TIMING IS EVERYTHING - Basic Kart Ignition Explained                          Printer-friendly format

By John Copeland

Few systems on your kart are more critical, or less understood, than the ignition system. For the most part, the ignition system does its job without much fuss. You hit the starter, or pull the recoil, and the engine fires up. If it doesn't, you put in a fresh sparkplug and everything is ok again. But a lot of us do not have a clue about how the ignition system really works. Over the next couple of months we're going to take a look at the ignition systems that are mounted on the Yamaha KT100S, the Briggs and Stratton, and on the current generation of Reed Valve engines. But to begin with, we need to have a basic understanding of how magneto ignition systems work.

All kart engines use magneto ignition systems. Unlike the ignition system in your car or truck, a magneto doesn't require a battery as a basic power source to run the ignition system. Instead, it relies on principles of electricity and magnetism. When you wrap wire around an iron or steel core, and then change the magnetic field in that core, you induce an electrical voltage in the wire. Magnetically speaking, as the magnetic field of the magnet approaches the field that the wire creates around the metal core on which it is wound, a "potential" develops. The rate at which that potential develops determines it magnitude. It's how a generator works. The more coils of wire you pass through the magnetic field, the more voltage you induce in the wire. Spinning the coils on the armature of the generator past the magnets induces voltage in the armature. That's what powers your trailer lights at the track, or your power tools, or whatever. But it doesn't matter which element, the magnet or the coil of wire, moves and which is stationary. In your engine, the magnets are located in the flywheel or ignition rotor attached to the crankshaft. The coils of wire into which we're going to induce the voltage are located in the coil, which in this case is stationary. So unlike the example above, the magnetic field rotates past the wire, rather than the other way around. But the effect is the same. Moving the magnets past the wire induces voltage in the wire.

"Terrific" you say. "We've created voltage in the coil by moving the magnets past it." But I'm afraid I have some bad news for you. First of all, the voltage you've generated is way too puny to jump across the spark plug gap. As a matter of fact, it takes Secondary winding about 5000 volts to make the spark jump across a .030 inch gap. And even if you could somehow muster enough voltage to fire the plug, you've got only the vaguest idea of when that energy is going to have enough oomph to make the jump and start the combustion process. No, you need to be able to take the relatively small voltage that you've induced in the coil wires and then tell it exactly when you want it to fire the sparkplug. And secondly, you've got to jack up the voltage so it has the muscle to light off the fuel/air mixture in the cylinder.

Actually, the second part of this project is the easier of the two. Your coil actually has two coils of wire in it. (See Figure 1) The first one, called the "primary winding" is the wire in which the magnetic field induces the voltage. It is not even connected to the sparkplug wire. The plug wire is connected to the "secondary winding" of the coil, and the secondary winding has a lot more coils of wire in it than the primary, more "turns" they call it. It basically works like a transformer. The more turns, the more voltage for a given voltage. Putting 12 volts through the primary winding that has 100 turns and a secondary winding that has 200 turns will give you 24 volts in the secondary winding. Only in your ignition coil, the ratio of turns from the primary to the secondary is a lot more than 2 to 1. That is how the small voltage that the magneto generates in the primary winding gets stepped up enough to make the sparkplug fire.

Like I said before, getting the voltage up enough to fire the plug is the easy part. Telling the coil just exactly when to unleash that energy is the tricky part. We'll be looking at the ignition systems used on the Briggs, the 100cc Yamaha, and the current crop of 2?cycle Reed engines. While each of these engines uses a slightly different technique to "trigger" the spark, they all rely on the same basic principle. As the magnets in the ignition rotor or flywheel approach the coil, the magnetic field begins to induce voltage in the primary winding. This voltage is grounded to the crankcase of the engine as a "dead short" circuit. The fact that the circuit is grounded, and therefore complete, and has the voltage potential we mentioned before, causes a current to flow. That ground may be through a wire coming out of the coil attached to the crankcase or it may be internal to the coil and not be visible. At some pre-determined point (remember, each engine type does this differently) this short circuit opens. That means the voltage can't ground out anymore. Instead, it "bounces" back through the coil, ti energizing the secondary winding, and that voltage grounds itself out by jumping the spark plug gap to ground on the engine. Think of it This way; if you've ever closed a water faucet real fast, you sometimes get a "shock wave" in the water pipes. They call it "water hammer" and it makes a noise like someone banging once on the water pipe. Sometimes it happens when the washing machine is running and one of its valves closes real fast. Anyway, the same thing can happen, more or less, to electrical current flowing through an inductor. If the current flowing to the ground in the primary circuit of the ignition system is suddenly stopped because the circuit is broken, the current "bounces" back. In electrical circles it's called "inductive fly-back" and it's what creates the voltage spike that the the coil then steps up to enough voltage to jump the sparkplug gap.

Certainly the location of the magnets in the flywheel, relative to the location of the coil affects when, relative to the position of the piston in the cylinder, the voltage rises and gets things going. But exactly when the primary circuit opens and initiates the process that actually fires the sparkplug involves several other, more sophisticated factors. Next we will look at those for each type of engine and why they are so important.

Remember, when the magnets in the rotor or flywheel on the crankshaft rotate past the windings of wire in the coil, they induce a voltage in those windings. But that little voltage doesn't have enough voltage to jump the gap on the sparkplug. We have to "muscle it up" to about 5000 volts or so to be able to consistently ignite the compressed fuel/air mixture in the cylinder. That is where the secondary winding, with a lot more "turns" of wire, comes in. Like a transformer, it boosts up the voltage to where we need it to get the job done.

