What does an ignition system do? Well, once the air-and-fuel mixture has been compressed in the combustion chamber of a small engine, the engine needs something to ignite the air and fuel. The engine’s ignition system performs this task. The ignition system produces a high voltage that’s used to cause a spark plug to fire. The spark from the spark plug is very hot, and this heat ignites the fuel and air mixture. The result-ing “explosion” in the combustion chamber forces the piston down and gets the crankshaft turning.
Remember the four stages of operation in a four-stroke
engine. During the intake stage, the piston moves down in the cylinder to take the air-and-fuel mixture in to the cylinder.
Then, the piston rises during the compression stage to com-press the air-and-fuel mixture in the combustion chamber.
When the piston reaches top dead center, the spark plug fires and ignites the compressed air-and-fuel mixture. The ignition of the air-and-fuel mixture forces the piston down in the cyl-inder, producing the power stage. The power produced by the ignition of the air-and-fuel mixture gets the crankshaft turn-ing, which in turn keeps the piston moving and the engine running. The ignition process will keep the engine running for as long as the fuel lasts and for as long as the spark plug keeps firing.
An ignition system must produce a very high voltage in order to force electric current (moving electrons) across the spark
spark. The spark must occur at exactly the right time in the engine cycle in order to ignite the fuel and air mixture prop-erly. Also, an engine requires many sparks per minute in order to keep running at the proper speed. For example, a single-cylinder four-stroke engine that’s operating at 3,600 rpm requires 1,800 ignition sparks per minute. (If an engine has more than one cylinder, multiply the number of cylinders times 1,800 to determine the number of needed sparks.) You can see that the ignition system has a very difficult job to do!
How does the ignition system produce a spark, time it per-fectly, and keep making sparks over and over again? Let’s find out. Figure 20 shows a simplified drawing of a basic ignition system. The main components of the system are the power source, the ignition coil, the spark plug, the spark plug wire, the triggering switch, and the stop switch. All ignition systems will contain these basic components.
First, let’s look at the ignition coil. All ignition systems con-tain an ignition coil. The coil is actually a type of transformer.
Remember that in the previous section of your text, we talked about the basic operation of transformers. A transformer con-sists of two wire windings wound around an iron core. The
FIGURE 20—A simplified drawing of the parts of an ignition system is shown here.
iron core is sometimes called an armature. The first wind-ing is called the primary windwind-ing, and the second windwind-ing is called the secondary winding. The secondary winding has many more turns of wire than the primary winding.
In the ignition coil, one end of the transformer’s primary winding is connected to a power source. Depending on the type of ignition system, the power source may be a battery or a magneto. Either type of power source will apply a voltage to the primary winding of the transformer. (We’ll discuss these power sources in more detail shortly.)
When a current passes through the primary winding, a magnetic field is created around the iron core. This magnetic field induces a voltage in the secondary winding (remember the concept of mutual inductance). If the current flow through the primary winding is stopped, the magnetic field collapses rapidly and produces a high voltage current in the secondary winding.
Because the secondary winding of the transformer has many more wire coils than the primary, the voltage produced in the secondary circuit is much higher than the original volt-age applied to the primary winding. In a typical small engine ignition system, the power source supplies about 12 volts to the primary winding of the ignition coil, and the ignition coil increases that voltage to 20,000 volts or more.
The secondary winding of the coil is connected to the spark plug wire. This is a heavily insulated wire that leads directly to the spark plug. When the magnetic field collapses and pro-duces the high voltage in the secondary, current runs directly to the spark plug and causes a spark to jump across the spark plug gap. The spark ignites the air-and-fuel mixture and the engine starts running.
Now, remember that the high voltage in the secondary wind-ing of the transformer is only produced when the current stops. This is a very important concept to understand. The current from the power source passes through the primary winding of the transformer, and when the current flow is stopped, the magnetic field collapses and a high voltage is produced in the secondary winding. This means that an igni-tion system needs some device that will keep turning the current from the power source on and off.
The device that turns the current on and off is a switch.
Look at Figure 20 again and note the position of the switch.
Remember the facts you learned earlier about open and closed circuits. The ignition system’s circuit is closed when the switch closes. So, when the switch closes, current flows from the power source to the transformer. When the switch opens, the circuit is opened and the flow of current immedi-ately stops. When the current stops, the magnetic field in the transformer collapses, producing the voltage needed to fire the spark plug.
Imagine that you’re standing near a light switch in your home, flipping the switch on and off. Each time you flip the switch on, the light comes on. When you flip the switch off, the light goes out. If you keep doing this, you’ll get an ON, OFF, ON, OFF pattern. This is very similar to the action of the switch in an ignition system. The switch is connected to one end of the primary winding. Each time the switch opens, the current flow to the primary winding stops and the spark plug fires. The spark plug keeps firing continually (about 1,800 times per minute) to keep the engine running.
Once an engine is started by an ignition system, the engine will keep running without stopping until it runs out of fuel.
Thus, if you want to stop the engine before that time, you’ll need to activate the stop switch. This switch may also be called the grounding switch or the kill switch. Different types of stop switches are found in different engines. In some engines, the stop switch will stop the flow of electricity to the spark plug. This type of switch will be a small metal lever con-nected near the spark plug. You simply push in the lever to stop the engine. In other engines, the stop switch is designed to prevent the flow of electricity through the primary windings of the coil. This type of stop switch will be connected to the throttle. When you move the throttle into the STOP position, the engine will stop.
The basic operation of small engine ignition systems is quite straight-forward. Although the explanation we’ve provided here is simplified, it gives you a basic idea of how all ignition systems work.
Now, to build on this basic foundation of knowledge, we’ll look at the components that vary in different ignition systems.
These components, as we’ve already mentioned, are power sources and switching devices. Let’s start with power sources.