Study Unit Small Engine Ignition Systems

By Robert L. Cecci iii PreviewPreview

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POWER CHECK ANSWERS POWER TROUBLESHOOTING IGNITION SYSTEMS TROUBLESHOOTING SERVICING IGNITION SYSTEMS SERVICING IGNITION SYSTEM OPERATION IGNITION SYSTEM SMALL ENGINE IGNITION SYSTEMS SMALL ENGINE INTRODUCTION Small Engine Ignition Systems

INTRODUCTION

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 procedures 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 systems 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 electrical power. The power source in a common household circuit is typically provided by a wall outlet. The power source in a

1 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 circuit path to reach the load. In contrast, in an open circuit, 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 copper wire. The load is a standard light bulb. In Figure 1A, the switch is open (turned to the OFF position). The electrical circuit 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 electricity 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 electricity can’t flow to the light bulb. In Figure 1B, the switch is closed, allowing electricity to reach the light bulb and light it

2 Small Engine Ignition Systems 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 negative 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 flashlight 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 contains 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 terminals 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 FIGURE 2—In a simple battery, of electricity. Opposite a chemical reaction takes place electrical charges (positive between the electrodes and the and negative) attract each electrolyte solution. This chemical reaction produces an electrical other very strongly and try to charge on each of the electrodes.

Small Engine Ignition Systems 3 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 battery 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 follow 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.

4 Small Engine Ignition Systems 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 element, 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 properties of the element. All atoms are made up of tiny atomic particles called protons, neutrons, and electrons. The electron is a very lightweight particle 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 electron 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 electrons, 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 (–).

Small Engine Ignition Systems 5 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 balance 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 reason, the free electron in a copper atom can easily be dislodged from its orbit.

FIGURE 4—This copper atom contains a single electron in its outermost orbit. This free electron can easily be dislodged from its orbit, which makes copper a good conductor of electricity.

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 current is produced by the movement of electrons. Well, in order to get the electrons moving, we have to remove them from atoms.

6 Small Engine Ignition Systems 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 surface, 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 produce 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 moving 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 electricity and are classified as insulators. Now, let’s see how electrons flow within an electrical circuit. 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.

Small Engine Ignition Systems 7 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.

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 circuit 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.

8 Small Engine Ignition Systems 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 formulas, current is usually represented by the letter I. Small amounts of current may be noted with the abbreviations 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 current. 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

Small Engine Ignition Systems 9 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 microamperes 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.

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 positive 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 abbreviation for volts is the letter V. So, the quantity “2 volts” would be abbreviated as “2 V.” In electrical diagrams and mathematical 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 current 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

10 Small Engine Ignition Systems 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 electrical diagrams and mathematical 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 prefixesk 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 resistance 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 measure circuit quantities in more detail later.) In order to better understand the relationship of current, voltage, 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 reservoir (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.

Small Engine Ignition Systems 11 FIGURE 6—Basic electrical principles can be visualized easily when you compare an electrical circuit to a water system.

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 waterwheel 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.

12 Small Engine Ignition Systems 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 produced 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.

FIGURE 7—The voltage level of a direct current (DC) remains exactly the same 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 maximum positive value. At the maximum positive point, the current reverses direction and falls back to zero. The current

Small Engine Ignition Systems 13 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.

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

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 relationship 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 stands for resistance in ohms.

14 Small Engine Ignition Systems 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 variations 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 current 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 measure 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.

Small Engine Ignition Systems 15 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 operation of a multimeter. The actual operation 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 voltage, 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 conductor 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 magnetic lines of force or magnetic flux. The magnetic field is strongest in the space immediately surrounding the conductor. The force of electromagnetism has many interesting and highly useful practical applications.

16 Small Engine Ignition Systems 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.

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

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

Small Engine Ignition Systems 17 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.

FIGURE 11—The magnetic 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.

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 voltage 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 magnetic induction. (Note that current won’t flow through the wire until the wire is connected in a complete circuit.)

18 Small Engine Ignition Systems The generator action of electromagnetic induction is illustrated 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.

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 conductor in the direction shown by the arrows. Note that the galvanometer is an instrument that measures electric current.

The generator action of electromagnetic induction is the basic property that’s used to operate electric generators. In an electric 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 positioned on both sides of the inside of the generator to produce a magnetic field. As the armature is turned within the magnetic 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.

Small Engine Ignition Systems 19 FIGURE 13—This illustration shows a very simple generator. As the coil of wire is rotated in the magnetic field, an electric current is generated.

The voltage and current produced by the simple generator 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 electromagnetic induction to power their ignition systems. For example, in some small engine ignition systems, coils are placed underneath the flywheel or outside next to the edge of theflywheel. 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 system. We’ll discuss these systems in more detail later in this study unit.

20 Small Engine Ignition Systems 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 discuss 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 electromagnetic induction causes the conductor to move.

Small Engine Ignition Systems 21 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. FIGURE 15—When the field windings are energized, the field magnets produce a powerful magnetic 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.

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.

22 Small Engine Ignition Systems 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 conductor 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 FIGURE 16—A basic transformer is shown to a battery through here. A change in voltage a switch and a resis- in the primary winding tor; a voltmeter is induces a voltage in the secondary winding. connected across the secondary winding.

Small Engine Ignition Systems 23 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 winding. The magnetic field spreads outward and cuts across the secondary winding, inducing a voltage on the secondary winding. 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 electronic devices. Electronic devices are components that are used to control the flow of electrons in a circuit. Many different electronic 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 materials that are used to make electronic components. Semiconductor devices are manufactured in laboratories under very special conditions. The semiconductor materials are specially processed and combined to form electronic devices such as diodes and transistors. Because of the way the semiconductor materials are processed during manufacturing, the finished diodes and transistors are capable of

24 Small Engine Ignition Systems controlling the flow of electrons. So, as a result of these special 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 cathode. 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.

FIGURE 17—Figure 17A shows the parts of a diode, and Figure 17B is the electrical symbol for 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 conductor. 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 semiconductor device that has three wire terminals. These terminals are called the base, the collector, and the emitter. Transistors

Small Engine Ignition Systems 25 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. FIGURE 18—Figure 18A shows the package of a transistor, and Figure 18B is the electrical symbol for a transistor.

A silicon-controlled rectifier (SCR) is another type of semiconductor 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 illustrates the package of an SCR, and Figure 19B shows the electrical symbol for an SCR.

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

26 Small Engine Ignition Systems 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.

Small Engine Ignition Systems 27 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.

Check your answers with those in the back of this book.

28 Small Engine Ignition Systems SMALL ENGINE IGNITION SYSTEMS

Now that we’ve reviewed all the important electrical and electromagnetic 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 resulting “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 cylinder, producing the power stage. The power produced by the ignition of the air-and-fuel mixture gets the crankshaft turning, 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 plug gap. As many as 20,000 volts are needed to make this

Small Engine Ignition Systems 29 spark. The spark must occur at exactly the right time in the engine cycle in order to ignite the fuel and air mixture properly. 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 perfectly, 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.

FIGURE 20—A simplified drawing of the parts of an ignition system is shown here.

First, let’s look at the ignition coil. All ignition systems contain 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 consists of two wire windings wound around an iron core. The

30 Small Engine Ignition Systems iron core is sometimes called an armature. The first winding is called the primary winding, and the second winding 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 voltage 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 produces 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 winding 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 ignition system needs some device that will keep turning the current from the power source on and off.

Small Engine Ignition Systems 31 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 immediately 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 connected 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.

32 Small Engine Ignition Systems 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 connected 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 electromagnetism 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, permanent 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 magnets move by the coil and induce a voltage in its primary winding.

Small Engine Ignition Systems 33 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 battery 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 generally 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 magneto or a battery.

Trigger Switching Devices

Different types of ignition systems use different types of switching devices. There are two basic types of trigger switching 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 ismovable. The movable contact is

34 Small Engine Ignition Systems 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.

FIGURE 22—This simplified drawing shows a set of breaker points.

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 similar 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.

Small Engine Ignition Systems 35 FIGURE 23—This figure illustrates the action of the breaker points in an ignition circuit. The points serve as a triggering switch. In Figure 23A, the points are closed; the ignition circuit is complete 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.

36 Small Engine Ignition Systems FIGURE 24—This figure illustrates the two different 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.

