the electronic ignition system. Magneto systems are usually used in equipment where electricity is only needed to power the spark plug, not a starter system or lights. Larger garden tractors and similar equipment that have starter systems and lights usually use battery ignition systems. Electronic igni-tion systems contain electronic components that perform the switching action in the ignition circuit. Electronic ignition sys-tems are often found on newer engines.
As you read through the material on these ignition systems, remember that all three systems contain some of the same components. All three types of systems use an ignition coil (a transformer), for example. The magneto system and the battery system are very similar in function—they just use dif-ferent power sources. Either a battery system or a magneto system may use breaker points or an electronic switch to perform the triggering switch function. Finally, an electronic ignition system uses electronic components to perform the switching function, but its power source will still be either a battery or a magneto. Refer back to the basic ignition system illustrated in Figure 17 for reference if necessary.
The Magneto Ignition System
The magneto is frequently used in small engine applications to provide a voltage source for the ignition system. As we noted earlier, in a magneto, permanent magnets are installed in the engine’s flywheel or rotor. The ignition coil is mounted in a stationary position near the flywheel. When the flywheel spins, the magnets will induce a voltage in the primary wind-ing of the ignition coil.
The basic parts of a flywheel magneto are shown in Figure 31.
Note the position of the magnets and the coil in the drawing.
The flywheel magneto illustrated here contains breaker points
and a condenser underneath the flywheel. However, note that the coil in a flywheel magneto system may also be located on a mounting bracket at the side of the flywheel.
Figure 32 shows a photo of a typical flywheel used in a mag-neto system. The flywheel itself is cast from an aluminum alloy. During this casting process, two or more permanent magnets are encased within the aluminum. These magnets will pass the ignition coil as the flywheel rotates.
FIGURE 31—The basic parts of a flywheel mag-neto ignition system are shown here. In this system, the ignition coil, breaker points, and condenser are located underneath the flywheel.
However, in many sys-tems, the ignition coil is located on a mounting bracket at the side of the flywheel.
FIGURE 32—Shown here are the magnets that are cast into the flywheel.
The position of the magnets on the flywheel is very important.
In order for the magneto to work properly, the voltage must be generated in the primary winding of the ignition coil at the correct moment of the flywheel’s rotation. To generate the voltage at the exact time needed, the magnets in the flywheel must be properly aligned. This means that the flywheel must be located in exactly the proper position on the crankshaft.
The flywheel is held in position on the crankshaft by a small bar of soft metal called a flywheel key. The flywheel key is inserted into matching slots that are cut into the crankshaft and flywheel. Together, these slots are called the keyway.
The key physically holds the crankshaft and the flywheel in alignment. In modern engines, the flywheel is held to the crankshaft with a special key called a shear key. This type of key will break off (shear) if the flywheel becomes jammed. If an engine with a jammed flywheel were to continue to run, it would be damaged. So, the shearing action of the flywheel key disengages the flywheel from the crankshaft and stops the engine. A shear key is shown in Figure 33.
FIGURE 33—The flywheel and crankshaft are held in alignment by a flywheel key. In this illustration, the flywheel has already been lifted off the crank-shaft. The key is the small part being removed from the keyway in the crankshaft.
In order for the magneto system to work, the ignition coil must be mounted in a stationary position close to the fly-wheel. Figure 34 shows a photo of an ignition coil near a flywheel. (You can compare this photo to the simplified draw-ing of a similar assembly shown earlier in Figure 21.) Note that this particular type of ignition coil is seen only on older Briggs and Stratton engines. All newer engines that use a fly-wheel magneto ignition will contain an electronic ignition coil.
However, we’ve included this illustration to familiarize you with the older style of ignition coil.
Note the small air gap between the flywheel’s edge and the ignition coil. The air gap is an important specification in an ignition system. The engine manufacturer will determine the proper width of this gap (in thousandths of an inch). The gap must be this exact determined width in order for the magneto system to work properly. This is one of the specifications that must be checked when you’re servicing an ignition system.
FIGURE 34—This photo shows how an ignition coil is positioned near the flywheel in a magneto ignition system. Figure 21 showed a simplified draw-ing of a similar assembly.
