Small Engine Ignition Systems

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Study Unit

Small Engine Ignition

Systems

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In your previous study units, you’ve learned about the com-ponents of an engine and how they affect engine operation. You’ve also learned about lubrication and cooling systems. This text will show you how an engine’s ignition system oper-ates. An ignition system generates a high voltage that’s used to cause a spark plug to fire. The spark from the spark plug ignites the air-and-fuel mixture and gets the engine running.

When you complete this study unit, you’ll be able to

n Explain the difference between voltage, current, and

resistance in a circuit

n Describe how a spark plug is constructed and how it

operates

n Identify the components of magneto, battery, and

electronic ignition systems

n Explain the basic operation of these different ignition

systems

n Explain how safety interlocks are incorporated into the

ignition system

n Describe the procedures involved in an ignition system

tuneup

n List the steps used in troubleshooting ignition systems

Remember to regularly check your student portal on your student homepage. Your instructor may post additional resources that you can access to enhance your learning experience.

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In this study unit, you’ll be learning about the different types of ignition systems that are used to start and run pieces of power equipment. We already looked at ignition systems briefly in an earlier study unit; however, in this study unit we’ll discuss the operation of these systems in detail. Later in the study unit, we’ll cover the maintenance and repair proce-dures used with ignition systems.

To start our discussion, we’ll begin with a review of some basic concepts about electricity and circuits. Note that you don’t need to be an electrician to work on the ignition systems that are used in outdoor power equipment. However, a basic knowledge of some electrical principles will make these sys-tems easier to understand and troubleshoot.

BASIC ELECTRICAL CONCEPTS

A Simple Circuit

In order to work effectively on ignition systems, you’ll need to know how electricity is generated, distributed, used, and controlled. Let’s start the learning process by looking at a simple circuit.A circuit is defined as a complete electrical path. A typical circuit includes a power source, conductors, a load, and a switch. A power source is simply a source of electri-cal power. The power source in a common household circuit

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cordless appliance circuit is a battery. The conductors are the wires that carry the electricity. The load is a device, such as a light or an appliance, that we want to run with electricity. The

switch is the device used to turn the circuit on and off.

Circuits may be closed or open. In a closed circuit, when the switch is turned on, electrical power from the power source flows through an unbroken path to the load, flows through the load, and then returns back to the power source. A closed circuit is complete—the power flows through the entire cir-cuit path to reach the load. In contrast, in an open circir-cuit, the switch is turned off. When the switch is turned off, the path of the circuit is broken and the power can’t reach the load.

A simple flashlight circuit is shown in Figure 1. The power source in this circuit is a battery. The conductors are cop-per wire. The load is a standard light bulb. In Figure 1A, the switch is open (turned to the OFF position). The electrical cir-cuit is therefore open, and power can’t flow through the wires to reach the bulb. In Figure 1B, the switch is closed (turned to the ON position). The circuit is therefore complete, and elec-tricity can flow through the wires to reach the bulb and turn it on.

FIGURE 1—This figure illustrates a simple electrical circuit. In Figure 1A, the switch is open, so electric-ity can’t flow to the light bulb. In Figure 1B, the switch is closed, allowing electricelectric-ity to reach the light bulb and light it

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Now that you understand what a basic circuit is, let’s take a closer look at electricity itself. What exactly is electricity? Electricity is a natural force produced by the movement of electrons. Electrons are tiny atomic particles that have a nega-tive electrical charge. In the circuit shown in Figure 1, moving electrons come from the battery. The battery produces a flow of electrons that moves through the wires to light the flash-light bulb.

Note that the battery has two different ends. The end of the battery that’s labeled with a negative or minus sign (2) is called the negative terminal. The opposite end of the battery that’s labeled with a positive or plus sign (1) is called the

positive terminal. The negative terminal of the battery has a negative charge—that is, it contains too many electrons. The

positive terminal of the battery has a positive charge—it con-tains too few electrons.

The negative and positive charges in a battery are produced by a simple chemical reaction. Figure 2 shows a simplified diagram of the parts of a battery. The battery contains a chemical solution called

electrolyte. The battery

ter-minals or electrodes are two strips of metal. Each electrode is made from a different type of metal. When the strips of metal are placed into the electrolyte solution, a chemical reaction occurs. As a result of this reaction, a negative charge forms on one electrode and a positive charge forms on the other electrode.

You’ve probably often heard the phrase “oppo-sites attract.” Well, this is definitely true in the world of electricity. Opposite electrical charges (positive and negative) attract each

FIGURE 2—In a simple battery, a chemical reaction takes place between the electrodes and the electrolyte solution. This chemical

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balance each other out. Because of this attraction, whenever lots of electrons are concentrated in one place, the electrons will try to move to a place where there are fewer electrons. This is the basic operating principle of a battery. The negative terminal of a battery has a high concentration of electrons, while the positive terminal has very few electrons. So, the electrons at the negative battery terminal will be strongly drawn toward the positive battery terminal. However, in order to actually move from the negative terminal to the positive terminal, the electrons need a path to follow. We can create a path for the electrons by connecting a wire between the bat-tery terminals.