Creating the potential and jacking up the voltage are the easy parts of this project. Getting the spark to happen at precisely the right moment, relative to the piston's position in the cylinder, is what makes all the difference. There are a variety of mechanisms to control when this happens. Remember, for a good part of the time that the magnets are moving past the coil and inducing voltage in the windings, that voltage is shorted out directly to the crankcase. But at some point, that short circuit has to be interrupted, causing the voltage to "bounce back" through the coil, energize the secondary winding, and fire the plug. In the older engines, like older Briggs', McCulloch 2?strokes, and the like, this job was handled mechanically. A set of "points", little spring-loaded contacts, actually opened and closed, actuated by a cam lobe located on the crank or the hub of the flywheel. The points stayed closed for most of the crankshaft rotation and the current flowed through them and grounded out to the crankcase. When the cam lobe came around and the points opened, the short circuit was broken and the sparkplug firing process began.

Today's engines, whether 2 cycle or 4 cycle, have moved past this rather primitive mechanical operation to electronic ignition. These solid-state systems are more durable and reliable than the old points systems, but more importantly, they give pin-point control of what engineers call the "spark event".

The Yamaha uses a special type of magneto system with the "trigger" mechanism housed in that little gold box that attaches to the outside of the engine. For as expensive as that X%*?@ little thing is (Yamaha calls it the TCI module), it is surprisingly simple inside. The primary current flows through the wire coming out of the coil, into the TCI module, and grounds through the case of the TCI. (See Figure 2) That is why it is so important to be sure you have a good connection between the TCI and the engine crankcase. Be sure it is mounted securely, either directly on the engine or to something connected to the engine. The TCI contains a small circuit board with a few resistors and capacitors, and a good-sized transistor. The transistor is the heart of the system and it, in effect, "measures" the amount of current flowing through it. As the rotor moves past the coil, the voltage builds, reaching a peak when the magnets are just about centered on the coil. Just before this peak, the transistor in the TCI reacts. Think of it as flipping an "electronic switch". The circuit opens and, with no place to go, the voltage bounces back. That voltage spike in the primary windings of the coil energizes the secondary windings and that high voltage then goes in search of someplace to go. The path of least resistance is across the sparkplug gap to ground on the cylinder head. Bingo! The plug fires and so does your engine. The only variable that affects when the spark occurs is when the rotor/coil combination generates the correct current to open the transistor in the TCI. There use to be a significant variance between the older TCI boxes, measured as current required to open the transistor. But, curiously, these differences did not translate into any measurable timing or performance differences. The newer TCI boxes, standard with Yamahas and PRDs, have virtually no variance in current value required. Since the tech rules dictate that the ignition timing key and rotor are fixed, this means that, regardless of the TO value, all Yamaha KT 100s have about the same ignition timing, measured in degrees of crank rotation before top dead center.

Again, the principle is pretty much the same. The rotating magnet on the ignition rotors moves its magnetic field through the field surrounding the fixed coil. That induces a potential voltage in the primary windings of the coil. That potential causes a low voltage current to flow from the coil via the primary wire, to the TCI, and through the TCI to ground against the engine. When the current reaches a pre-determined value engineered into the transistor in the TCI box, that transistor opens and the current suddenly stops flowing. "flyback" effect causes a voltage spike to occur in the primary winding and that energizes the secondary winding. With many many times more wire turns in the secondary than the primary, the voltage potential induced in the secondary winding is much higher. High enough, in fact, to jump the sparkplug gap to ground itself, causing the spark, which is what we really wanted all along.
The critical factors are:

  • Solid connections between the coil primary wire and the TCI box.

  • Solid connection between the TCI box and the engine casting.

No shorts or other interruptions to divert the current from the TCI box.
This last point is important because it is how you hook up a kill switch to the Yamaha, if you wish to.

A simple wire from the connection between the primary wire and the TCI, running to a switch does the trick. Just hook the other side of the switch to a wire that is grounded and, when the switch is flipped, the current from the primary wire will take the path of least resistance and bypass the TCI box and go through the switch to ground. Bingo! No current to the TCI box, no spark.

Most of the Reed Valve 2 cycles in use today, including Gearbox engines, using some variation of the CDI systems first developed for Italian kart engines in the 70s. CDI stands for "Capacitive Discharge Ignition. Once again we have a rotating magnet and a fixed coil or "stator". In contrast to the Yamaha system, however, the control module is housed in the stator with the primary windings, and the coil is external. Those primary windings called the Figure 3 "charge coil" in the stator are mirrored in the external "ignition" coil, where they share space with the secondary windings. But also in the stator is the "pickup" or "pulser" coil. (See Figure 3) So, as the magnets in the rotor move past the pickups in the stator, they induce voltage in the primary windings of the charge coil in the stator.

When the control circuitry in the stator, triggered by a signal from the pulser coil, disables the current's path to ground, that current flows through the wires connecting the stator to the coil where it flows through matching windings. These, in turn, power up the secondary windings and the voltage is discharged across the sparkplug gap. There are important differences to note between these systems, whether they be Selecta, ltalsystems, or whoever. Some also include an external Capacitive Discharge module in addition to the external ignition coil. But in each case the stator-to-coil connection consists of at least two wires; a primary and a ground. This means that you don't have to worry about mounting the coil on the engine itself. As long as the wires reach, you can mount the coil wherever it is convenient. Often this means mounting it in a location more protected or less subject to vibration. The second difference, and the one that can make an important performance difference, is that using three sets of windings means that the voltage can be "stepped up" a bit more than the two set configuration in the Yamaha ignition. That can yield a hotter spark and more efficient and complete combustion.

Next month we'll conclude this look at ignition systems by looking at the Briggs & Stratton "Magnetron" system and examining what adjustments and tuning features each different ignition system offers and what effect those adjustments can make. See you then.

Timing is Everything - Part 2


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