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 secondary winding. For these reasons, the condenser is used to control the current 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 winding. Therefore, the condenser prevents the electricity from jumping the air gap between the opening points. When the magnetic field collapses 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 lead connected to it. The wound strips are then inserted in a

Small Engine Ignition Systems 37 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.

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

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 condenser, the spring, and the plunger. The other type of switching device used in small engine ignition systems is an electronic switch. In an electronic switch, solid-state electronic components are used to turn the current 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 voltage 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.

38 Small Engine Ignition Systems 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.

Small Engine Ignition Systems 39 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 cylinder 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 porcelain 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.

40 Small Engine Ignition Systems There are two types of spark plug wire connections. In one method, the spark plug wire is connected to the terminal nut of the spark plug with an exposed metal clip. This type of connection is usually seen on older mowers. The newer type of connection is an insulated boot-type connection. A boot-type connector has a synthetic rubber cap that fits over the terminal nut. Figure 28 shows both types of connections on real engines. Figure 28A shows the exposed metal clip connector and Figure 28B shows the boot-type connector. If you look quickly at a group of spark plugs, they may all look very much alike. However, there are many small differ- ences in the way spark plugs are manufactured that allow them to perform well in different types of engine applications. The correct type of spark plug must be used in each engine to allow the engine to work efficiently and economically over a long period of time. Spark plugs are carefully manufactured to precise specifications. Each plug is identified by a specific manufacturer’s number. When replacing a spark plug, always use the same type of replacement plug. Now, let’s look at some of these different spark plug specifications. The first specification is called reach. Thereach of a spark plug is the length of the metal threads at the end of the plug. The reach of a spark plug is shown in Figure 29. The correct spark plug reach is very important to proper engine operation. If the spark plug reach is too long, the threaded part will extend down into the combustion chamber and hit the piston each time it rises, causing serious damage. If the reach is too short, the spark will occur too high up in the cylinder head. This will cause the air-and-fuel mixture to begin burning too slowly in the combustion chamber and delay the start of the power stroke. This will in turn cause a loss of power and very hard engine starting. Another important consideration in spark plug operation is temperature. Heat from the fuel combustion process is absorbed by the spark plug during engine operation and is con- ducted upward through the plug. Combustion temperatures are normally in the range from 1,000° F to 1,500° F. Thus, spark plugs must be able to withstand these temperatures.

Small Engine Ignition Systems 41 FIGURE 28—The engine shown in Figure 28A uses an exposed metal clip to connect the spark plug wire to the terminal nut of the spark plug. You can see the silver metal of the exposed terminal nut. The engine in Figure 28B uses a boot-type connector that covers the entire terminal nut.

42 Small Engine Ignition Systems FIGURE 29—The length of the threaded area of a spark plug is called the reach.

Spark plug manufacturers make different series of plugs to withstand different heat ranges. A plug is called a cold plug if it can easily transfer combustion heat from the firing end to the shell and the cylinder head. In a hot plug, the center electrode is more isolated from the shell and the cylinder head. Therefore, a hot plug tends to retain its heat. A spark plug with the correct heat range must be installed in an engine. A cold plug would therefore be installed in an engine that has high combustion temperatures. A hot plug would be installed in an engine that runs at cooler temperatures. If a hot plug is installed in a hot running engine, the spark plug may overheat. If a cold plug is installed in a cool running engine, heavy carbon deposits will form on the electrodes, making it difficult for the spark plug to fire. When the plug is in the correct temperature range, the heat from combustion will burn the by-products of combustion off the electrodes and keep them clean without causing overheating. Different plugs have different types of electrodes. Some plugs use a copper/steel alloy center electrode. Other plugs use a platinum alloy electrode. Platinum alloy electrodes tend to operate hotter, burning off combustion deposits at lower temperatures. Some spark plugs place a small ceramic element in the center electrode. This element acts as a resistor and is used to suppress radio frequency interference. Radio frequency interference is caused by the firing of the spark plug. This interference causes a popping noise in radios, televisions, and in some types of communication systems. The resistance element in the plug will help remove this interference.

Small Engine Ignition Systems 43 The length of the grounding electrodes in spark plugs also varies. Some grounding electrodes bend and extend over the entire width of the center electrode. This is sometimes called an automotive gap spark plug. Another type of grounding electrode extends only partway over the center electrode. This is called a clipped gap plug. These two types of plugs are shown in Figure 30.

FIGURE 30—Figure 30A shows the end of an automotive gap spark plug. Figure 30B shows a clipped gap spark plug.

Now, take a few moments to review what you’ve learned by completing Power Check 2.

44 Small Engine Ignition Systems Self-Check 2

1. True or False? The power source in a small engine ignition system supplies a voltage directly to the secondary winding of the ignition coil.

2. The side electrode of a spark plug is also called the ______.

3. A spark plug that can easily transfer combustion heat from the firing end to the shell and the cylinder head is called a ______plug.

4. What are the two possible power sources in a small engine ignition system? ______

5. True or False? The secondary winding of the ignition coil is connected to the spark plug wire.

6. True or False? In a small engine ignition system, triggering (switching) may be performed by a set or breaker points or by a battery.

7. During the operation of a breaker points assembly, what component stores and releases an electrical charge? ______

8. A spark plug that tends to retain its heat is called a ______plug.

9. What part of the spark plug does the spark plug wire connect to? ______

10. True or False? The metal section at the bottom of a spark plug is called the insulator.

Check your answers with those in the back of this book.

Small Engine Ignition Systems 45 IGNITION SYSTEM OPERATION

Now, let’s take a closer look at the construction of some different types of ignition systems. The three basic types of ignition systems used in outdoor power equipment applications are the magneto ignition system, the battery ignition system, and 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 ignition systems contain electronic components that perform the switching action in the ignition circuit. Electronic ignition systems 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 different 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 winding 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

46 Small Engine Ignition Systems 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 31—The basic parts of a flywheel magneto ignition system are shown here. In this system, the ignition coil, breaker points, and condenser are located underneath the flywheel. However, in many systems, the ignition coil is located on a mounting bracket at the side of the flywheel.

Figure 32 shows a photo of a typical flywheel used in a magneto 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 32—Shown here are the magnets that are cast into the flywheel.

Small Engine Ignition Systems 47 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 thekeyway. 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 crankshaft. The key is the small part being removed from the keyway in the crankshaft.

48 Small Engine Ignition Systems In order for the magneto system to work, the ignition coil must be mounted in a stationary position close to the flywheel. Figure 34 shows a photo of an ignition coil near a flywheel. (You can compare this photo to the simplified drawing 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 flywheel magneto ignition will contain an electronic ignition coil. However, we’ve included this illustration to familiarize you with the older style of ignition coil.

FIGURE 34—This photo shows how an ignition coil is positioned near the flywheel in a magneto ignition system. Figure 21 showed a simplified drawing of a similar assembly.

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.

Small Engine Ignition Systems 49 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.

FIGURE 35—This is a simplified illustration of the operation of a magneto. A permanent magnet is mounted near the edge of the flywheel. As the flywheel turns, the magnet passes near the ignition coil and induces a voltage in the primary winding.

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

50 Small Engine Ignition Systems 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 magnetic 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 further 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 current 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 illustration, the movement of the magnet causes the magnetic lines of force to change direction. As a result, a current flows in the primary winding.

Small Engine Ignition Systems 51 The voltage in the primary winding will induce a low voltage in the secondary winding due to the action of mutual inductance. 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 collapses 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 primary winding is broken. Any remaining current in the primary is absorbed by the condenser. The magnetic field then collapses through the secondary winding, inducing a high voltage in the secondary. 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.

52 Small Engine Ignition Systems As the magnetic field collapses through the secondary winding, 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 firing 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.

The Battery Ignition System

Now, let’s look at a battery ignition system. In a battery ignition system, a battery is used to provide power to the ignition coil instead of a magneto. The battery ignition system, however, contains the same type of electronic switching components (or breaker points and condenser) and spark plug that a magneto system does. Battery ignition systems are used in garden tractors, boats, snowmobiles, and other larger devices. The battery used in this type of system is a lead-acid storage battery, similar to the type used in an automobile. However, the battery in a small engine ignition system will generally be much smaller than the battery in a car or truck. This is because the small engine is easier to start than a car or truck engine. Besides providing electricity to power the ignition coil, the battery may also be used to power lights, horns, and other accessory circuits.