Therefore, if we attach the two ends of a piece of wire to the two battery terminals, we create a path for the electrons to fol-low between the terminals. By attaching the wires, we actually build a circuit. In order to use the electrons to perform useful work, we can connect a light bulb to the circuit. We can also connect a switch to our circuit so that we can turn the circuit on and off.

When we turn on the switch, the circuit is closed, and the electrons from the negative battery terminal will move to the positive battery terminal. As the electrons flow through the light bulb, they cause the bulb’s filament to heat up and glow, producing visible light. The flow of electrons through a circuit is called electric current.

A simple circuit is shown in Figure 2. Electrons flow from the negative battery terminal to the positive terminal through the conductors that are attached to them. Note that the flow of electricity produced by the battery will continue as long as the chemical reaction in the battery keeps up. After some time, the chemical reaction in the battery will stop and the battery will stop functioning. At that point the battery will need to be recharged or replaced.

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Atoms and Electrons

You’ve just learned that electrons are atomic particles. What exactly does this mean? To answer that question, we’ll need to look at the structure of an atom in a little more detail.

All matter in the universe is formed from about one hundred or so different substances called elements. Each different ele-ment, such as hydrogen, gold, or uranium is made up of its own unique hydrogen, gold, or uranium atoms. An atom is the smallest particle of an element that still keeps the proper-ties of the element.

All atoms are made up of tiny atomic particles called protons, neutrons, and electrons. The electron is a very lightweight par-ticle that has a negative electrical charge. Protons are much heavier than electrons and have a positive electrical charge.

Neutrons have no electrical charge at all—they’re neutral.

Electrons are the smallest type of atomic particle; one electron is much smaller than the atom as a whole.

Figure 3 shows a drawing of a hydrogen atom, which is the simplest atom known. (The element hydrogen is a gas that’s found in the atmosphere.) A hydrogen atom contains one elec-tron and one proton. The proton is located at the nucleus (the center) of the atom. The electron orbits around the nucleus in a circle, just like the moon orbits around the earth. All atoms are constructed in this same general way, but the number of elec-trons, protons, and neutrons varies in each different element.

FIGURE 3—This single atom of hydrogen contains one proton and one electron. The proton is represented by the circle with the plus sign (+). The electron is represented by the circle with the minus sign (–).

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The hydrogen atom contains one positively charged proton and one negatively charged electron. The positive charge of the proton and the negative charge of the electron bal-ance each other out. Thus, as a whole, the hydrogen atom is perfectly balanced electrically. Because opposite electrical charges attract each other, the electron in a hydrogen atom is very strongly attracted to the proton and is attached tightly to it. The electron can’t be easily removed from the atom.

Now, in comparison, let’s look at the copper atom shown in Figure 4. (The element copper is a metal.) The copper atom contains 29 electrons and 29 protons. The electrons orbit the nucleus of the copper atom in several layers called shells. The outermost shell contains only one electron; this electron is called a free electron. Since the free electron is alone and very far away from the atom’s nucleus, it’s not strongly attached to the nucleus like the hydrogen’s electron was. For this rea-son, the free electron in a copper atom can easily be dislodged from its orbit.

In general, protons and neutrons can’t be easily removed from an atom. However, in some atoms, electrons can be easily removed from their orbits. You already know that electric cur-rent is produced by the movement of electrons. Well, in order to get the electrons moving, we have to remove them from atoms.

FIGURE 4—This copper atom contains a single electron in its outermost orbit. This free electron can easily be dis-lodged from its orbit, which makes copper a good conduc-tor of electricity.

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The idea of removing electrons from an atom may seem strange and impossible. However, we remove electrons from atoms all the time without realizing it. For example, if you shuffle across a carpet and then touch a metal surface, what will happen? You’ll probably receive a small shock, and maybe even see a spark. This happens because as you scuffed your shoes along the carpet, you actually rubbed free electrons off the carpet. Your body held onto these electrons, and you became negatively charged. When you touched the metal sur-face, the free electrons from your body jumped over to the metal, restoring your body to a neutral charge. The discharge of electrons caused the small spark that you felt.

Thus, you can see that it’s not impossible to get electrons moving from one place to another. However, it’s easier to get electrons moving in some materials than in others. The structure of an individual atom will determine how easily an electron can be removed from it. For example, you saw that the structure of the hydrogen atom makes it very difficult to remove an electron from its orbit. So, it’s very difficult to pro-duce a flow of electricity in hydrogen. However, in a copper atom, the outermost electron can easily be dislodged from its orbit. Therefore, it’s very easy to get a flow of electricity mov-ing in copper. This is why copper is used to make electrical wires and cables.

Any substance in which electrons can move freely is called an electrical conductor. Copper, silver, gold, and other metals are good electrical conductors. (In fact, silver and gold are better electrical conductors than copper, but because silver and gold are so expensive, they aren’t used to make electrical wires.) Materials in which the electrons are very tightly bonded to the nucleus are called insulators. Plastic, nylon, ceramic, and other such materials are very resistant to the flow of electric-ity and are classified as insulators.

