Once the mixture of air and fuel has been compressed by the piston in the combustion chamber, the engine needs some way to ignite the mixture. This task is performed by the engine’s ignition system. The ignition system produces a high voltage that’s used to create a spark in the cylinder. The heat from the spark ignites the air and fuel mixture, and the resulting explosion in the combustion chamber forces the piston down and gets the crankshaft turning.

The main components of the ignition system are the ignition coil, the spark plug, the spark plug wire, the power source, and the triggering switch. All ignition systems will contain these basic components.

A 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. The basic parts of a spark plug are the terminal nut, the ribs, the porcelain insulator, the shell, the threads, the center electrode, the gap, and the lower electrode.

A spark plug screws into a hole in the center of the cylinder head so that it will be located directly over the combustion chamber. The threads that screw into this hole are located directly above the plug’s electrodes. This threaded part is made of steel, and so are the two electrodes. The small space between the two electrodes is called the gap. When a voltage is applied to the spark plug, electricity will jump across this gap (from one electrode to the other) to produce a 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).

Note that the lower electrode is connected to the threaded part of the spark plug. The center electrode passes through the body of the spark plug and 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 spark plug wire is a heavily insulated electrical wire that leads from the ignition coil to the spark plug.

Before you begin any type of work on a small engine, you should always disconnect the spark plug wire from the terminal nut of the spark plug to prevent the engine from starting accidentally.

Let’s look at the ignition coil. This coil is actually a type of transformer. A transformer is an electrical device that’s able to convert a small voltage to a much higher voltage. A transformer contains two wire windings that are wound around an iron core. The first winding is called the primary winding and the second winding is called the secondary winding.

One end of the ignition coil’s primary winding is connected to a power source (the power source in small gas powered equipment will be either a battery or a magneto. For more information on these components, see below). When current from the power source flows through the primary winding, a magnetic field forms around the iron core of the transformer. If the current flow through the primary is suddenly shut off, the magnetic field will collapse and produce a high voltage in the secondary winding. In simple terms, this is the basic operating principle of a transformer.

In the ignition coil, the secondary transformer winding is connected to the spark plug wire. The spark plug wire runs directly from the ignition coil to the spark plug. When the high voltage is produced in the secondary winding, the electricity flows through the spark plug wire to the plug, down through the plug, and jumps across the spark plug gap, producing a spark. The spark then ignites the fuel and gets the engine running.

Note that in order to keep the engine running, the ignition system must keep on producing one spark after another very rapidly. Since the high voltage in the secondary coil of the transformer is only produced when the current flow from the power source is stopped, in order to keep producing sparks, the ignition system needs a device that will keep turning the current from the power source on and off.

This action is performed by the triggering switch in the ignition system. The triggering switch keeps turning the current from the power source on and off in order to make the transformer produce one high voltage spark after another. Every time the triggering switch opens, the current flow from the power source stops and the spark plug fires. As the switch keeps opening and closing, the rapid series of sparks produced keeps the engine running. Depending on the type of ignition system, the switch may be a set of mechanical contacts or an electronic switch.

There are two basic types of power sources used with ignition systems in small engine applications. These are the battery and the magneto. Larger garden tractors, snowmobiles, and similar equipment may use batteries to power their ignition systems. The battery used in this type of system is a lead-acid battery, similar to the type used in an automobile. Besides providing electricity for the ignition system, the battery may be used to power key start systems, lights, horns, and other accessories.

A magneto is used in equipment where electricity is only needed to power the spark plug, not a starter system or lights. A magneto is an electromagnetic device that can generate electricity. In a magneto, permanent magnets are attached to either the flywheel or to a rotor that turns the crankshaft. A permanent magnet always has a force called a magnetic field around it. When a magnetic field is moved near a conductor wire, electricity will be generated in the wire. If the conductor is wound into a coil that has many turns of fine wire, and a magnetic field is moved near the coil, a very large amount of electricity will be generated in the coil. This is the basic principle of operation that runs a magneto.

In most magneto systems, the wire coil remains stationary (in a fixed position) while the magnet moves. That is, when the starter rope or cord is pulled, the flywheel or the rotor spins. The spinning action moves the magnets near the ignition coil, which in turn generates electricity to produce a voltage and current.