Small Engine Ignition Systems 53 A typical lead-acid storage battery is made up of several individual compartments called cells. Each cell is made up of a series of lead plates. Small spaces between the plates are filled with anelectrolyte solution. The electrolyte is usually made from sulfuric acid diluted with water. Each cell produces approximately 2 V when the battery is fully charged, so a 12 VDC battery will contain six cells. A diagram of a typical storage battery is shown in Figure 38.

FIGURE 38—This figure shows a storage battery that contains six 2 V cells. The total voltage of the battery is 12 V.

The acid used in storage batteries can cause burns or destroy clothing. Always use extreme caution when working near a lead-acid battery. Normally, the storage battery in a battery ignition system has a total output voltage of 12 volts of direct current. The current produced by the battery will be measured in units called ampere/hours (Ah). In a battery ignition system, a generator, alternator, or coils within the flywheel may be used to recharge the battery as the engine operates. Figure 39 shows a pictorial view of a typical battery-powered ignition system. The system shown uses a breaker points assembly for triggering. In this system, the battery provides the voltage needed to energize the primary winding of the ignition coil. The voltage to the ignition coil is switched on and off by means of the ignition switch. This switch is often operated by a key in garden tractors and similar equipment. (In this illustration, the ignition switch is shown with only one set of

54 Small Engine Ignition Systems contacts. In some garden tractors and riding lawn mowers, the ignition switch may have multiple sets of contacts to engage the starter solenoid and other such options or accessories.)

FIGURE 39—This pictorial illustration displays the components of a typical battery ignition system.

When the ignition switch is turned on, the switch contacts close and the ignition circuit closes. When the circuit closes, electric power from the battery passes through the ignition switch and through the primary winding of the ignition coil. The opposite end of the primary winding is connected to the breaker points and condenser. The breaker points assembly, the secondary winding, and the spark plug all operate in exactly the same way as in the magneto system. The only difference in the battery system is that the battery energizes the primary winding of the ignition coil. When the ignition switch is turned off, the switch contacts open, and the flow of power from the battery to the primary winding is stopped. As a result, the engine stops.

Magneto/Battery Systems

Note that many pieces of outdoor power equipment may contain both a magneto and a battery. This may sound a bit confusing at first, but it’s really simple to understand. In such

Small Engine Ignition Systems 55 a machine, the magneto is the power source for the ignition coil. However, a separate power source—the battery—is used to operate an electric starter circuit and any accessory circuits. An electric starter is simply a small DC motor that has a movable gear on its output shaft. The starter is generally activated by turning a key, or by turning a key and pushing a button. When the key is turned in the ignition switch, the circuit from the battery to the starter motor is closed, and current flows from the battery to the motor. As a result, the starter motor’s armature turns. (Note that we’ll discuss the starter motor in more detail in a later study unit.) When the motor’s armature turns, the gear on the armature pushes outward and mates with gear teeth on the flywheel. The turning gear on the motor’s armature will therefore cause the flywheel to start turning. When the flywheel starts turning, the small gear on the motor armature retracts. Remember that this type of machine contains a magneto. Therefore, when the flywheel starts turning, the magneto ignition system begins working, just as we described earlier. The only difference in this system is that you can start the machine with a key rather than by pulling a starter rope. That is, the starter motor makes the flywheel turn rather than a rope and pulley. Many modern garden tractors and other large pieces of equipment contain systems like this.

Electronic Ignition Systems

Breaker points and condenser systems have been used for many years on power equipment. You’ll still occasionally see these systems when customers bring their older lawn mowers, chainsaws, garden tractors, and other machines to your repair shop. However, breaker points and condenser systems have been replaced in all newer engines by electronic ignition systems. This is because mechanical breaker point contact sets are flawed in that they will eventually wear out and fail. This will cause poor engine performance at first, and ultimately engine failure. Electronic ignition systems use electronic diodes, transistors, and SCRs in place of mechanical switching components, so they last for a very long time.

56 Small Engine Ignition Systems An electronic ignition system in a piece of outdoor power equipment will almost always use an electronic ignition module and a flywheel magneto. An electronic ignition module is a sealed plastic unit (often black) that contains the ignition coil, the electronic trigger switching devices, and (usually) a capacitor. The module is usually mounted on a bracket very close to the outer edge of the flywheel. A heavy wire (the spark plug wire) leads from the module directly to the spark plug. In addition, you’ll see a small grounding wire on the module. The end of the grounding wire usually connects to a grounding terminal on the mounting bracket for the kill switch. A typical ignition module is shown in Figure 40. Note how the ignition module is mounted very close to the flywheel.

FIGURE 40—This electronic ignition module contains the charging coil, transformer, diodes, and SCR in one compact assembly. The ignition module is mounted near the flywheel on a bracket and is held in place by two retaining screws.

Electronic ignition systems have no moving parts, except for the flywheel and its magnets, so the performance of the system won’t decrease through operation. Electronic ignition modules are very resistant to moisture, oil, and dirt. They’re very

Small Engine Ignition Systems 57 reliable, don’t require adjustments, and have very long life spans. They provide easy starting and smooth power during operation. Also, these units are generally quite inexpensive. There are two basic types of electronic ignition configurations that we’ll discuss in this section of your text: 1. The capacitor discharge (CD) ignition system 2. The transistor-controlled (TC) ignition system Let’s take a closer look at these two types of systems now.

The Capacitor Discharge Ignition System

The electronic ignition system most often used is the capacitor discharge ignition system. The basic components of a CDI system may be configured in several different ways. Usually, the components are all contained in a sealed ignition module like the one shown in Figure 40. However, in certain small engine applications, the CDI components may be configured and positioned differently. Although the various CDI systems may have different arrangements of wiring and parts, all CDI systems operate in much the same way. Figure 41 shows how the components of a CDI system are arranged when they’re contained in a sealed ignition module. Note that the CDI system contains two coils that are triggered by magnets in the flywheel. The larger coil is thecharging coil and the second, smaller coil is called the trigger coil. The trigger coil performs the function of the breaker points in this system and controls the ignition timing. As the flywheel rotates past the charge coil, electrical energy is produced in the module. The capacitor then stores this energy until it’s needed to fire the spark plug. As the flywheel magnet rotates past the trigger coil, a low voltage signal is produced. This signal causes an electronic switch to close, which in turn allows the energy in the capacitor to pass through the switch to the transformer. At the transformer, the voltage is increased to 25,000 volts. This high voltage travels through the high tension lead to the spark plug, creating a spark at the electrode and igniting the air-and-fuel mixture.

58 Small Engine Ignition Systems FIGURE 41—This illustration is an electronic diagram of the components of a CDI ignition system. Note that all the components are contained in a sealed module mounted near the flywheel.

Now, let’s look at a different type of CDI system. Figure 42 shows a diagram of a CDI system in which the charging coil and the trigger coil are mounted underneath the flywheel. The ignition coil and other components are located away from the flywheel, closer to the spark plug. This type ofsystem is sometimes called an externally mounted CDI system. A CDI like this one may be seen in motorcycle ignitions, but not usually in lawn mowers and other such equipment. However, you should be aware of how this type of system operates. The secondary power source for this CDI system is a stored charge in the capacitor. Magnets in the flywheel are used to energize both the charging coil and the trigger coil. As the flywheel turns and the magnet passes over the charging coil, one complete cycle of alternating current is produced (Figure 42A). This current is passed to the group of four diodes, which rectifies it to direct current and passes it on to the capacitor. The capacitor then charges to its full capacity. At this point, the ignition system is prepared to fire.

Small Engine Ignition Systems 59 FIGURE 42—The operation of an externally mounted CDI system is shown here. In Figure 42A, the magnet passes over the charging coil to produce one complete cycle of alternating current. The current is passed to the group of four diodes, which rectifies it and passes it on to the capacitor. The capacitor then charges to its full capacity. In Figure 42B, the magnet passes the trigger coil and generates a brief pulse of current. This current pulse is passed to the SCR, which turns on and allows the capacitor to discharge its stored charge to the ignition coil.

The flywheel continues to turn. As the magnet in the flywheel passes the trigger coil, a brief pulse of current is generated. This current pulse is passed to the gate terminal of the SCR (Figure 42B). The SCR turns on and allows the capacitor to discharge its stored charge to the ignition coil. Remember that the ignition coil is a transformer. As the current from the SCR rushes into the ignition coil, a very strong magnetic field builds up in the coil’s primary winding. The magnetic field in the primary winding induces a strong voltage in the secondary winding, and this voltage discharges across the spark plug gap.

