Small Engine Ignition Systems

Basics The ignition system is responsible for providing the spark to ignite the fuel mixture. Most small engines have a magneto ignition system This system uses a rotating magnet generator, coil, condenser, breaker points and a spark plug. Newer engines use solid state (electronic) ignition systems that get rid of the breaker points

Another style of ignition systems uses a 12 volt battery to produce the spark, with breaker points or solid state electronics

Magneto Ignition Systems that use Breaker Points A strong spark across the spark plug is required to ignite the air fuel mixture. Electricity is produced through the spinning magnet mounted on the flywheel. The spark occurs slightly before TDC.

2 Circuits The ignitions system of a small engine uses 2 main circuits The first is called the primary circuit. This contains the primary winding and 2 leads. 1 lead connects to the ground, while the other is attached to the breaker points The secondary circuit contains secondary coil winding and its 2 leads (one of which is grounded the other connects to the spark plug), a spark plug wire, and a spark plug.

Components Flywheel Magnet This a permanent magnet mounted in the flywheel rim. Other magneto ignition components can be mounted either inside or outside of the flywheel. As the magnet spins close to the armature legs of the leg, a electric current is produced. This current is eventually used to create the spark for the spark plug.

Armature The purpose of the armature and its legs is to pick up and build up the magnetic field from the moving magnet. The armature is not wired to the magnet, but the a magnetic field is created through a process called induction. The armature has a iron core, in which the magnetism is concentrated and rapidly reversed from one direction to another. Main adjustment is the space between the armature and the magnet. This is called the armature air-gap. For some engine, this is adjustable, for others it requires flywheel replacement to change. Bad main bearings can cause problems also as the flywheel will move around.

Coil The main function of the coil is to increase the voltage so the spark will jump across the spark plug. It takes a low voltage and turns it into a high voltage. Coil is made up of primary and secondary windings. Primary winding is about 175 turns of heavily insulated wire. The secondary contains about 10,000 turns of very thin, insulated wire wound outside of the primary windings.

Breaker Points These are part of the primary circuit. The 2 contact points are normally closed providing a path for the electricity to ground The points are mechanically opened by the camshaft or crankshaft. They are closed again by a spring. If opened by the camshaft, it opens once every 2 revolutions. If opened by a crankshaft, it opens every revoltion, thus creating a waste spark.

Breaker Points

Breaker Points Function When the points are close electricity flows through the primary circuit to ground which is the frame of the vehicle When a spark is needed, the points open which breaks the flow of electricity. This causes the magnetic field to reverse, and electricity flows through the secondary windings. Adjustment needed is the gap between the breaker points when they open, this is called the breaker point gap. This is the widest gap that can be created by the points A flat feeler gauge is used to measure this gap.

Breaker Point Adjustments A wider gap advances the spark, whereas a narrower gap retards the spark. A good ground is essential for the system to work. To test the breaker points, use a ohm meter set to Rx1. Good grounding rates of .2 to .3 with the gaps closed indicating the points are working

Engine Kill Switch A “kill” lead, with a normally open switch connected to ground is often used to short out the primary circuit. Using a kill switch causes very little arc burn on the points, as compared to pulling the spark plug lead which does burn the points. A remote kill lead also allows manufactures to install a kill switch on the handle bars for more convenient stopping. The kill terminal is also a good place to connect a tachometer.

Condenser This is a storage reservoir or “surge chamber”. It is made of 2 strips of foil with paper insulation in between them. The foil strips are not connected. The strips are layed out so that one is grounded and the other connects to the coil on the primary side. The condenser receives and stores the surge of current on the primary circuit to prevent excessive arcing across the breaker points. It also helps in the rapid collapse and reversing of the magnetic field of the coil Have condensers checked before installing, even if new. They do go bad, and cause other component failure.

