Often all the mechanical work is complete on a fine little model gasoline engine, or a grand old restored engine, and then comes the question: 'What am I going to do for an ignition system?' If that is your dilemma, then here is an answer.
To help understand the operation of an ignition system a little better, first we'll have a very basic course on electromagnetism. Whenever electrical current flows through a conductor such as a wire, circular magnetic lines of force from around that conductor. (See Fig. #1 in the Image Gallery). Now if you take that wire and wrap it into a coil, the magnetic flux lines complement each other. (See Fig. #2)
If an iron core is placed in the coil, the flux lines will travel through the iron. The flux lines like the iron core because they can travel through it much easier than through air. We have now created an electro magnet. (See Fig. #3).
There are some peculiar things about electricity and magnetism. If an electrical current passes through a conductor, magnetic flux lines form around the conductor as we have just seen, and inversely if magnetic lines cross a conductor they cause a current to flow in that conductor. The latter is how we generate electricity.
Suppose we wrap two coils around an iron core and induce an electrical current into one coil. We'll call it the "primary coil." This will cause a magnetic field or flux lines to build around the iron core. As the flux lines are building and cut across the wires in the "secondary" coil, electricity will flow in the secondary coil. This is called induction. (See Fig. #4)
Another peculiar thing about the above apparatus is that the voltage in each of the coils is proportional to the number of turns in the two windings. In an ignition coil, for instance, the primary coil energized by, say, a six volt battery would have relatively few windings, and the secondary coil would have thousands of times more windings of very small diameter wire in order to get the 15,000 to 25,000 volts needed to jump the gap in the spark plug.
Let's apply the knowledge we have learned and make a diagram of an ignition system for a one-cylinder gasoline engine.
In the diagram, (See Fig. #5) we have put a round head rivet in the cam gear. The rivet acts as a contact to complete the circuit from the battery and around through the primary winding. This contact point or switch needs to be adjustable so that the timing of the spark will fire just as the piston comes to the top of the compression stroke. This adjustment can be as simple as just bending the contact finger that touches the rivet head. On large engines it is very desirable to have some means to retard the timing for starting, and then advancing it for running.
The timing point or contact does not necessarily need to be located on the cam gear. It can be located on any moving part of the engine that will cause the switch to close at the proper time.
In the diagram (Fig. #5) we are using a buzz coil. This is the same sort of circuit as was used on the old Model T Fords. At rest the buzz coil points are closed. As the circuit is energized the iron core magnetizes and pulls the buzz coil points apart which breaks the circuits and lets the buzz coil points snap back together again, which energizes the circuit and the process starts all over again. The result is an almost continuous arc at the spark plug as long as the timing contacts are closed. If the buzz coil points are not used as in modern engines, then there is just one snap spark at the spark plug each time the timing contacts closes and completes the circuit.
The condenser (or capacitor) is an insignificant-looking but necessary little contraption. A condenser has the ability to store and release small amounts of electricity. The best way to describe its function is to compare it to an air dome on a piston-type water pump. Believe it or not, electricity flowing in a circuit has kinetic energy similar to that of a flywheel. It takes effort to get it to start moving, and to stop it once it is moving. The condenser absorbs the surge as the points open and the flow stops. Arcing at the points is greatly reduced. Then as the points close again the stored electricity flows back into the circuit along with electricity from the battery and adds that extra "Umpf!" to help fire the spark plug.
Now that we know all about electricity and how to wire it up to our engine, let's proceed to make a "Sparker" for our engine. First we will need to acquire a magneto coil from a small engine such as a lawn mower engine. Later models will have a more compact coil than those that were made over 15 or so years ago. These coils may vary in shape from the one illustrated, but they will all work.
You will want to check the coil to make sure that it is good. This can be done by touching the primary leads to a "C" or "D" size flash light battery while holding thumb and finger across the high-tension lead and the ground wire. You will get a mild shock if the coil is good.
With just a flashlight battery for power, the shock that you get is more of a "startle" than a "zap your finger off" jolt that you get when you touch the spark plug wire of a running engine.
If you don't want to make this test yourself, you might get your wife to hold the high tension lead and the ground wire while you manipulate the primary leads to touch the battery, explaining to her that you cannot hold all those wires at the same time. I can assure you that this method will verify whether the coil is good, however I need to caution you that your bodily injuries might be far less if you just perform the test by yourself.
Once the coil is proven good, we can proceed to make the sparker as shown in Fig. #6 and Fig. #7. The simplest high tension ignition system is the single spark ignition system as shown in the lower diagram in Fig. #7. You can build a small hardwood box of your own design to house the coil, condenser and battery. If all you desire is a single flash spark ignition then you can stop here. If you desire to build a buzz coil spark ignition system, then continue on.
Note: A condenser from any automotive or small gas engine ignition system will work in the systems shown in the Image Gallery. Direction of hookup is not critical.
For a buzz coil ignition system, start with a mounting board about 1/2" x 3-1/2" x 1/8" to 1/4" thick. The board can be any non-conductive material such as plastic, masonite, plywood or hardwood, hardwood being the classic material.
This article originally appeared in the June 1988 issue of Gas Engine Magazine. The text of this article was written by Rex Whiting, and Birk Petersen did the illustrations. Rex says that "by way of batteries, I used the batteries from a battery powered vacuum or battery powered grass trimmer that is rechargeable but small and compact."