60 Small Engine Ignition Systems Transistor-Controlled Ignition Systems

Another type of electronic ignition system that may be found in modern small engine applications is the transistor-controlled ignition system (TC). In this type of system, transistors are used to perform the trigger switching function. The electronic components of a transistorized ignition system are generally contained in one small unit that can be mounted directly to the ignition coil. Because of the construction of this type of unit, there are no test points to monitor in the event of ignition system failure. A diagram of a typical transistor system is shown in Figure 43.

FIGURE 43—In this figure, you can see how a transistor unit is mounted directly to the ignition coil in a small engine ignition system.

A transistorized ignition system operates by controlling the flow of electricity to the primary coil. Typically, two transistors are contained within the electronic control unit. One transistor is used to supply electricity to the primary coil. When the voltage level in the primary reaches a certain level, the second transistor turns off the first transistor. This causes the magnetic field around the primary coil to collapse, creating a high voltage across the secondary coil. This high voltage is then discharged across the spark plug. Transistorized and SCR ignitions should be stopped by using a ground terminal on the ignition module. The ignition coil will have the electronic module attached to the coil and armature assembly. One or more external terminals may be included on

Small Engine Ignition Systems 61 the transistor or SCR module. If these terminals are grounded, the module will stop supplying electricity to the coil. This in turn will stop the spark causing the motor to stop.

Safety Interlock Switches

A safety interlock switch is a special type of electronic switch that’s used on outdoor power equipment to protect an operator from injury. A safety interlock may prevent an engine from starting in an unsafe mode, stop the engine or the mowing blade when the operator lets go of a lawn mower handle, or stop the engine or the blade when the operator rises out of the seat (in rider mowers). For example, most newer lawn mowers have a safety bar located near the top of the handle. The bar must be held against the handle as you pull the starter cord in order for the engine to start. If you let go of the handle, the engine will automatically stop. The bar is attached to a cable that connects to a brake assembly. If you let go of the bar, the brake assembly stops the crankshaft from revolving. The brake assembly contains a switch that grounds the ignition module when the brake is engaged. Garden tractors and riding lawn mowers often have safety interlock switches located under the operator’s seat. If the operator rises from the seat, the safety switch closes and grounds out the ignition module, stopping the engine. On garden tractors, some switches won’t allow the engine to start unless the transmission is in neutral, a person is sitting on the seat, the blade is disengaged, and so on. The safety switches must be in their open contact state to allow the engine to start. However, once the engine has started, the switches are closed as the transmission is shifted into gear or the blade is engaged under the deck. The safety interlock module allows the engine to run with these switches closed. A diagram of a safety interlock module is shown in Figure 44.

62 Small Engine Ignition Systems FIGURE 44—A safety interlock circuit is shown here.

The electronic safety interlock module operates by sensing and monitoring the voltage from the ignition coil. If no voltage is present, the engine isn’t running. However, once the engine starts and the ignition coil produces a voltage, the deck switch and neutral switch are bypassed by the safety interlock module. The ignition switch and the safety seat switch are still in the coil circuit. Closing either switch will cause the engine to stop. Now, take a few moments to review what you’ve learned by completing Power Check 3.

Small Engine Ignition Systems 63 Self-Check 3

1. The power source in a battery ignition system is a/an ______.

2. True or False? The main benefit of a CDI system is that it uses breaker points to control the trigger switching.

3. The secondary power source in an externally mounted capacitor discharge ignition system is a/ an ______.

4. True or False? In an ignition system, an electric starter is a small DC motor that’s used to start the flywheel turning.

5. The flywheel is held in position on the crankshaft by a small bar of soft metal called a ______.

6. True or False? In an ignition system that contains both a battery and a magneto, the battery is the power source for the ignition coil.

Check your answers with those in the back of this book.

64 Small Engine Ignition Systems SERVICING IGNITION SYSTEMS

The ignition systems used in small engine applications are generally very durable, but they do need periodic maintenance. An ignition system tune-up includes several specific maintenance services on different parts of the ignition system—the spark plug, the flywheel and magneto, the breaker points, and the ignition coil. An ignition tune-up is generally performed on a small engine once per season. For example, an ignition tune-up would be performed on a lawn mower in the spring or early summer when it’s taken out of winter storage. If you live in an area where mowers and other pieces of outdoor power equipment are used all year long, more than one tune-up may be needed each year. The manufacturer’s manual for each particular engine will tell you how often tune-ups should be performed. Note that modern electronic ignition systems don’t have moving parts that can wear out. Therefore, they require very little maintenance, and there are very few or no adjustments that can be made on them. A tuneup on an engine that has an electronic ignition system may involve nothing more than a visual check of the components and a spark plug change. However, breaker points assemblies and other moving parts in older engines can wear out. So, these parts in older systems must be checked and replaced periodically. In this section of your study unit, we’ll go over ignition maintenance procedures in detail. We’ll also discuss some typical ignition system problems and how to troubleshoot them. As you read through this information, you may want to refer back to some of our earlier discussions on ignition system operation for reference or to review terms.

Starting the Tune-Up

When you begin an ignition system tune-up, you should note that you don’t need to remove the engine from a piece of power equipment in order to check the ignition system. You should be able to access all the ignition components easily from the top or side of the engine. However, a few minor disassembly steps will be needed. Let’s take a look at these steps.

Small Engine Ignition Systems 65 The first step you must take before working on any engine is to disconnect the spark plug wire. When the wire is disconnected, it’s impossible for the engine to start accidentally. If the spark plug wire connection has a boot-type rubber cap, gently turn and pull the cap off the terminal nut of the plug. If the connection uses a metal clip, pull the clip off the terminal nut. Ground the wire by fastening it to the engine block. On many engines, a special grounding stub is located near the spark plug wire. If you attach the spark plug wire to this special stud, it won’t fall off. Next, it’s a good idea to thoroughly clean the engine before you start working on it. Remove any loose dirt, grass clippings, and so on with a soft brush or with a blast of compressed air. The next step is to remove the blower housing from the engine. The blower housing is the metal cover that fits over the top of the engine and protects the components underneath from dirt. The blower housing may also contain the engine’s starter rope and handle. The flywheel and the coil are usually located under the blower housing; if the engine has a breaker points assembly, it will be located underneath the flywheel. Several screws or bolts will generally be used to hold on the housing. Remove these screws or bolts and lift off the cover as shown in Figure 45. When the blower housing is removed, it’s a good idea to clean the housing and inspect the components underneath. Grass clippings and dirt may collect on the fins of the cylinder and cylinder head and in other areas. Before you begin to check the ignition system, take the opportunity to clean off all visible components. A clean engine will operate much better than a dirty one. Now, you’re ready to begin the tune-up. We’ll start our discussion with a general visual inspection of the system.

66 Small Engine Ignition Systems The General Inspection

The first step in an ignition system tune-up is to make a general visual inspection of the system. The biggest enemies of the ignition system are dirt, dampness, and oil. Dirt can hide trouble signs such as damaged or broken wiring or wire insulation, loose or corroded terminals or connections, and cracked or damaged components. Dampness or moisture can cause shorting or current leakage from ignition coils or spark plug wires. Oil can rot wire insulation. It’s therefore extremely important that the components of an ignition system be kept clean—especially the wiring. Begin your visual inspection with the wiring. If the wire insulation is cracked, rotted, or burned, the wire should be replaced. Repairing broken insulation with electrical tape isn’t recommended except as a temporary emergency measure.

Small Engine Ignition Systems 67 Wires and cables should be located where they can’t be damaged by heat from the cylinder head, cylinder block, hot exhaust gases, or spinning engine components. All wiring connections should be clean, free of corrosion, and secure. Inspect ignition coils for damaged or corroded components. A coil case, for example, could be cracked or rusted. A cracked ignition coil will cause a current leakage that will result in a weak spark or spark failure. Any damaged components should be replaced.

Spark Plug Service

Because of the way in which small engines are designed, the spark plug is usually one of the easiest components to get to during a tune-up. The spark plug is located in the middle of the cylinder head and is usually clearly visible from the outside of the engine.