Now, let’s see how electrons flow within an electrical cir-cuit. Figure 5 shows a simple circuit in which a copper wire is attached to a battery. One section of the copper wire is enlarged so that you can see how electrons would flow through the wire.

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In the figure, the circuit is closed, and the electrons from the negative battery terminal are drawn to the positive terminal. Remember that the outermost electron in each copper atom is easily dislodged from its orbit. The flow of current starts at the negative battery terminal. An electron is drawn from the negative battery terminal into the copper conductor wire. This electron then collides with a free electron in a copper atom, bumping the copper electron and taking its place. The displaced copper atom moves to a neighboring copper atom, bumps another free electron out of orbit, and takes its place. As this “chain reaction” continues, each free electron bumps its neighbor out of orbit and takes its place. (When we refer to the electrons bumping each other, you might think of the balls on a billiards table. One ball strikes another, causing it to move.) This chain reaction of moving electrons is electric current. In reality, of course, atoms are much too small to see, so we can’t follow the movement of just one electron through a wire. Many millions of copper atoms make up a wire. When a cir-cuit is closed, millions of electrons move through the wire at the same time, and at a very high rate of speed. The more electrons moving through a circuit, the higher the current in the circuit.

FIGURE 5—In this simple circuit, a section of the conductor wire has been enlarged so that you can see how electrons would flow through the wire. A free electron from the battery enters the wire. As the battery electron enters the wire, it dis-places free electrons from the copper atoms in the wire, creating a “chain reaction” of moving electrons.

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Current, Voltage, and Resistance

Electrical and electronic circuits have three basic quantities associated with them: current, voltage, and resistance. These quantities have a very important relationship in a circuit. As you’ve already learned, current is the flow of electrons through a conductor. When a complete conducting path is present between two opposing electrical charges, electrons will begin to flow between the two points. Current is measured in units called amperes or amps. The abbreviation for amperes is the letter A. So, the quantity “3 amperes” would be abbreviated “3 A.” In electrical drawings, diagrams, and mathematical for-mulas, current is usually represented by the letter I.

Small amounts of current may be noted with the abbre-viations mA (milliamperes) or mA (microamperes). One

milliampere of current is equal to one one-thousandth of an

ampere, or 0.001 A of current. One microampere of current is equal to one-millionth of an ampere, or 0.000001 A of cur-rent. The following table shows you how to convert between these different values.

Table

ELECTRICAL QUANTITIES

Unit Abbreviation Value Ampere A 1 ampere Milliampere mA 0.001 ampere Microampere µA 0.000001 ampere Volt V 1 volt Megavolt MV 1,000,000 volts Kilovolt kV 1,000 volts Millivolt mV 0.001 volt Microvolt µV 0.000001 volt Ohm Ω 1 ohm Megohm MΩ 1,000,000 ohms Kilohm kΩ 1,000 ohms

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Now, let’s look at the electrical quantity of voltage. Remember that in a battery, one terminal has a negative charge and the other terminal has a positive charge. Whenever a pos-itive charge and a negative charge are positioned close to each other in this way, a force is produced between the two charges. This force is called electrical potential. Electrical potential is simply the difference in electrical charge between the two opposing terminals. The bigger the difference between the two opposing charges, the greater the electrical potential will be.

Voltage is a measure of the amount of electrical potential in a

circuit. Voltage is measured in units called volts. The abbrevi-ation for volts is the letter V. So, the quantity “2 volts” would be abbreviated as “2 V.” In electrical diagrams and mathemat-ical formulas, voltage is usually represented by the letter E. The last electrical quantity we’ll look at is called resistance.

Resistance is a force of opposition that works against the flow

of electrical current in a circuit. You’ve already seen that cur-rent flows easily through copper wires in a circuit. However, frayed wires, corroded connections, and other such obstruc-tions will slow down the movement of electrons through a circuit. That is, the circuit will resist the flow of current

Conversion Examples

To convert megohms to ohms, multiply the number of megohms by 1,000,000.

To convert kilohms to ohms, multiply the number of kilohms by 1,000. To convert ohms to megohms, divide the number of ohms by

1,000,000.

To convert ohms to kilohms, divide the number of ohms by 1,000. To convert microamperes to amperes, divide the number of microam-peres by 1,000,000.

To convert milliamperes to amperes, divide the number of milliamperes by 1,000.

To convert amperes to microamperes, multiply the number of amperes by 1,000.000.

To convert amperes to milliamperes, multiply the number of amperes by 1,000.

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through it. When a lot of resistance is present in a circuit, a higher voltage is needed to get the flow of electrons moving through the circuit.

Resistance is measured in units called ohms. The abbreviation for ohms is the Greek letter omega, represented by the symbol Ω. Resistance is usually represented by the letter R in electri-cal diagrams and mathematielectri-cal formulas.

Standard abbreviations are used to describe large values of resistance. The value 10,000 ohms, for example, may be noted as either 10 kΩ or 10 kilohms. The prefixes k and kil stand for kilo (one thousand). The value 20 million ohms may be noted as 20 MΩ or 20 megohms. The prefixes M and meg stand for mega (one million).

