Different types of ignition systems use different types of switching devices. There are two basic types of trigger switch-ing devices used in small engine ignition systems. Some ignition systems use a set of electrical contacts called breaker points and a condenser to do the switching. Other systems use electronic components to perform the switching. Either way, however, the result on the ignition coil and the spark plug is the same.
Breaker points are mechanical contacts that are used to stop and start the flow of current through the ignition coil. The points are usually made of tungsten, a very hard metal that has a high resistance to heat. One breaker point is stationary (fixed), and the other point is movable. The movable contact is
mounted on a spring-loaded arm. The spring pressure is used to hold the points together. A simplified drawing of a set of breaker points is shown in Figure 22.
When the two breaker points touch, the ignition circuit is complete and the primary winding of the transformer is energized. Then, when the end of the spring-loaded movable breaker point is pressed, its contact end moves apart from the stationary breaker point. When the points move apart, the circuit opens and the flow of current stops. Each time the breaker points move apart, the spark plug fires. This action is shown in Figure 23. In Figure 23A, the points are closed. In Figure 23B, the points are open and the spark plug is firing.
The movable breaker point is moved to the open position by a turning lobe or a plunger. Remember that the camshaft in a four-stroke engine has lobes that lift the valves. Well, a simi-lar type of lobe is used in a breaker points system to press the movable breaker point into position. Depending on the engine design, this lobe may be located on the flywheel or on the end of the camshaft. Or, a plunger device that’s operated by the crankshaft or camshaft may move in and out to press on the movable point.
In any case, each time the lobe turns or the plunger extends, the device presses the movable breaker point away from the stationary point. As a result, the spark plug fires. The spring mounted under the movable point then returns the point to its original position. Thus, the cam or plunger is responsi-ble for the timing of the spark. The two different methods of moving the spring-loaded breaker point (lobe and plunger) are shown in Figure 24.
FIGURE 22—This simplified drawing shows a set of breaker points.
FIGURE 23—This figure illustrates the action of the breaker points in an ignition circuit. The points serve as a trigger-ing switch. In Figure 23A, the points are closed; the ignition circuit is com-plete and the primary winding of the ignition coil is energized. In Figure 23B, the points are open; the flow of current in the ignition coil stops and the spark plug fires.
Another important component of a breaker points system is the condenser. Remember that each time the breaker points touch, current flows through them. Unless this current flow is controlled in some way, a spark or arc will occur across the breaker points as they move apart. If this sparking was allowed to occur, the breaker points would burn up. They would also absorb most of the magnetic energy in the ignition coil and prevent it from producing a high voltage in its sec-ondary winding.
For these reasons, the condenser is used to control the cur-rent as it flows through the breaker points. As soon as the breaker points begin to separate, the condenser absorbs the current so that it can’t jump between the points and make a spark. When the spark plug fires, the condenser releases the current back into the primary circuit.
A condenser is actually a type of capacitor. A capacitor is an electrical component that can store an electrical charge.
So, when current is applied to the condenser, the condenser absorbs the current and stores it. Then, as the points open, the capacitor absorbs the electricity created by the collaps-ing magnetic field in the primary windcollaps-ing. Therefore, the condenser prevents the electricity from jumping the air gap between the opening points. When the magnetic field col-lapses in the secondary, the spark plug fires. At that same instant, the condenser releases its charge back into the primary winding.
The construction of a typical condenser is shown in Figure 25. A condenser is a cylinder-shaped component made of two aluminum foil strips wound together and separated by an insulating paper strip. One aluminum strip has an electrical
FIGURE 24—This figure illustrates the two dif-ferent devices that are used to move the breaker point. Figure 24A shows how a turning lobe is used to open the points, and Figure 24B shows how a plunger is used to open the points.
cylindrical metal case. A grounding connection is attached to the outside of the case. In an ignition circuit, a condenser is connected across or parallel to the breaker points.
The breaker points and condenser work together to form one switching device. The breaker points and condenser switching system is used in both magneto ignition systems and battery systems. An illustration of a real breaker points system is shown in Figure 26. Note the position of the points, the con-denser, the spring, and the plunger.
The other type of switching device used in small engine igni-tion systems is an electronic switch. In an electronic switch, solid-state electronic components are used to turn the cur-rent flow to the primary winding on and off. An electronic switch completely eliminates the need for breaker points and a condenser. We’ll discuss electronic ignition systems in more detail later in the study unit.