Removing the Plug

The first step in servicing a spark plug is to remove it from the engine. Remember that the spark plug wire must always be disconnected from the plug first! Also, if an engine has recently been running, you should always allow the engine to cool before removing the spark plug. The heat of the engine causes the metal of the cylinder head and the spark plug shell to expand, and the spark plug may be very tightly locked in its hole. If you try to remove a plug before the engine has cooled, the spark plug may seize in the hole and damage its threads. When the engine and spark plug are cool, the plug will be much easier to remove and there’s less chance of damaging the threads in the cylinder head. Make sure that any loose dirt on the cylinder head near or around the plug is removed. A small clean paintbrush is good for this job. It’s very important to prevent any dirt from getting into the engine through the hole in the cylinder head. Dirt in the moving parts of an engine can cause serious damage. Once the area around the spark plug is clean, you can remove the plug. To remove the plug, use the correct size of spark plug socket. A spark plug socket is a special socket

68 Small Engine Ignition Systems wrench that’s made just for removing and installing spark plugs. Spark plug sockets are deep and have rubber inserts. The depth of the socket allows it to fit over the entire top of the spark plug to reach the hexagonal area of the shell. The rubber insert protects the porcelain of the spark plug from breakage as you turn the wrench. If a spark plug is very tight in its hole, it must be removed very carefully to prevent it from breaking. To remove a very tight spark plug, fit the end of the socket wrench over the spark plug. Turn the wrench one halfturn counterclockwise. Next, reverse the ratchet and lightly tighten the spark plug back into the cylinder head threads. Reverse the ratchet again and turn the spark plug out of the threads one full turn. Reverse the ratchet once again and tighten the spark plug about one half-turn. Once the spark plug has been loosened, you can remove it with your fingers. This alternating loos- ening and tightening action helps protect the threads from binding and becoming damaged.

Inspecting the Plug

Once the plug has been removed, you should inspect it to determine its condition. The condition of a spark plug can tell you a lot about how an engine is operating. In fact, most outdoor power equipment technicians will remove the spark plug first when troubleshooting a faulty engine. One of the first things to check is whether the spark plug is the correct type for the engine. Then, check the condition of the electrodes. Figure 46 displays several different engine conditions that are revealed by spark plugs. Figure 46A shows a new spark plug. Note that the bottom surface of the center electrode is flat and the surfaces of the lower electrode are squared. A used plug in normal condition will look much the same, but the electrodes will be colored an ashy gray or light tan from carbon deposits. (Carbon deposits are produced during normal fuel combustion.) An oil-fouled plug is shown in Figure 46B. Oil fouling will cause the end of a plug to be saturated with wet, sooty, black oil deposits. In a four-stroke engine, an oil-fouled plug may indicate that the piston rings aren’t sealing the cylinder properly. Or, oil may be passing through the valve stem

Small Engine Ignition Systems 69 area. Sometimes, a clogged breather can cause an oil-fouled plug. (Remember that a breather is a vent in the crankcase.) The clogged breather will prevent the crankcase from venting properly, and as a result, pressure will build up in the crankcase. This pressure will cause oil to be pushed up past the piston rings and into the combustion chamber. The oil in the combustion chamber will then foul the spark plug.

FIGURE 46—Shown here are three of the different conditions that are possible when a spark plug is removed from an engine. Figure 46A shows a new, clean plug. Figure 46B shows an oil-fouled plug. Figure 46C shows a fuel-fouled plug.

On two-stroke engines, oil fouling of spark plugs is quite common. Remember that in a two-stroke engine, the oil and the fuel are mixed together in the crankcase. Therefore, oil fouling is a normal by-product of engine operation in the two-stroke engine. Oil fouling in a two-stroke engine plug may also be caused by too much oil in the fuel-and-oil mixture. For example, if an engine is designed for a 40;1 fuel-and-oil mix and your customer is using a 10;1 mixture, the plug can easily

70 Small Engine Ignition Systems become oil fouled. (Note that in either a two-stroke or four- stroke engine, oil fouling may also be displayed at the exhaust pipe as excessive smoke.) Figure 46C displays a spark plug that was fouled by excessive fuel. Fuel fouling (also called carbon fouling) is indicated by dry, black, fluffy deposits on the spark plug electrodes. However, the plug won’t have the caked or lumpy appearance of an oil-fouled spark plug. Fuel fouling is most often caused by extended operation with an air-and-fuel mixture that’s too rich. This is usually a car- buretor problem, although a blocked exhaust or faulty valve can also cause fuel fouling. You’ll probably be able to smell fuel on the spark plug if the fuel fouling problem is severe. Another possible cause of fuel fouling is weak ignition. If the high-tension cable, points, condenser, electronic module, or coil is faulty and the spark is too weak, a plug can become fuel fouled. Fuel fouling can also be caused by using too cold a spark plug in an engine. Both oil fouling and fuel fouling can cause a spark plug condition known as a bridged gap. In this situation, carbon or oil deposits build up in the spark plug gap until it becomes completely blocked. A bridged gap will seriously affect the engine’s ignition efficiency. Note that the deposits caused by fuel and oil fouling can usually be cleaned off a spark plug, and the plug can then be reinstalled in the cylinder head. However, this isn’t usually a cost-effective practice. Spark plugs are inexpensive, and they should always be replaced during an engine tune-up. After many hours of use, spark plug electrodes will begin to erode. New electrodes have flat surfaces; however, an eroded center electrode will appear rounded, while an eroded side electrode will have a curve on its inside surface. Plugs with eroded electrodes should be replaced. When inspecting a spark plug, you may find that the plug’s electrode or insulator is damaged. The electrodes may be heavily pitted and the insulator broken or cracked in extreme situations. This damage is usually caused by too hot a plug being used in an engine. A physical impact can also damage a plug. For example, if a piston or ring part breaks and hits

Small Engine Ignition Systems 71 the spark plug, you may find damaged or bent electrodes or cracked and broken insulators. Or, if the spark plug reach is too long, the piston head may strike the electrodes. The most common cause of physical damage, however, is debris or foreign objects in the cylinder. Sometimes, a bolt or washer may loosen and actually be “sucked in” to the cylinder. The foreign object will then strike the spark plug electrodes when the piston rises. Plug heat ranges are changed depending on the condition of the plug that’s removed from the cylinder head. A hotter plug is generally installed if the plug looks dirty. A cooler plug is installed if the plug displays heat damage such as the cracking or chipping of the insulator. The manufacturer’s manual will provide recommendations about the type of plug that should be used in their engine. You should always follow these recommendations to prevent the types of problems we described. Never sand, sandblast, or file a spark plug and then replace it in an engine. Using sandpaper or a file will leave tiny grooves on the electrodes. These grooves will either burn off or will collect deposits as the engine operates. Also, sandblasting and filing will leave tiny particles of sand or metal behind on the electrodes. These particles will get into the engine’s cylinder and cause serious damage. In the past, some spark plug manufacturers have produced small sandblasting cleaning machines that were designed to be used with their spark plugs. However, most small engine manufacturers now recommend against using these machines for the reasons we described. Remember, spark plugs are inexpensive. If you’re ever in doubt of a plug’s quality, simply replace it.

Checking the Plug Gap

The next step in the ignition tune-up process is to check the spark plug gap. The width of the air gap between a spark plug’s electrodes is a precision measurement that’s determined by the spark plug manufacturer. In order for the plug to work properly, the gap between the electrodes must be the correct width. Therefore, before you install a spark plug in

72 Small Engine Ignition Systems an engine, you should measure the air gap between the electrodes. The service manual or owner’s manual for the engine will list the proper air gap for the spark plug. The spark plug gap can be checked by using a special measuring tool called a gapping tool. (The plug gap can also be measured with a feeler gage or a ramp gage, although these tools may be less accurate than the gapping tool.) A gapping tool is a device that contains small wire prongs of different thicknesses. The wire prongs are designed to measure in thousandths of an inch, and each one is labeled with its thicknesses. To measure the plug gap, first check the manufacturer’s manual to determine what the correct gap width is. For the purposes of this discussion, imagine that you’re measuring a plug gap and that the manufacturer’s manual says the correct gap width should be 0.030 inch. On the gapping tool, find the wire that’s marked 0.030 and fit it into the gap between the plug’s electrodes. This process is shown in Figure 47.

FIGURE 47—If the gapping tool wire slides between the electrodes with a slight resistance, you’re measuring the gap correctly.