Spark Plug This is a vital part of the spark system It provides the gap across which the spark created in the secondary windings, travels to ignite the fuel. The spark plug consists of the shell, ceramic insulator, center electrode, and ground electrode. The 2 electrodes are usually separated by a gap of .025 to .055 inches, depending on the engine Breaker points are generally .02 to .03, while solid state is .03 to .045. Electricity flows down the center of the plug, across the gap, and to the ground electrode.

Spark plugs are designed to operate at 700 to 1500 degrees. Most small engines run satisfactorily with plugs rated at 900 to 1350 degree. Some engines use a spark plug kill. This is a spring loaded plate that is pressed down onto the spark plug to short it out. This is hard on breaker points as it burns them from the arcing created

The Magneto Cycle When the magnet is away from the coil, it has little to no effect on the coil. But as the magnet moves closer to the coil, it “feels” the increasing magnetic field. The magnet is acting on the coil, causing electricity to flow in the primary circuit. This current passes through the breaker points into the ground. When the magnet is near the legs of the armature, the current produced is at its highest.

The Magneto Cycle When current output is at highest, the piston is nearing TDC. When the points open, the flow of electricity is stopped and the magnetic field collapses and is reversed. This causes high voltage to move through the secondary winding and causes the spark to jump across the gap.

Battery Ignition Systems with Breaker Points This type of systems operated very similar to as breaker points in a magneto system. Major difference is that the battery serves as the source of electricity to energize the coil rather than a magnet passing by an armature. Inside the coil, the primary and secondary windings run parallel to each other. The windings start with the positive terminal and end at the chassis ground. When the points are closed, the current passes through the points, to the ground. When the points open, the current cannot move to the ground, but it needs to go somewhere. The only remaining connected ground is the spark plug.

Battery Ignition with Breaker points

Magneto Breaker Points without Breaker Points This type of system is called a solid state ignition. 2 types CDI- Capacitor Discharge Ignition Traditional Coil with Solid State Trigger Coil

CDI Solid State This type of system has a different design, but still has the same job, deliver a spark to the spark plug. Testing of these coil types requires specialized equipment that is different for each engine manufacturer. This type of system creates AC current, so a rectifier is used to change the output to DC. The changed current than travels to a capicator for storage. The flywheel turns 180degree past the coil and triggers a trigger switch, which creates a small current that turns on the rectifier When the rectifier closes, the current than passes to the primary windings A step up transformer than ups the voltage of the current.

Traditional Coil with Solid State Trigger Coil Basically, it is traditional coil/breaker point set up, minus the breaker points In there place a trigger coil is placed on the traditional 2 legged armature. The trigger coil has a trade name of Magnatron. It is activated by the spinning magnet on the flywheel. A problem with this systems is that if the flywheel key is sheared or partially sheared, a good spark will still be made, but the ignition timing will be off, and the engine will spark at the wrong time. Ignition timing is controlled by the flywheel key, and a good strong key is essential.

Testing of the soil is done with a spark plug tester. The engine must be tested at 350 rpm, which is about fast the engine is spun with a typical rewind starter. Breaker points engines can be “retro fitted” with the newer style electronic ignition.

Ignition Timing This needs to happen before the engine is at TDC of the compression stroke. How far before depends on the engine type and use. Typically anywhere from 25 to 5 degrees before TDC. Most small engines have a precise moment they are to spark. Breaker Points will spark the moment the points move away from each other Electronic systems fire as soon as the trigger coil is activated

Ignition Timing Problems If the spark happens to soon, the engine may knock, because the pressure of the burn is working against the compression of the motor. This can result in serious engine damage, cracked heads, broken rods and pistons Engine kickback apon starting can also be a result of too advanced timing.

Ignition Timing Problems Retarded or delayed spark can create a loss of power, as the fuel is not being ignited as it is fully compressed and the piston is already moving down due to the crankshaft. Automotive engines automatically adjust the ignition timing through computers, vacuum operated distributers, or weights in the distributer. Some small engines use counterweights to do the same thing and adjust the ignition timing. This si typical with a 2 stroke engine.