This is a very basic description of the operation of ignition systems in equipment that uses small gas powered engines.


Once the air and fuel mixture compressed in combustion chamber of small gasoline engine, engine needs to ignite it. Ignition system performs this task. Ignition system produces high voltage to cause spark plug to fire. Spark from plug very hot, ignites fuel and air. Resulting explosion forces piston down, gets crankshaft turning.

During compression stage of engine operation, piston rises to compress air and fuel mixture in combustion chamber. When piston reaches top dead center, spark plug fires, ignites compressed mixture. Igniting mixture produces power stage. Power produced by ignition gets crankshaft turning, keeps piston moving, engine running. Ignition process will keep engine running for as long as fuel lasts and plug keeps firing.

Ignition system must produce very high voltage to force current (moving electrons) across spark plug gap. As many as 20,000 volts needed. Spark must occur at right time in engine cycle to ignite mixture properly. Engine requires many sparks per minute to keep running at proper speed. Four stroke engine operating at 3,600 rpm requires 1,800 sparks per minute. If engine has more than one cylinder, multiply number of cylinders by 1,800 to determine needed sparks.

Main components of ignition system are power source, ignition coil, spark plug and spark plug wire, triggering switch, and stop switch.

Consider ignition coil. All ignition systems contain coil. Coil a type of transformer. Transformer consists of two wire windings wound around iron core. This core called an armature. First winding is primary winding, the second winding the secondary winding. Secondary winding has many more turns of wire than primary.

In ignition coil, one end primary winding connected to power source. Power source may be a battery or a magneto.

When current passes through primary winding, magnetic field created around iron core. This magnetic field induces voltage in secondary winding via concept mutual inductance. If current flow through primary is stopped, magnetic field collapses and produces high voltage current in secondary winding.

Because secondary winding has many more wire coils, voltage produced in secondary is much higher than original voltage applied to primary. Power source supplies about 12 volts to primary, ignition coil increases that voltage to 20,000 or more.

Secondary winding of coil connected to spark plug wire. When magnetic field collapses and produces high voltage in secondary, current runs to spark plug, causes spark to jump across spark plug gap. Spark ignites air and fuel.

High voltage in secondary winding only produced when current stops. Current from power source passes through primary winding of transformer, and when current flow stopped, magnetic field collapses, high voltage is produced in secondary. This means ignition system needs device to keep turning current from power source on and off.

Device that turns current on and off is switch. Ignition system’s circuit closed when switch closes. When switch closes, current flows from power source to transformer. When switch opens, flow of current stops, magnetic field in transformer collapses, producing voltage needed to fire spark plug.

If you want to stop engine, need to activate stop switch (also may be called grounding switch or kill switch). In some engines, stop switch will stop flow of electricity to spark plug. This type switch will be small metal lever connected near spark plug. Simply push in lever to stop engine. In other engines, stop switch designed to prevent flow of electricity through primary windings of coil. This type switch connected to throttle. Move throttle to stop position, engine will stop.

Consider components that vary in different ignition systems. These components are power sources and switching devices.


In outdoor power equipment, two power sources used, battery and magneto. In battery ignition system, lead acid storage battery connected to ignition coil. Battery provides voltage to operate coil.

Magneto systems more common than battery. Magneto ignition systems use electromagnetism to produce voltage. Recall generator action of electromagnetic induction. By this principle, when a conductor wire moved through magnetic field, voltage induced in conductor. If magnet moved near conductor, voltage induced in conductor. If conductor connected to complete circuit, current will flow in conductor wire. This is principle of magneto.

In magneto system, magnets installed in flywheel. Coil mounted at stationary point near flywheel. As flywheel spins, moving magnets cause voltage in primary winding of coil. Magneto has several advantages over battery. When magneto used, no on board battery necessary. No separate charging system for battery required. Machines with magneto also require less seasonal maintenance.

Battery system also have advantages. Battery can also be used to run accessories like headlights and electric start. Most magneto systems can only provide power to spark plug. Machines that contain battery systems can be started with key. Magneto systems activated by cord or rope.