Small Engine Ignition Systems 73 The wire should fit snugly between the electrodes. If the gap is too large or too small, use the metal tab on the side of the gapping tool to gently bend the grounding electrode into its correct position. A plug’s gap can also be measured using a flat feeler gage. However, if the spark plug’s electrodes are worn, the flat blades of the feeler gage may not give an accurate reading. An illustration of this is shown in Figure 48A. Note that when the plug’s electrodes are worn from use, the wire gapping tool will give a more accurate measurement of the gap (Figure 48B). Also, note that if the plugs electrodes are very worn, it should probably be replaced with a new plug anyway.

FIGURE 48—A feeler gage can be used to measure the gap of a spark plug, as shown in Figure 48A. However, if the electrodes are worn, the feeler gage blade may not give an accurate reading, as shown in Figure 48B.

74 Small Engine Ignition Systems Note that the gap of a spark plug should always be measured before you install it. This also applies to a brand new plug. A new plug’s electrodes may have been bent out of shape and need to be adjusted.

Installing a Spark Plug

When the spark plug gap has been checked (and corrected, if necessary), it’s time to replace the spark plug into the cylinder head. Hold the plug with your fingers and gently screw the plug into the cylinder. Don’t force the plug to turn. The plug should turn at least two full turns into the cylinder head. Now, use a spark plug socket to tighten the plug into the cylinder head. A spark plug should be tightened to the proper manufacturer’s specifications. This is normally in the range of 15 foot-pounds. A torque wrench, such as those shown in Figure 49, should be used to tighten down the plug. These wrenches have special dials or gages on them that indicate the tightening force you’re applying. When using a torque wrench, turn the plug socket with the torque wrench until the dial pointer reads 15 foot-pounds.

FIGURE 49—Two types of torque wrenches are shown here.

Small Engine Ignition Systems 75 One of the biggest problems with spark plug installation is the possibility of cross-threading the plug. In cross-threading, the plug is screwed into the cylinder head at a slight angle, damaging the threads inside the hole in the cylinder head. Aluminum cylinder heads are very easily damaged. If a plug is cross-threaded into the cylinder, it’s possible to repair the threads. The best repair method is to remove the cylinder head and screw a tap of the appropriate size into the hole. Or, if you don’t have a tap, you can remove the cylinder head and screw a spark plug with a long reach backwards through the hole. Either method will clean up the top threads, allowing you to reinstall the cylinder head and screw the correct plug back in from the top of the cylinder head. (Use a new cylinder head gasket whenever you remove and replace the cylinder head on an engine.) A special tool called a thread chase can also be used to clean up damaged threads. Again, remove the cylinder head and screw in the thread chase (just as you would install the spark plug). The thread chase will cut away the faulty thread area, leaving good threads behind. If the threads are heavily damaged, you can use a thread insert to replace the existing threads. In this case, you would drill the plug hole oversize. The oversized hole is then tapped to match the thread size of the outside of the thread insert. The insert is then threaded into the oversized hole in the cylinder head, and the spark plug is threaded into the insert.

Magneto Service

Now, let’s look at the procedures involved in maintaining a magneto system. If a magneto system has electronic switching components, there will be very little to check. You can perform a general visual inspection of the wiring and terminals, but otherwise, an electronic system is basically maintenance-free. However, in older systems that have breaker points, the points will often wear out and fail. Therefore, systems that contain breaker points must be inspected carefully. The breaker points can be located in two different locations on the engine. In most

76 Small Engine Ignition Systems two-stroke engines, the breaker points will be mounted under the flywheel. In four-stroke engines, the points may be located under the flywheel or under a timing cover. If the breaker points and condenser are located under the flywheel, the first step is to remove the flywheel. Flywheels are loosened by using special tools—either a knock-off tool or a flywheel puller. The type of tool used to loosen a particular flywheel will depend on the engine’s construction. We’ll discuss the removal of the flywheel in more detail in a later study unit. For this discussion of the ignition system, simply assume that we’ve removed the flywheel by lifting it off the crankshaft as shown in Figure 50.

FIGURE 50—The flywheel is being lifted off the crankshaft in this illustration. In some engines, the breaker points and condenser system would be located underneath the flywheel.

As long as the flywheel is removed, you can inspect it. Check for rusting, corrosion, and broken fins. (If a flywheel has a broken cooling fin, the entire flywheel should be replaced.) Test the magnets in the flywheel by placing a metal socket on each magnet. The socket should stick to the magnet when you

Small Engine Ignition Systems 77 shake the flywheel. If the magnets have lost their power, the magneto system won’t work; in such a situation, the flywheel must be replaced. With the flywheel removed, thebreaker points cover will be exposed. This metal cover protects the breaker points and condenser from liquids and dirt. This cover can be removed by removing the two retaining screws and lifting off the cover as shown in Figure 51. If the breaker points and condenser are mounted behind a timing plate cover in the upper cylinder head area, simply remove the cover screws and the cover.

FIGURE 51—Once the flywheel is removed, you can remove the breaker points and condenser cover.

The breaker point contacts open and close many thousands of times during engine operation. In addition, there’s always a slight amount of arcing between the contacts as they begin to open or begin to close. Therefore, there’s usually a great deal of wear and pitting in the contact area.

78 Small Engine Ignition Systems In the first stages of point contact wear, the points will begin to pit. Next, the pit will become larger. The material from one contact’s pit may be deposited on the second contact. This is shown in Figure 52. Pitting can become so bad that the contact sets will stick or weld together. Obviously, this will cause the engine to stop running.

FIGURE 52—After a period of use, the breaker points will often fail due to pitting of the points’ contact areas.

Replacing the Points

If the breaker points are worn, they can easily be replaced. A procedure for changing the points will usually be provided in the service manual for the engine. However, a general procedure would include the following steps: Step 1: Locate the retaining screw. Remove the screw and the points. Note the position of the points so that you can reinstall them correctly. Step 2: As you lift out the points, you’ll see that a wire from the primary side of the coil and a second wire from the condenser will be attached to the points. Remove the nut that holds these wires on the points and remove the wires. Step 3: Locate and remove the condenser retaining screw. Remove the screw and then lift out the condenser. A condenser removing tool can be used to lift out the condenser once the screw is removed (Figure 53). Make note of the position of the condenser when you remove it. (Note that in the particular system shown in Figure 53, the stationary contact point is actually a part of the condenser.)

Small Engine Ignition Systems 79 FIGURE 53—A special tool can be used to remove the condenser as shown here.

Step 4: Before installing the new points and condenser, compare the old parts to the new parts. Make sure that they’re the same size and that their mounting holes are positioned in the same locations. Step 5: Install the new condenser and tighten its retaining screw. Step 6: With the condenser mounted, connect the wire from the condenser and the wire from the ignition coil to the points and tighten down the retaining nut. Step 7: Mount the points inside the enclosure.

Setting the Point Gap

The small distance between the breaker point contacts is called the point gap. This is a precision measurement that’s determined by the manufacturer, just like the spark plug gap. Therefore, this gap must be measured to ensure proper functioning of the points.

80 Small Engine Ignition Systems A blade-type feeler gage can be used to measure the point gap. This tool is illustrated in Figure 54. Each feeler gage blade is labeled with its thickness. Determine the proper gap width by checking in the service manual for the engine. (This gap will normally be in the range of between 0.020 inch and 0.030 inch.) Then, rotate the crankshaft of the engine until the points are at their full-open position. Find the feeler gage blade that matches the gap width specified in the manual. Then, insert the blade between the point contacts as shown in Figure 55.

FIGURE 54—A blade-type feeler gage is shown here.

Small Engine Ignition Systems 81 FIGURE 55—In this illustration, the point gap is being measured with a feeler gage.

If necessary, you can combine two or more blades together to get the exact specified width needed. Simply rotate the proper blades out of the tool and hold them together to measure the gap. If the point gap width is correct, you should feel a slight drag between the point contacts and the gage blade. If the gap is too large or too small, the point gap should be adjusted until the width is correct. To adjust the point gap, loosen the point gap adjusting screw and move the points until the proper gap is reached. Or, in some engines, you can insert a screwdriver into a slot in the points and twist the screwdriver to open or close the points as necessary (Figure 56). Tighten down all screws to complete the job, recheck the gap, and then reinstall the cover. Some manufacturers recommend different methods for setting the point gap in their engines. These methods may involve the use of a tool called a dial indicator and other tools or instruments. These techniques will be described in detail in their service manuals. We won’t describe all these different methods here because you’ll seldom see breaker points systems

82 Small Engine Ignition Systems in your day-to-day work. Therefore, you probably won’t be setting a point gap very often. However, be aware that these methods are described in manufacturer’s manuals if you need further information.

