
timing chain. The gears and cams insure that
the contact switch is opened, and the ignition spark occurs at just the
of the engine
Here's a computer animation of the action of the levers and contact switch:
In this animation, we have cut open cylinder #3 so that you can watch the
motion of the valves, cams, rocker arms, and electrical contacts and switches.
The spring which moves the electrical contact inside cylinder #3
is partially hidden by the cylinder itself. The spring is barely visible
behind the blue exhaust valve spring. You can better see the action of the
electric cam and spring on the adjoining cylinder #4 to the right. But notice
that the timing of the motion of the switches and valves is different between
adjoining cylinders. On the animation, we have cut the bus bar to allow us
to see inside cylinder #3; the bar wraps around cylinder #3 in the
same way that it wraps around cylinder #2 to the left.
How Does It Work?
To understand how the electrical system works, we have drawn a
simplified wiring diagram of the engine:
We have numbered the cylinders (and combustion chambers) from 1 to 4
going from the front of the engine to the back.
The magneto, wires, contact switches, and grounded cylinders produce
an electrical circuit, which you have heard about in school.
This particular type of circuit is called a parallel circuit
because there are parallel lines running through the four
cylinders. The contact switch on any cylinder can be opened or closed
without affecting the neighboring cylinders. (If the cylinders were
wired in series, opening any switch would cut the current
to all the cylinders.)
Throughout nearly all of the cycle for a given cylinder,
the contact switch is held opened and no current flows through the system.
But when the cam pushes the levers, the contact switch in one cylinder
is initially closed which produces a current of
electricity from the magneto through the bus bar, switch, and
levers, to the crankcase and back to the magneto.
This condition for cylinder #1 is shown at the top of the figure.
As the cam continues to move, the contact switch is suddenly pulled open,
as shown at the bottom of the figure. A small spark occurs as the
switch is opened (you can see this effect if you pull the plug on an
operating lamp in your home.)
Inside the combustion chamber, this spark is used to ignite the fuel/air
mixture at the end of the
The contact switch is kept open inside
the cylinder until the next firing. The opening of the switch is called
an electrical break (of the circuit) and this firing technique is
called a "make and break" system. The four cylinders of this engine fire
one at a time in a firing order which is repeated. The brothers
used a 1 - 3 - 4 - 2 firing order to balance out the firings and make
the engine run as smooth as possible.
Historical note -
Notice that in the "make and break" system there
are moving parts located inside the combustion chamber.
Modern internal combustion
engines do not use this method, but instead use a spark plug to produce the
ignition spark. A spark plug has no moving parts, which is much safer than the
method used by the brothers.
Modern systems also use very high voltage relative to the brothers' system.
But the brothers had one advantage on modern systems. Their contact points
moved during the engine cycle, so they stayed relatively clean.
Modern spark plugs can foul because of oil and dirt present in the
combustion chamber collecting in the gap of the plug. The "make and
break" system does not have this problem.
- Beginner's Guide Home Page