Three types ignition systems in outdoor power equipment: the magneto ignition system, the battery ignition system, the electronic ignition system. Magneto system used in equipment where electricity only needed to power spark plug. Garden tractors that have starter systems and lights use battery systems. Electronic ignition systems contain electronic components that perform switching action in ignition circuit. Electronic systems often found on newer engines.

All three systems use ignition coil (a transformer). Magneto and battery systems similar in function, just use different power sources. Battery or magneto system may use points or electronic switch to perform switching. Electronic system uses electronic components to perform switching, but power source will still be battery or magneto.


Following two types ignition systems used most often in outboards:

1)Conventional flywheel magneto system with breaker point contacts operated by a crankshaft lobe.
2)The flywheel magneto system with electronic ignition module (capacitor discharge type).

As technician working on outboards, should know how to service and repair breaker points due to fact that service life outboard motors usually considerably longer, giving technician far greater chance of encountering older model than might be the case with other kinds power equipment. Newer outboards of course employ electronic ignition systems. In outboards, the electronic ignition module often called a pulse pack or power pack. Power pack usually contains the capacitor, SCRs, diodes, and any other components needed to trigger.


Electromagnetism important to operation of ignition systems. Electromagnetism is the magnetic effect produced when current flows through a conductor. When conductor wire carrying electric current, wire will be surrounded by a magnetic field. Magnetic field is space around a magnet or magnetic object that contains a force of attraction. This force of attraction called magnetic lines of force or magnetic flux. Magnetic field strongest in space immediately surrounding conductor.

If insulated piece conductor wire looped around to form a coil, resulting device called a magnetic coil. When current flows through magnetic coil, each loop of wire develops small magnetic field. Small magnetic fields around each loop of wire combine to form larger stronger magnetic field around entire coil. Coil develops a north pole and a south pole. Magnetic field at center of magnetic coil is stronger than fields above or below coil.

An electromagnet is device made by inserting piece of magnetic material (usually iron or soft steel) into a magnetic coil. Piece of metal around which conductor is coiled called the core. When current applied to coil, core becomes magnetized, develops north and south poles. Addition of metal core to coil increases magnetic force of coil. Electromagnet much stronger than a magnetic coil of similar size.

Some electromagnets have special movable cores. This is called a solenoid. Inside solenoid coil, core is movable round metal piece called a plunger. When solenoid coil energized by flow of current, resulting magnetic field pulls plunger into coil. When current stops, spring above plunger presses plunger back into original position. Solenoids used in starter systems of garden tractors and riding mowers.


When current flows through conductor, magnetic field produced around conductor. Also true that if conductor moved through magnetic field, voltage produced on conductor. If conductor wire connected in complete circuit, current will flow through conductor wire. This effect called generator action of magnetic induction. Note that current won’t flow through wire until wire connected in complete circuit.

Consider a conductor wire being moved between north and south poles of two magnets. Magnetic lines of force will be moving from right to left. When conductor moved upward through magnetic field, a voltage will be induced on conductor, and current will flow through wire. Current will only flow, however, if conductor is part of closed circuit.

Generator action of electromagnetic induction basic property used to operate electric generators. In electric generator, component called armature turned in magnetic field to produce electric current. Armature made up of many loops conductor wire. Magnets are positioned on both sides of inside of generator to produce magnetic field. As armature is turned within magnetic field, electric current produced.

If we wound many loops of wire into a coil, turned coil in magnetic field, a much larger voltage and current would be produced. The amount of voltage and current produced by generator based on three things:

1)The number of turns in coil and diameter of wire
2)The strength of magnetic field
3)The speed at which wire coil passes by magnets

Small engines use generator action of electromagnetic induction to power ignition systems. Coils placed underneath flywheel or next to edge of flywheel. Magnets are embedded in edge of flywheel. As flywheel spins, magnets pass by coils and generate necessary voltage and current to operate ignition system.

Some larger pieces outdoor power equipment use generator action of electromagnetic induction to charge batteries. Belt driven generators or alternators charge the batteries, and energy from the batteries used to power ignition systems.