FIGURE 56—In some breaker points assemblies, a screwdriver is inserted into an adjustment slot to adjust the gap. (Courtesy of Kohler Co.)

Adjusting the Magneto Air Gap

The air gap of a magneto is the very small distance between the rotating magnets and the armature of the ignition coil. The air gap is another precision measurement that’s determined by the manufacturer. Ideally, the air gap should be as small as possible, because the closer the moving magnets are to the armature, the greater the amount of current that will be induced in the primary winding. However, the magnets should never come in contact with the armature during the rotation of the flywheel or rotor. In some engines, the magneto air gap can be adjusted. However, it’s not possible to adjust the air gap on all engines, particularly on those that contain electronic components. When the ignition coil is mounted outside the flywheel, the air gap is quite easy to measure and adjust. Look up the proper specified gap measurement in the service manual. Then, measure the gap with an air gap gage. An air gap gage is simply an index-like card made of plastic or cardboard. The card is manufactured to a precise thickness. Figure 57 shows an air gap gage being used to measure a magneto air gap.

Small Engine Ignition Systems 83 FIGURE 57—The magneto air gap can be set by placing an air gap gage between the armature and the flywheel. The gage shown here is a small cardboard card of a precise thickness.

Note that you may also use a nonmagnetic feeler gage to measure a magneto air gap. You won’t get an accurate measurement if you use a steel feeler gage to measure the air gap because the magnets in the flywheel will pull on the gage blades. Instead, use a brass gage to make the measurement. If the magneto air gap isn’t correct, it can be adjusted by taking the following steps. Step 1: Align the flywheel magnets with the ignition coil armature. Step 2: Place the proper size of gage between the coil and the flywheel. Step 3: Loosen the armature mounting bolts. The magnets will pull the armature toward the flywheel until it rests against the gage. Step 4: Retighten the armature mounting bolts. Step 5: Remove the gage and check the clearance. Rotate the flywheel a few times to make sure no part of it contacts the armature.

84 Small Engine Ignition Systems Note that if the flywheel does contact the armature, the flywheel may be warped or loose, the flywheel key or the keyway may be worn, or the crankshaft may be bent. If the armature has no provision for adjusting the air gap, any of these conditions may be the cause of an improper air gap. If the coil is located under the flywheel, the following steps should be taken to adjust the magneto air gap. Step 1: Remove the flywheel. Step 2: Place a piece of electrical tape on the inside rim. Step 3: Replace the flywheel (finger tight). Step 4: Rotate it 10 or 12 times (removing the spark plug will make the engine easier to turn over by hand). Step 5: Remove the flywheel and examine the tape. Step 6: If the tape is scuffed, air gap clearance is too small. Step 7: If the tape isn’t scuffed, add another strip and repeat the test. If the tape is still not scuffed, the air gap clearance is too great. Step 8: Adjust the position of the armature, if possible, until the air gap clearance is such that one layer of tape isn’t scuffed but two layers are, when the flywheel is rotated.

Electronic Ignition System Service

As we discussed earlier, an electronic ignition module is a sealed plastic unit that contains both the ignition coil and the electronic switching components. Because electronic components themselves are very long-lasting, and because the components are all sealed in a protective plastic shell, electronic ignition modules are very dependable and don’t often fail. These modules also require almost no maintenance, and most modules can’t even be adjusted in any way. However, in some engines with electronic ignition modules, it’s possible to adjust the air gap. This procedure is performed in exactly the same way as we described earlier for the nonelectronic magneto ignition system.

Small Engine Ignition Systems 85 It’s also a good idea to perform a standard visual check on the module to make sure the wiring and terminals are all in good condition. Electronic ignition modules aren’t designed to be repaired—you can’t replace components inside the module. Therefore, if any problems are detected, the electronic module or unit should simply be replaced. The units are relatively inexpensive and readily available from parts suppliers for all types of engines.

Ignition Timing

In a small engine, the ignition of the fuel must happen at the exact proper time in order for the engine to run at full power. Ideally, the fuel would be completely burned so that all the expanding gases from combustion would force down on the piston when it begins the power stroke. However, since the fuel takes some time to begin burning, the spark must occur a little bit before the piston starts the power stroke. In most real engines, the spark occurs when the piston is still moving upward on the compression stroke. Most small engines have one ignition timing setting that’s determined by the manufacturer and listed in the service manual. If this setting varies, the engine will lose efficiency and power. However, it’s possible to adjust the ignition timing in many engines. Some engines (such as those manufactured by Tecumseh and Kohler) have a provision for adjusting the timing with the engine running. This requires the use of a special device called a timing light (Figure 58). This type of timing light is connected between the spark plug and the spark plug wire (in the first cylinder of multicylinder engines). The spark plug isn’t disconnected. Each time the spark plug fires, the timing light will produce a flash of light.

86 Small Engine Ignition Systems FIGURE 58—The timing light shown here is used to test ignition timing with the engine running. The timing light will flash each time the spark plug fires.

Engines that can be timed while running usually have spark advance mechanisms. If so, they’ll have two marks: a TDC mark and a timing mark. To check and adjust the timing with the engine running, perform the steps that follow. Step 1: Adjust the point gap according to the manufacturer’s specifications. If this involves removing the flywheel, theflywheel will have to be replaced before the timing can be checked. Step 2: Locate the timing marks. On some engines, particularly those with separately mounted (self-contained) magnetos, an inspection plate may have to be removed to expose the flywheel timing marks. It’s recommended that the timing marks be accented with chalk to make them easier to see. Step 3: Install the timing light as described above. Step 4: Start the engine and run it at about 1,500 rpm. This speed may vary, depending on the engine. Consult the manufacturer’s service manual or owner’s manual for the proper speed. The best results will be obtained if a tachometer is used to determine engine speed.

Small Engine Ignition Systems 87 Step 5: Aim the timing light beam at the timing marks. As the light flashes each time the spark plug fires, the light creates a stroboscopic effect. In other words, the timing marks will appear to be stationary because the only time they’re seen is when they’re illuminated by the timing light beam. If they’re perfectly aligned, the timing is okay. If they’re not aligned, adjustment will be necessary. Step 6: Engines with a provision for checking the timing while running will also usually be provided with a means of adjusting the position of the breaker assembly relative to the cam while the engine is running. The method varies depending on the engine and the type of ignition system. Externally mounted points, or the points of separately mounted magnetos, are usually adjusted by loosen- ing the point locking screw and rotating the point assembly around the cam with a screwdriver. Other systems may be equipped with a point position adjusting screw. In either case, make the necessary adjustments to advance or retard the spark until the timing marks are perfectly aligned in the timing light beam.

TROUBLESHOOTING IGNITION SYSTEMS

Now that we’ve covered basic maintenance procedures, let’s look at some troubleshooting information. Troubleshooting procedures aren’t part of a routine tune-up; these procedures are only performed when something is clearly wrong with the ignition system. The first consideration when troubleshooting an engine that doesn’t start or doesn’t run properly is whether or not there’s a spark or a consistent, properly timed spark. The second consideration is if the fuel is being properly delivered. Spark, fuel, and compression are needed to allow an engine to operate properly.

88 Small Engine Ignition Systems Testing for Spark

There are several different methods that can be used to test to see whether the ignition system of a small engine is producing a spark. When an engine won’t start, this is the first troubleshooting test you would generally perform. The simplest method of testing for spark (and the one that’s most often used by technicians) is to disconnect the spark plug wire from the spark plug terminal. Then, holding the wire with insulated pliers, place the end of the plug wire near the engine block. Next, have a second person turn over the engine (pull the starter cord or turn the key). A spark should jump from the end of the spark plug wire to the engine block. If the engine wasn’t starting but you get a spark during your test, the plug may be bad, the ignition timing may be off, or the engine simply may not have an ignition problem. Note that the spark you see should be strong, sharp, and blue-white in color. If the spark is weak or yellow-orange in color, there may be an ignition problem. If no spark at all appears, the ignition system has a problem, and you can proceed to further testing. (Most engine service manuals will contain detailed troubleshooting procedures for ignition systems.) The ignition spark can also be tested by using a small test light like the one shown in Figure 59. This tester is a special type of dual-purpose test light. Remember that the spark is created by a high voltage that’s passing through the spark plug wire and through the spark plug electrodes. This high voltage can be measured with the tester. If the tester lead is held near the spark plug wire, the high voltage in the spark plug wire will induce a voltage in the tester. This induced voltage will cause the bulb in the tester to glow each time the high voltage is present. However, remember that this high voltage is a shock hazard. Never place the tip of the tester on any metal parts of the ignition system. Also, never puncture the spark plug cable with the pointed tip of the tester.

