Wednesday, August 15, 2012

Engine Electrical System

This is a computer drawing of the electrical system of the Wright brothers' 1903 
aircraft engine. This engine powered the first, heavier than air, self-propelled, mane
uverable, piloted aircraft; the Wright 1903 Flyer at Kitty Hawk, North Carolina, in 
December, 1903. To generate thrust for their aircraft, the brothers used twin, 
counter-rotating propellers at the rear of the aircraft. To turn the propellers, the 
brothers designed and built a water-cooled, gasoline powered, four-stroke, four c
ylinder,internal combustion engine.
Mechanical Operation


The figure at the top shows the major components of the electrical system on 

the Wright 1903 engine. In any internal combustion engine, fuel and oxygen are 
combined in a combustion process to produce the power to turn the crankshaft 
of the engine. The job of the electrical system is to provide the spark which initiates 
combustion.
Electrical power is generated by the magneto at the rear of the engine. The magneto 

relies on the physics principle ofelectrical inductance to produce electricity; 
when a wire is moved through a magnetic field an electrical current is inducedin 
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 crankcase, and 
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 proper moment of 
the engine cycle. Here's a computer animation of the action of the levers 
and contact switch:


Computer animation of the Wright 1903 aircraft engine valves

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
ll 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 compression stroke.
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.

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