Engine service manuals often provide electrical specifications in ohms. For example, if you were measuring the amount of resistance in an ignition module’s pins, the service manual would tell you the correct value you should measure in ohms. The service manual may tell you, for example, that the resis-tance you should measure between Pin 1 and Pin 3 on an electronic control module should be 300 Ω. (Note that we’ll discuss ignition components, specifications, and how to mea-sure circuit quantities in more detail later.)

In order to better understand the relationship of current, volt-age, and resistance in a circuit, we can compare an electrical circuit to a simple water system. Electric circuits and water distribution systems have many of the same properties. In Figure 6, a simple electrical circuit is compared to a water circuit. The water pipes form a path for the water to follow, so the pipes are like the conductors in the electrical system. The water valve turns the flow of water on and off, so the valve is like the switch in the electrical system. The waterwheel is being operated by the flow of water, so the wheel compares to the light bulb (the load) in the electrical circuit. The water res-ervoir (the water source) can be compared to the battery (the power source) in the electrical circuit. The flow of water can be compared to the flow of electrons. The water pump pushes the water into the pipes, so the pump can be compared to the voltage or potential in the electrical circuit.

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In Figure 6A, both the water circuit and the electrical circuit are turned off. Both the water valve and the electric switch are in the off position, so no water or current flows. The water-wheel doesn’t turn and the light bulb doesn’t light up.

In Figure 6B, the water valve is turned on. Water is pumped out of the reservoir and into the pipes; the water flows through the pipes, turns the waterwheel, and then returns to the reservoir. In the electrical system, the switch is also turned on. Electric current flows out of the battery through the wires, lights the bulb, and returns to the battery.

FIGURE 6—Basic electrical principles can be visualized easily when you compare an electrical circuit to a water system.

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In this example, you can think of resistance as being like a blockage or a clog in the water pipe. If some debris was stuck in the pipe, the flow of water through the pipe would be reduced. In a similar way, a resistor in an electrical circuit reduces the flow of current through the circuit.

DC and AC Voltage and Current

There are two different types of current that you should be aware of. Direct current (DC) is the flow of electrons in one direction only. A DC voltage is nonvarying and is usually pro-duced by a battery or a DC power supply unit. If we were to graph a DC voltage of 9 volts or 12 volts over a period of time, the graphs would appear as shown in Figure 7. Whatever the voltage value, a DC voltage remains constant and unchanging over time.

In contrast, alternating current (AC) is the flow of electrons first in one direction, and then in the opposite direction. Alternating current reverses direction continually and is produced by an AC voltage source. Alternating current is the type of current found in household electrical systems and wall outlets. A graph of an alternating current over time is shown in Figure 8. The current starts at zero, then rises to a max-imum positive value. At the maxmax-imum positive point, the

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continues to drop until it reaches the maximum negative value. The current then reverses direction again and rises back to zero. One complete transition of the current from zero to the positive peak, down to the negative peak, and back up to zero is called a cycle. These alternating current cycles repeat continuously for as long as the current flows.

Small engines in lawn mowers, snow blowers, garden tillers, and other such equipment that don’t contain a battery may use AC voltages and currents for starting and operation. Larger machines that contain batteries will use the DC voltage produced by the battery to power the starter, lights, horn, and other accessories. These machines will also use AC voltages for the ignition and charging circuits.

Ohm’s Law

The values of resistance, current, and voltage have a very important relationship in a circuit. A resistance of one ohm (1 Ω) permits a current flow of one ampere (1 A) of current in a circuit that has a source voltage of one volt (1 V). This rela-tionship is summarized by Ohm’s law and is expressed with the following mathematical formula:

E 5 I 3 R

In this formula, the variable E stands for voltage in volts, the variable I stands for current in amperes, and the variable R

FIGURE 8—The voltage level of an alternating current (AC) changes con-stantly over time.

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Two useful variations of the Ohm’s law formula are the following:

I 5 E 4 R R 5 E 4 I

Ohm’s law is a very useful formula that you should know. The Ohm’s law formula is frequently used to analyze circuits and troubleshoot problem areas. By using these three given vari-ations of the Ohm’s law formula, it’s easy to find the proper voltage, resistance, and current values for a circuit. Any time you know two of the three circuit values (voltage, current, or resistance) you can calculate the third, unknown circuit value by using the Ohm’s law formula.

Note that as the resistance in a circuit increases, the cur-rent will decrease. If the resistance in a circuit decreases, the current will increase. All circuits are designed to carry a particular amount of current. In fact, many circuits are pro-tected by fuses that are rated in an amperage value that’s just slightly higher than the current value of the circuit. If a problem develops in a circuit, the circuit will draw too much current from the battery and the fuse’s elements will melt (the fuse will blow), opening the circuit.

Measuring Electrical Quantities

There are several testing tools that technicians use to mea-sure circuit quantities. The most common testing instrument is the multimeter or voltohm-milliammeter (VOM). This one instrument enables you to measure voltage, current, and resistance. The multimeter is a box-like device that has two wire test leads connected to it. The ends of the wire leads hold

probes that are used to make the actual circuit tests. A dial

on the front of the multimeter is used to select the quantity you want to measure. The multimeter also has a display face where it displays the circuit information it reads. Depending on the type of multimeter, the display may be a moving metal needle or a digital display.