Small Engine Ignition Systems 89 FIGURE 59—The low-voltage circuit tester shown here is being used to check for ignition problems.

A timing light can also be used to check the condition of the ignition system. An inexpensive timing light won’t require an external voltage source. Instead, the timing light has two wire test leads. Remove the spark plug wire from the spark plug terminal. Then, connect one of the timing light’s test leads to the end of the spark plug wire, and connect the other test lead to the spark plug terminal. Then, turn the engine over (either pull the starter rope or turn the key). The timing light’s bulb will then flash each time the timing light receives a pulse of high voltage. Other special tools are available that can be used to test for spark. For example, an automotive timing light may be used. These timing lights must be connected to a power source (such as a 12 VDC battery in a garden tractor or riding mower). Once connected to a battery, the third lead from the timing light simply clips onto the spark plug cable. This is an inductive pickup lead. The high voltage in the spark plug cable will trigger the timing light through this pickup and create a burst of light from the timing light bulb.

90 Small Engine Ignition Systems Problem of No Spark

Once you’ve determined that an engine’s ignition system isn’t producing a spark, the next step in the troubleshooting procedure will depend on the type of ignition system the engine has. If the ignition system uses a breaker points assembly, the points and condenser are the most likely cause of the “no spark” condition. To check this problem, remove the starter drive, flywheel, and breaker points cover to examine the points. Look for pitting, dirt, or moisture between the points contacts. In some small engines, the points are operated by a plunger rod that’s driven off the crankshaft. The plunger passes through a bushing in the crankcase. Sometimes, these bushings will wear and allow oil to seep past into the points and condenser area. The remedy for this situation is to remove the points and condenser and clean the area. Then, replace the bushing and plunger rod and install new points and a condenser. In an electronic ignition system, the problem of no spark may be caused by several factors. However, these are all simple to check for. First, check to make sure that the kill switch wire or grounding wire is properly connected and not shorted out. Then, measure the air gap to make sure this precision measurement is correct. If these items appear to be good, the problem is probably a failure in the electronic module. Replace the module with a known good component and test the engine. If it starts, you can assume that the electronic module was the problem. (If the engine still won’t start, the flywheel key may be sheared or the flywheel magnets may have lost their magnetism. Remove the flywheel to check for these conditions.) In most engines, it’s very easy to remove and replace electronic modules, and the components are relatively inexpensive. Note that there’s a testing device that’s used to test the condition of ignition modules. However, this piece of equipment is quite expensive and most small engine shops don’t have them. However, the low cost of replacement modules makes it easier to simply replace the module and test the engine.

Small Engine Ignition Systems 91 If an electronic ignition module is found to be faulty, remove the blower housing and locate the ignition module. Next, remove the two bolts as shown in Figure 60. With the two retaining bolts removed, you can gently lift the module away from the mounting bracket as shown in Figure 61. Install the new ignition module, but leave the retaining bolts somewhat loose. Check the magneto air gap by using an air gap gage, then tighten the bolts to complete the installation. In a battery ignition system, a weak battery can cause a no-spark condition. The battery can be checked with a voltmeter to see if the proper voltage (approximately 12 V) is present. The ignition switch, safety interlock switches, or safety interlock module can also cause a no spark condition. Note that we’ll discuss the testing of these circuits and devices in a later study unit.

92 Small Engine Ignition Systems FIGURE 60—An electronic ignition module can be removed by removing the two retaining bolts as shown here.

FIGURE 61—Once the retaining bolts are removed, simply lift off the ignition module.

Small Engine Ignition Systems 93 Weak Spark Problems

A weak spark condition can be caused by many factors. In a breaker points and condenser system, a weak spark is often caused by pitted or dirty points or a faulty condenser. A point gap that’s too large or too small can also cause a weak or mistimed spark. A faulty coil can also cause this problem. In a battery ignition system, a weak spark may be due to low battery voltage. This low voltage won’t allow the proper magnetic fields to be created across the primary and secondary windings of the coil. Bad battery contacts, bad ignition switch contacts, or a faulty connection at any wire in the ignition system can also cause a weak spark condition. In a magneto system (including those with electronic switching components), a weak spark can be caused by weak flywheel magnets. The permanent magnets used in a flywheel rarely fail. However, these magnets can lose their magnetism over time, or as a result of an impact to the magnets (if the flywheel is hit or dropped on the floor, for example). You can test the magnets by placing the blade of a large screwdriver about one inch away from the magnets. At this distance, you should feel a strong pull on the blade of the screwdriver. If the pull is weak, the flywheel should be replaced. A flaw in an electronic ignition module may also cause a weak spark. As we mentioned earlier, these modules can be tested by using a special testing device. However, this piece of equipment is rather expensive. Instead, the best method is to simply replace the module with a known good module and then try to start the engine. Before condemning any ignition module, always make sure that the module is in its proper position near the flywheel and that the air gap is the proper width.

Mistimed Ignition Spark

A mistimed ignition spark will usually be noticed as a hard- to-start or a “pinging” engine. In a breaker points system, the point gap is critical to ignition system timing. The point gap must be set to the manufacturer’s specifications as given on the engine plate or service manual.

94 Small Engine Ignition Systems On electronic ignition systems, the position of the module near the flywheel and the width of the air gap play an important part in ignition timing. Some older electronic ignition systems had slots in their armature coils that allowed both up-and-down and side-to-side motion. This type of coil is said to have both an air gap adjustment and an edge gap adjustment. In such an engine, the edge gap should be adjusted using the timing marks provided by the manufacturer. Another possible problem with a mistimed electronic ignition is that the flywheel key hassheared. A partially sheared key will be bent, while a completely sheared key will be cut in half. A partially sheared key will cause the flywheel to be out of alignment with the crankshaft, resulting in a mistimed spark. (If the key is completely sheared, the engine won’t start at all.) The solution is to replace the sheared key with a new key. A partially sheared key will appear as if the top and bottom section of the key are offset from each other (Figure 62). A normal key will usually look like a rectangular bar of metal. The best way to check the condition of a flywheel key is to compare its appearance to its picture in a parts manual.

FIGURE 62—This diagram shows the appearance of a partially sheared flywheel key.

Now, take a few moments to review what you’ve learned by completing Power Check 4.

Small Engine Ignition Systems 95 Self-Check 4

1. True or False? In an ignition system tune-up, you don’t need to remove the engine from the equipment.

2. The most accurate way to measure the width of a spark plug gap is to use a ______tool.

3. True or False? Electronic ignition systems have a high failure rate, so they should always be replaced during an ignition tune-up.

4. The electrodes of a used spark plug in normal condition will be colored ______or ______from carbon deposits.

5. True or False? The best way to clean a spark plug is to sandblast it.

6. A spark plug gap that’s completely blocked by carbon or oil deposits is called a/an ______.

Check your answers with those in the back of this book.

96 Small Engine Ignition Systems 97 AnswersAnswers Power Check Answers 2 Power Check Answers Power Check Answers 1 Check Answers Power False diode current True

False voltage grounding electrode amperes cold False A battery or a magneto negative, positive True True False resistance The condenser magnetic hot ohms The spark plug wire connects to the terminal nut of the spark plug. True

1. 1. 2. 2. 3. 3. 4. 4. 5. 5. 6. 6. 7. 7. 8. 8. 9. 9. 10. 10. 11. 12.

Power Check Answers 3

1. battery 2. False 3. stored charge in the capacitor 4. True 5. flywheel key 6. False

Power Check Answers 4

1. True 2. gapping 3. False 4. gray, tan 5. False 6. bridged gap

98 Self-Check Answers