Now, let’s take a closer look at the operation of a magneto system. Figure 35 shows a simplified drawing of a magneto system in operation. Note the position of the flywheel, the ignition coil, the breaker points, the condenser, the spark plug, and the spark plug wire. The breaker points system is located underneath the flywheel. As you can see, only the edge of the flywheel is visible; the rest is cut away so that you can see the breaker points assembly underneath.
Remember that the ignition coil is a transformer that contains two windings of conductor wire. In a typical transformer, the primary winding consists of about 150 turns of fairly heavy copper wire, and the secondary transformer consists of about 20,000 turns of very fine copper wire. Note that in order to keep Figure 35 simple, we haven’t shown the primary and secondary windings in detail. However, you can imagine them positioned inside the ignition coil.
The flywheel is turning in a clockwise rotation. Naturally, in real life, the flywheel would be turning at a high rate of speed.
However, for the purposes of our discussion here, imagine that the flywheel is turning in slow motion. As the flywheel
FIGURE 35—This is a sim-plified illustration of the operation of a magneto.
A permanent magnet is mounted near the edge of the flywheel. As the fly-wheel turns, the magnet passes near the ignition coil and induces a voltage in the primary winding.
turns, the permanent magnet mounted near the edge of the flywheel begins to pass by the ignition coil. As the magnet passes by the coil, magnetic lines of force from the permanent magnet move into the armature of the ignition coil. The mag-netic lines of force move from the north pole of the magnet through the armature and back out to the south pole of the magnet. The magnetic field induces a voltage in the primary winding of the transformer.
In Figure 36, the flywheel magnet has moved a little bit fur-ther in its rotation. When the flywheel magnet reaches this position, the magnetic lines of force from the magnet will sud-denly move through the armature in the opposite direction.
This happens because of the sudden change in the position of the north pole and the south pole of the magnet. The change in the direction of the magnetic lines of force will cause a cur-rent to flow in the primary winding of the transformer. The primary winding is connected to the breaker points. Since the breaker points are closed at this time, the current flows through the points.
FIGURE 36—In this illus-tration, the movement of the magnet causes the magnetic lines of force to change direction. As a result, a current flows in the primary winding.
The voltage in the primary winding will induce a low voltage in the secondary winding due to the action of mutual induc-tance. However, the voltage in the secondary is still too low to jump across the gap of the spark plug.
At this point, the turning cam lobe in the breaker points assembly turns and begins to open the points (Figure 37). As the points separate, the current flow in the primary circuit is broken. The magnetic field around the primary winding col-lapses through the secondary winding of the transformer. As this occurs, any current left in the primary circuit is absorbed into the condenser. The absorbing action of the condenser prevents the remaining voltage in the primary circuit from arcing across the breaker points.
FIGURE 37—As the breaker points open, the current flow in the pri-mary winding is broken.
Any remaining current in the primary is absorbed by the condenser. The magnetic field then collapses through the sec-ondary winding, inducing a high voltage in the sec-ondary. At the same time, the condenser discharges back into the primary winding. The high voltage in the secondary causes a current to flow through the spark plug wire and jump across the gap of the spark plug.
As the magnetic field collapses through the secondary wind-ing, a high voltage is induced in the secondary winding. At the exact same time, the electric charge absorbed and stored in the condenser flows back into the primary winding. This discharging action helps increase the voltage in the secondary circuit. The very high potential of the voltage induced in the secondary winding causes a current to flow through the spark plug wire and arc across the spark plug gap.
After the high voltage in the secondary winding is released as a spark, the flywheel continues to turn until the magnet positions itself by the ignition coil again, and the process repeats itself.
Note that the various actions we’ve described here occur very, very quickly in real time. The movement of the magnet on the flywheel, the reversal of the magnetic lines of force, the opening of the points, the collapse of the magnetic field, the charging and discharging of the condenser, and the fir-ing of the spark plug all occur in almost the same instant.
Remember that an engine may require as many as 1,800 sparks per minute in order to operate. You can imagine, then, how quickly the various components of the magneto system must react in order to produce so many sparks.