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To operate a multimeter, you would take the following basic steps:

Step 1: Select the quantity you want to measure by turning the dial.

Step 2: Take the two test leads in your hands and touch the probes to two points in a circuit.

Step 3: Read the resulting information on the meter’s display. Note that this is just a very basic description of the opera-tion of a multimeter. The actual operaopera-tion of a multimeter is somewhat more involved, and electrical safety precautions must be observed. You could destroy a multimeter if you use it improperly; more importantly, you could receive a serious electrical shock. (We’ll discuss how to use and operate a VOM in detail in a later study unit. These instruments are used to test the electrical systems found in certain types of outdoor power equipment. However, you won’t usually need to use a VOM to perform any tests on a small engine ignition system, which is why we won’t talk about it at this time.)

Note that when a multimeter is set to read resistance, it’s sometimes called an ohmmeter. When it’s set to measure volt-age, it’s called a voltmeter. When it’s set to measure current, it’s called an ammeter. You may see these different terms from time to time.

Electromagnetism

Now, let’s take a look at electromagnetism. This concept is very important to the operation of ignition systems.

Electromagnetism is the magnetic effect produced when

electric current flows through a conductor. When a con-ductor wire is carrying an electric current, the wire will be surrounded by a magnetic field. A magnetic field is the space around a magnet or magnetic object that contains a force of attraction. This force of attraction is sometimes called

mag-netic lines of force or magmag-netic flux. The magmag-netic field is

strongest in the space immediately surrounding the conduc-tor. The force of electromagnetism has many interesting and highly useful practical applications.

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If an insulated piece of conductor wire is looped around to form a coil, the resulting device is called a magnetic coil (Figure 9). When current flows through a magnetic coil, each separate loop of wire develops its own small magnetic field. The small magnetic fields around each separate loop of wire then combine to form a larger and stronger magnetic field around the entire coil. The coil develops a north pole and a south pole. The magnetic field at the center of a magnetic coil is stronger than the fields above or below the coil.

An electromagnet is a device that’s made by inserting a piece of magnetic material (usually iron or soft steel) into a magnetic coil (Figure 10). The piece of metal around which the conduc-tor is coiled is called the core. When current is applied to the coil, the core becomes magnetized and develops a north and south pole. The addition of the metal core to the coil increases the magnetic force of the coil. So, an electromagnet is generally much stronger than a magnetic coil of a similar size.

FIGURE 9—This figure shows a basic magnetic coil and the magnetic lines of force that surround it.

FIGURE 10—This figure shows the construction of a basic electromagnet. A piece of magnetic material is inserted into a magnetic coil.

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Some electromagnets have special movable cores. This type of electromagnet is called a solenoid. Inside the solenoid coil, the core is a movable round metal piece called a plunger. In most cases, when a solenoid coil is energized by a flow of current, the resulting magnetic field pulls the plunger into the coil. When the flow of current stops, a spring above the plunger presses the plunger back into its original position. Solenoids are sometimes used in the electrical starter systems of garden tractors and riding mowers, so you should be familiar with them. Figure 11 shows a view of a basic solenoid and plunger.

Electromagnetism in Generators

Now, let’s look at another important electromagnetic property. Remember that when current flows through a conductor, a magnetic field is produced around the conductor. Well, it’s also true that if a conductor is moved through a magnetic field, a volt-age will be produced on the conductor. If this conductor wire is connected in a complete circuit, current will flow through the conductor wire. This effect is called the generator action of

mag-netic induction. (Note that current won’t flow through the wire

until the wire is connected in a complete circuit.)

FIGURE 11—The mag-netic field of a solenoid coil produces a force on the iron plunger, pulling it into the center of the coil. Solenoids are often used to control valves and switches in electrical systems.

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The generator action of electromagnetic induction is illus-trated in Figure 12. In this figure, a conductor wire is moved between the north and south poles of two magnets. Note that the magnetic lines of force are moving from right to left. When the conductor is moved upward through the magnetic field, a voltage will be induced on the conductor, and current will flow through the wire in the direction indicated by the arrows. Note however, that the current will only flow if the moving conductor is part of a closed circuit.

The generator action of electromagnetic induction is the basic property that’s used to operate electric generators. In an elec-tric generator, a component called an armature is turned in a magnetic field to produce an electric current. The armature is made up of many loops of conductor wire. Magnets are posi-tioned on both sides of the inside of the generator to produce a magnetic field. As the armature is turned within the mag-netic field, an electric current is produced. Note that in some generators, the armature is turned with a manually operated handle or crank. Other generators, however, use other inter-nal components to turn the armature. A very basic illustration of the parts of an electric generator is shown in Figure 13.

FIGURE 12—In this figure, the conductor is being moved upward through the magnetic lines of force. The generator action of electromagnetic induction produces a current that flows through the con-ductor in the direction shown by the arrows. Note that the galvanometer is an instru-ment that measures electric current.

