This is a computer drawing of the timing system of the Wright
1903 aircraft engine.
This engine powered the first, heavier than
air, self-propelled, maneuverable, piloted aircraft; the Wright
at Kitty Hawk, North Carolina, in December, 1903.
for their aircraft, the brothers used twin, counter-rotating
at the rear of the aircraft. To turn the propellers, the
brothers designed and built a
internal combustion engine.
The figure at the top shows the major components of the timing system
on 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.
To produce useful work, the combustion must take place
at the end of the
of the engine
the exhaust valve must be opened to clear the cylinder of spent
The job of the timing system is to cause the various operations of the
engine cycle to occur in the correct sequence at the correct time.
The timing system consists of several mechanical components. The main
drive sprocket is attached to the engine crankshaft outside the
on the front of the engine. The drive sprocket has six teeth that engage
holes on the timing chain. The chain runs around the drive sprocket
and the larger cam shaft sprocket. The arrangement is
exactly like the chain on a bicycle from the pedals to the rear wheel.
The large cam shaft sprocket has twelve teeth, so two revolutions of the
crankshaft produce one revolution of the valve cam shaft.
This is the required ratio for a
engine in which the piston (attached to the crankshaft) makes two passes
through the cylinder during each cycle. To keep the
proper tension on the chain, there is a small adjustable tension
wheel on the outside of the chain.
The timing chain turns the valve cam shaft which is located on the
bottom of the engine. In the figure at the top of this page and in this
computer animation, we view the engine from below.
There are four valve cams attached to the valve cam shaft.
The cams rotate with the shaft and the
surface of each cam rides on a rocker arm of the
of each cylinder. Because of the design of the surface
or the cam, the rocker arm is pushed down, and the valve pushed open,
at specific times and for specific intervals during the shaft rotation.
This motion ensures that the valve is opened only during the exhaust stroke
of the cylinder. Notice in the animation that the four rocker arms
move at different times. This motion supports the
firing order of the cylinders.
There is a small gear located on the valve cam shaft near the front
of the shaft, to the right in the figure.
This gear engages another gear on the ignition cam shaft.
The rotation of these ignition shaft gears cause the ignition cam
shaft to rotate in an opposite direction from the valve cam shaft, but to
rotate at the same speed. Located on the ignition cam shaft are four
ignition cams which engage the spring switches of the
electrical system. On the animation, the ignition
cams and shaft are colored green. The combination of valve cams and ignition
cams insure that valves are opened and closed at the correct time in the engine
cycle and that ignition occurs when the valves are closed and the volume
of the cylinder is the smallest.
How Does It Work?
To better understand the action of the cams, here is a diagram describing
how cams work:
A cam is a metal disk for which the distance from the center of rotation
of the disk to the edge surface
varies as you move around the edge surface.
The cam rotates on a shaft and the surface of the cam rides on an
object called a follower. (For our engine, the rocker arm is the
follower). As the cam rotates from position 1 to position 2, the point
on the surface that touches the follower changes. Since the distance
from the center of rotation varies between points on the cam surface,
the follower moves.
Depending on how the follower is configured, the follower can rotate, or
translate, or close a switch, or perform a variety of tasks. The cam
eventually rotates back to position 1 and the task is repeated.
Because the actual combustion takes a finite amount of time, the firing of
the ignition system usually does not occur at exactly the top of the piston
motion. To allow some variation, there is a small handle on the leg of the
engine which connects to the ignition shaft gear. Moving this handle, causes
the gear to move slightly on the shaft so that the cam engages the switch at
a slightly different time relative to the moving of the valves (and piston).
This is called ignition advance and is used even on modern
automobile engines. For the Wright aircraft, the advance was set before
flight and could not be altered by the pilot in flight.
- Re-Living the Wright Way
- Beginner's Guide to Aeronautics
- NASA Home Page