We took detour from developing the mechanical parts of the engine in Part 4, to develop the magneto and the ignition system. It meant revisiting the theories used to describe one of the fundamental interactions in nature; electromagnetic phenomena.
At its heart, an ignition system is quite a simple device. It has to provide a high enough voltage to produce a nice, hot arc across the spark gap in the spark plug.
The temperature of the spark needs to be high enough to ignite a homogenous mixture of fuel and air. The spark has to occur at the right time in the engine’s cycle, so that as much as possible of the energy of the combustion can be converted to useful mechanical work.
It has to do this, repeatably, several thousand times a minute!
So that’s really only three things;
1. Produce a voltage
2. Transform that voltage to a voltage high enough to jump the spark gap
3. Time the spark
How to produce a voltage?
An English scientist, Michael Faraday, first realized that if you move a magnet close to quite close to piece of wire, a voltage is ‘induced’ in the wire. And that the faster you move the magnet, the more voltage is induced in the wire.
This then, is the fundamental working principle of ALL magnetos. In fact it’s the way almost all the electricity we use in daily life is produced.
For the purpose of our magneto, we mounted a magnet in a part of the engine, called a flywheel.
Like this…
Then we mounted a coil on the crankcase in close proximity to the magnet, so that every time the flywheel went around, the coil would experience the magnetic flux.
Like this;
But here’s the thing; making a magnet move in proximity to a coil won’t make a high enough voltage. It’s the rate of change of magnetic flux that determines how much voltage is produced. So here’s a neat trick all magnetos use to get a very high rate of change of magnetic flux.
Because you can’t see magnetic flux, here’s a graphical representation, using a Free, Open Source Magnetics analysis software called FEMM;
Look at what happens as the flywheel turns clockwise; the flux through the coil goes first one way (see the first picture). In the second picture, there’s almost no flux linked with the coil. In the third picture, the flux is now going in the opposite direction.
Under the influence of this flux reversal, the coil experiences a very high rate of change of flux and therefore produces a high voltage.
But when I say high voltage, I mean about 100V. That’s not even close to being enough to jump the spark gap.
To jump the spark gap, in the high pressure environment inside the combustion chamber, you need a minimum voltage in the vicinity of 6000V!
How to step up the voltage enough to jump the spark gap?
Enter Heinrich Friedrich Emil Lenz. This Russian – German Physicist first figured out the direction of the magnetic flux in a current carrying conductor and the direction of induced current in wire inside a magnetic field. Basically, the directions will be such as to oppose the very change that caused them.
Here’s what I didn’t tell you before;
That coil in picture, is actually 2 coils wound one on top of the other, around a soft iron core.
The inside coil (called the primary winding) has a few hundred turns of relatively thick wire, to get a sufficiently large voltage with a fairly high current. The outer coil, the secondary has several thousand turns of very fine wire.
This kind of arrangement is called a transformer.
So here’s the summary;
The magnet goes round and round on the flywheel, inducing a voltage in the primary. The reversal in the magnetic flux causes a high voltage to be induced.
This fast changing high voltage in the primary causes the primary to behave like a magnet with a varying flux field.
This varying flux field causes a voltage to be induced in the secondary.
Because the secondary has a very large number of windings of very thin wire, the voltage is transformed into a very high voltage at a very low current. This transformed voltage is now sufficiently high to ionize the air between the electrodes of the spark plug and appears as a spark.
But What about Timing?
The spark needs to occur at particular point in the engine’s cycle, so as to maximize the work done by the burning fuel and air mixture. This time point is somewhat before the piston reaches the top of its travel. This gives the fuel sufficient time to ignite and burn so that the maximum force can exerted on the piston head when it is on its way back down.
Up until the 1970’s engines used to have a ‘contact breaker’. The contact breaker is exactly what it sounds like. It literally breaks a closed circuit in which includes the primary coil.
As the flywheel, R, turns a current is induced in the primary winding. This current flows through the circuit. By opening the switch, the current can be interrupted. The switch is suitably linked to the engine mechanically, such that it opens at the appropriate time.
Now this is where things get interesting. Coils of wire don’t like to have the current flowing through them interrupted. If the current is interrupted, they produce a current of their own, to counter this interruption. This is called and inductive spike and can reach very high values.
Remember how the voltage in the primary is increased by reversing the flux? It’s increased again, by interrupting the current.
So now the voltage induced in the secondary is very high indeed! For example in the Ushtara engine, the voltage in the secondary during sparking goes up to 16000V.
But there’s a problem with contact breakers. Like all mechanical components, they wear out. Even more so in this case, because of the currents passing through them.
So we used electronics.
This circuit does exactly the same thing as the contact breaker. The current flowing through the device Q2 is interrupted when the voltage in the circuit becomes high enough to turn on the transistor Q1, thus causing an inductive spike in the primary.
So does it work?
I’m glad to say, it does…