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The voltage and current produced by the simple genera-tor shown in Figure 13 would be quite low. However, if we wound many loops of wire into a coil and turned the coil in the magnetic field, a much larger voltage and current would be produced. This is the arrangement in a real generator. The amount of voltage and current produced by a generator is based on three things:

1. The number of turns in the coil and the diameter of the wire

2. The strength of the magnetic field

3. The speed at which the wire coil passes by the magnets Many small engines use the generator action of electromag-netic induction to power their ignition systems. For example, in some small engine ignition systems, coils are placed under-neath the flywheel or outside next to the edge of the flywheel. Magnets are embedded in the edge of the flywheel. Then, as the flywheel spins, the magnets pass by the coils and generate the necessary voltage and current to operate the ignition sys-tem. We’ll discuss these systems in more detail later in this study unit.

FIGURE 13—This illustra-tion shows a very simple generator. As the coil of wire is rotated in the magnetic field, an electric current is generated.

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Some larger pieces of outdoor power equipment that contain batteries (such as garden tractors and riding mowers) use the generator action of electromagnetic induction to charge the batteries. In such machines, beltdriven generators or alterna-tors charge the batteries, and the energy from the batteries is then used to power the machines’ ignition systems. We’ll dis-cuss these systems in more detail later in another study unit.

Electromagnetism in Motors

You’ve just learned that when a conductor moves through a magnetic field, a voltage will be produced in the conductor. Now, suppose that a current-carrying conductor is placed in a magnetic field. What happens? Well, the interaction between the magnetic field and the moving electrons in the conductor causes a physical force to be applied to the conductor. If the conductor is free to move, this physical force will cause the conductor to move for as long as the conductor current and the magnetic field are maintained. This property is called the

motor action of electromagnetic induction.

The motor action of electromagnetic induction is shown in Figure 14. In the figure, a conductor is connected to a battery to form a complete circuit. Current is already flowing in the conductor when it’s placed in the magnetic field between the two magnets. The reaction between the magnetic field and the moving electrons in the conductor causes the conductor to move upward as shown by the arrow in the figure.

FIGURE 14—Because the current-carrying conductor has been placed in a magnetic field, the motor action of elec-tromagnetic induction causes the conductor to move.

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The motor action of electromagnetic induction is the basic property that’s used to operate electric motors. A very simple illustration of the parts of an electric motor is shown in Figure 15. In a motor, the armature is a rotating component that’s mounted on a shaft and positioned between the motor’s field magnets. Loops of conductor wire called armature windings are connected to the armature’s commutator. Note that for simplicity, only one winding is shown in the figure.

The brushes are electrical contacts that slide over the surface of the commutator as the armature rotates. The brushes are connected to an electrical power source outside the motor (usually a battery). Electrical wires called field windings are wound around the field magnets. When current flows into these wires, the field magnets become electromagnets and produce a powerful magnetic field inside the motor. When current is applied to the brushes, the current moves through the brushes and into the commutator and armature windings. The current flowing through the armature windings produces magnetic fields around the windings.

The interaction of all these powerful magnetic forces causes the armature to spin. The output shaft of the armature will be connected outside the motor to a machine or load to perform useful work.

FIGURE 15—When the field windings are ener-gized, the field magnets produce a powerful mag-netic field in the motor. When current runs through the armature windings, magnetic fields are produced around the windings. The interaction of these magnetic fields causes the armature to spin in a clockwise direction.

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Some larger lawn tractors contain small electric motors in their starter systems. The output shaft of the electric motor in such a system would generally be connected to gears that engage the flywheel. The spinning motion of the electric motor’s armature would be transferred through these gears to the flywheel and the crankshaft of the gasoline engine. (Note: Some people may use the word “motor” when talking about either the electric starter motor or the gasoline engine. Don’t confuse the starter motor with the gasoline engine!)

Mutual Inductance

The final electromagnetic property we’ll look at is called mutual inductance. If two conductors are placed close together, and current is applied to one of the conductors, a voltage will be induced in the other conductor. That is, because the two conductors are physically close to each other, the energy in the “live” conductor will stimulate the other con-ductor to become energized, too. This effect is called mutual

inductance, and it can be used to operate transformers. Note

that if the conductors are moved apart from each other, the effect of mutual inductance will become less strong. If the conductors are moved very far apart, the energy of the “live” conductor won’t be strong enough to influence the second conductor, and the mutual inductance effect will stop.

A basic transformer is shown in Figure 16. The transformer is a device that consists of two windings of wire wound around an iron core. The first winding is called the primary winding and the second winding is called the secondary winding. In the figure, the primary

winding is connected to a battery through a switch and a resis-tor; a voltmeter is connected across the secondary winding.

FIGURE 16—A basic transformer is shown here. A change in voltage in the primary winding induces a voltage in the secondary winding.

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When the switch is open (as shown), no current flows through the primary winding; thus, no magnetic field is produced, and no voltage is induced on the secondary winding. However, when the switch is closed, current flows through the primary winding, producing a magnetic field around the primary wind-ing. The magnetic field spreads outward and cuts across the secondary winding, inducing a voltage on the secondary wind-ing. The voltage will register on the voltmeter attached to the secondary winding.

Later in the study unit, we’ll show you how the principle of mutual inductance is used to help operate a small engine ignition system.

