Small Engines Ignition System

Ignition System Function Ignite the fuel and air mixture at the proper time. Advance and retard the ignition timing as needed. “Ground-out” the ignition system so the engine will stop running.

Ignition Parts Battery type Battery Ignition coil Ignition switch Low voltage wires (battery volts) Ignition Pick-up ( points or electronic) High Voltage wire(s) Spark Plug

Ignition Principles Electromagnetic Induction How does the Ignition Coil work? Primary winding: creates a magnetic field by running current through it. When we open the circuit current stops, and the electromagnetic field collapses.

Ignition Coil Parts Primary Winding Secondary Winding Iron Core

Switching current in Primary Breaker Points and condenser Points: Mechanical switch Condenser: makes the switch last longer so the points don’t “ burn out”

Self Induction When you run current through a coil of wire, you create a magnetic field. When you “open” the current flow through that coil of wire you collapse that magnetic field into itself.

Coil output You produce about 1 volt per turn of wire Example: 225 turns of coil wire can produce up to 225 volts, however your current output drops at the same rate that you voltage increases.

Induction Principle Coil of wire + Current flow = Magnetic field Magnetic field + Coil of wire = Current flow *Remember you must have the magnetic field or the coil of wire moving to have induction

Mutual Induction When you have two coils of wire in one housing. You create a magnetic field with one coil (primary) You collapse the magnetic field into both coils Primary and Secondary

Mutual Induction Output You can increase or decrease voltage output of your induction coils, by changing the primary and secondary windings ratio. Example: primary 250 turns secondary 20,000 turns Secondary output could reach up to 20,000 volts!! This is called a Step-up transformer

Continued 250 turns primary and 20 turns secondary. Maximum output 20 volts. This is a “step-down” transformer.

“Magnetron Ignition”

Magnetron Coil The magnetron coil actually contains 3 separate coils of windings. In addition to the primary and secondary windings, there is a 3rd winding, called the "trigger coil”.

Magneto Ignition

Building the magnetic field." When the leading edge of the magnets in the flywheel approach the coil, the magnetic field surrounding those magnets generates a small voltage (about 1 volt) that powers a solid-state switching device called a Darlington Transistor, turning the transistor "on." That "on" mode completes the circuit on the primary winding and a current of about 3 amps flows to ground on the crankcase

Collapsing the magnetic field As the flywheel continues to rotate that current builds the magnetic field around the secondary winding.When the trailing edge of the magnets the flywheel reaches the trigger coil, a second small current is induced in the trigger coil and that current tells the transistor to "turn off", effectively breaking the circuit of the primary winding to ground (Figure 3), the sudden break causes the magnetic field to collapse.

Since current (amperage) and voltage are inversely proportional, suddenly stopping the current flow causes the voltage to "spike" as it compensates. It's that spike that builds the voltage required in the secondary winding to jump the sparkplug gap and ignite the mixture. By the way, in the Magnetron coil, the primary winding has 74 turns of wire and the secondary has That's a 59.5:1 ratio and that helps insure that there's enough voltage to get the job done.

All this, the initial current in the trigger coil, the buildup of potential, the second trigger coil pulse, and the collapsing field with the resulting voltage spike and spark, in on about 10 degrees of crank rotation. That means that at 6000 RPM the whole thing, start to finish, only takes about seconds. Pretty amazing. This simple, but very effective, spark control system even advances the ignition timing, causing the plug to fire earlier (in degrees of crank rotation) as RPMs go up.