Basic Electronic Devices

Now that you have a good basic understanding of electrical and electromagnetic principles, let’s take a brief look at some elec-tronic devices. Elecelec-tronic devices are components that are used to control the flow of electrons in a circuit. Many different elec-tronic components are used in circuits, but we’ll just look at those that are used in ignition systems. These devices are the diode, the silicon-controlled rectifier (SCR), and the transistor. Let’s start by defining a few terms. You’ll remember that a conductor is a material that allows electrical current to flow through it easily. Copper is an example of a conductor. An insulator is a material that resists the flow of electricity through it. Porcelain, plastic, and nylon are insulators. In contrast, electronic devices are made from materials called semiconductors. A semiconductor is an element or a com-pound (a combination of elements) that conducts electricity “part-time.” That is, sometimes a semiconductor acts like a conductor, and sometimes it acts like an insulator. Silicon, germanium, and selenium are common semiconductor mate-rials that are used to make electronic components.

Semiconductor devices are manufactured in laboratories under very special conditions. The semiconductor materi-als are specially processed and combined to form electronic devices such as diodes and transistors. Because of the way the semiconductor materials are processed during manu-facturing, the finished diodes and transistors are capable of

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controlling the flow of electrons. So, as a result of these spe-cial manufacturing processes, the conducting and insulating properties of semiconductor materials can be used to perform useful work in a circuit.

Now, let’s look at the diode. A diode is a simple electronic device that has two terminals called the anode and the

cath-ode. The body of the diode is shaped like a small cylinder, and

the terminals are thin wires that protrude from the ends of the cylinder. Figure 17A illustrates the parts of a diode, and Figure 17B is the electrical symbol for a diode. This symbol is used on circuit diagrams to represent a diode.

When a voltage is applied to the cathode end of the diode, electric current will move through the diode and come out at the anode end. In this situation, the diode acts like a

con-ductor. However, if current is applied to the anode end of the

diode, the diode will completely resist the flow of current. No current will flow through the diode. So, in this situation, the diode acts like an insulator. Thus, diodes will allow current to flow through them only in one direction. If a diode is connected into a circuit, the diode will keep current flowing in just one direction. For this reason, the diode is sometimes called a “one-way street” in a circuit.

A transistor is another type of electronic device that’s

sometimes used in ignition systems. A transistor is a semicon-ductor device that has three wire terminals. These terminals are called the base, the collector, and the emitter. Transistors

FIGURE 17—Figure 17A shows the parts of a diode, and Figure 17B is the electrical symbol for a diode.

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are used to control the flow of current in a circuit. Figure 18A illustrates the package of a transistor, and Figure 18B shows the electrical symbol for a transistor.

A silicon-controlled rectifier (SCR) is another type of semicon-ductor component. The SCR has three terminals called the

anode, the cathode, and the gate. Note that the construction

of an SCR is similar to that of a diode, except that the SCR has an additional terminal called a gate. Figure 19A illus-trates the package of an SCR, and Figure 19B shows the electrical symbol for an SCR.

FIGURE 18—Figure 18A shows the package of a transistor, and Figure 18B is the electrical symbol for a transistor.

FIGURE 19—Figure 19A shows the package of an SCR, and Figure 19B is the electrical symbol for an SCR.

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SCRs are used as switching devices in electronic circuits. If a small amount of voltage is applied to the gate of the SCR, a current will flow through the SCR between the cathode and the anode. The current will continue to flow until the voltage is removed from the anode. Thus, the SCR can be switched on by applying a voltage to the gate and off by removing voltage from the anode.

A bit later in the study unit, we’ll look at how these electronic components function in electronic ignition system circuits. Now, take a few moments to review what you’ve learned by completing Power Check 1.

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Self-Check 1

At the end of each section of Study Unit Title, you’ll be asked to pause and check your understanding of what you’ve just read by completing a “Self-Check” exercise. Answering these questions will help you review what you’ve studied so far. Please complete Self-Check 1 now.

1. The measure of the amount of electrical potential in a circuit is called the _______.

2. Electrical current is measured in units called _______.

3. True or False? The terminals of a transistor are called the anode, the cathode, and the gate.

4. When a conductor wire is connected to the terminals of a battery, electrons move from the _______ terminal to the _______ terminal.

5. True or False? Electrons are the smallest type of atomic particle.

6. Opposition to the flow of electricity in a circuit is called _______.

7. When electricity flows through a conductor, a _______ field is created around the conductor.

8. Electrical resistance is measured in units called _______.

9. True or False? If a current-carrying conductor is placed in a magnetic field, the conductor will move. 10. A _______ is an electronic device that will allow current to flow through it in only one direction. 11. The flow of electrons through a circuit is called _______. 12. True or False? If a conductor is moved through a magnetic field, a voltage will be induced on the conductor.

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SMALL ENGINE

IGNITION SYSTEMS

Now that we’ve reviewed all the important electrical and elec-tromagnetic concepts you should know, you’re ready to begin your study of ignition systems. We’ll start with a basic review of the function and components of the ignition system. As we discuss the different types of small engine ignition systems and components, try to imagine and visualize the “unseen forces” of electricity and electromagnetism working in the system.

Basic Ignition System Operation

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

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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.

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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.

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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.

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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.

Power Sources

In outdoor power equipment, there are just two different power sources that are used to run ignition systems. These power sources are the battery and the magneto. In a battery

ignition system, a lead-acid storage battery is simply

con-nected to the ignition coil. The battery provides the voltage needed to operate the coil.

Magneto systems are far more common than battery systems. The magneto ignition system uses the principles of electro-magnetism to produce a voltage. Remember that earlier in the text we discussed the generator action of electromagnetic

induction. According to this principle, when a conductor wire

is moved through a magnetic field, a voltage will be induced in the conductor. It’s also true that if a magnet is moved near a conductor, a voltage will be induced in the conductor. If this conductor wire is connected to a complete circuit, current will flow in the conductor wire. This is the operating principle of the magneto in a small engine.

In a magneto system, permanent magnets are installed in the engine’s flywheel. The ignition coil is then mounted at a stationary point near the flywheel (Figure 21). As the flywheel spins, the moving magnets cause a voltage to be induced in the primary winding of the ignition coil.

FIGURE 21—In a magneto ignition system, perma-nent magnets are often installed in the engine’s flywheel. The ignition coil in such a system is mounted at a stationary point near the flywheel as shown here. When the flywheel spins, the mag-nets move by the coil and induce a voltage in its pri-mary winding.

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The magneto has several advantages over the battery as a power source. First, when a piece of power equipment uses a magneto, no on-board battery is needed. Batteries are heavy and bulky, and would be very inconvenient on machines like small lawn mowers and weed trimmers. Also, no separate charging system is required with a magneto, while batteries require a charging system to keep them working. Machines that have magneto ignition systems also require less seasonal maintenance than machines with batteries.

Battery systems also have some advantages. First, the bat-tery that powers an ignition system can also be used to run other devices, such as headlights and electric starter systems. In contrast, most magneto ignition systems can only supply power to fire the spark plug. Since a battery can be used to run a starter system, machines that contain battery systems can be started with a key. Magneto ignition systems are gen-erally activated by pulling a cord or rope. Therefore, larger garden tractors and similar equipment generally use battery systems, while smaller machines use magneto systems. We’ll look at the design and operation of both the magneto system and the battery system in detail a little later in the study unit. For now, just keep in mind that the power source for a small engine ignition system is provided by either a mag-neto or a battery.

Trigger Switching Devices

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

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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.

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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.

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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.

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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.

Spark Plugs

The spark plug is a device that’s designed to let a volt-age jump across a gap, producing a spark that will ignite the engine’s fuel. Both four-stroke and two-stroke gasoline engines will contain one spark plug for every cylinder. An external view of a spark plug is shown in Figure 27A. The basic parts of a spark plug are shown in Figure 27B.

FIGURE 25—The con-struction of a typical condenser is shown here. The condenser is a type of capacitor—an elec-trical component that can absorb and store an electrical charge. The condenser prevents current from arcing (jumping) across the breaker point contacts when they open.

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FIGURE 26—This photo shows a real breaker points system. The major components of the system have been labeled for you. All the components of the breaker points system work together to form one switching device.

FIGURE 27—Figure 27A shows an external view of a typical spark plug. Figure 27B shows the parts of a spark plug.

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The metal section at the bottom of the spark plug is called the

shell. The top section of the shell is molded into a hexagon

shape. This shape allows a wrench or socket to be used to install or remove the spark plug. The lower section of the shell is threaded. Remember that a spark plug screws into a hole in the center of the cylinder head. The threads on the bottom of the spark plug mate with threads inside the hole in the cylin-der head.

A spark plug has two metal electrodes or terminals. The metal electrodes are conductors that permit current to flow through them. One electrode runs down through the entire length of the spark plug. This is called the center electrode. The second electrode is connected to the threaded part of the spark plug. This electrode is sometimes called the side electrode or the

grounding electrode. The grounding electrode bends around

to bring it very close to the end of the center electrode. The small air space between the two electrodes is called the gap. The gap is very small and is usually measured in thousandths of an inch. The correct gap measurement is very important to the correct operation of the spark plug.

The top end of the center electrode connects to the terminal

nut of the spark plug. When the spark plug is screwed into

the cylinder head, the terminal nut will be connected to the spark plug wire. The high voltage produced by the ignition coil travels through the spark plug wire and enters the spark plug through the terminal nut. The electricity then flows down the spark plug through the center electrode and jumps across the gap from one electrode to the other to produce the spark. The body of the spark plug is encased in a porcelain shell. Porcelain (a china-like substance) is used for the shell

because porcelain is an electrical insulator (it doesn’t conduct electricity). This porcelain insulator electrically isolates the voltage inside the spark plug. The spark plug manufacturers’ name and identifying number are usually printed on the por-celain insulation.

Note that the porcelain covering is ribbed. The ribs extend from the terminal nut to the shell of the plug to prevent a condition called flashover. In flashover, current jumps or arcs from the terminal nut to the metal shell on the outside of the plug instead of traveling down through the center electrode.

Small Engine Ignition Systems

Study Unit