- A small engine ignition system consists of few parts; the flywheel, ignition coil, sparkplug cable and the sparkplug. Newer engines may also have an ignition module to amplify the voltage created by the flywheel magnet and ignition coil.
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A small engine flywheel has a magnet cast in to its outer edge. As the engine rotates this magnet passes by the two poles of the ignition coil at a distance, called an air gap, of a few thousands of an inch. This generates an electrical current which is amplified within the ignition coil to a voltage sufficient to fire the sparkplug.
Multi-cylinder engines have two ignition coils, each firing one or two cylinders simultaneously. -
An ignition spark must take place at a precise moment or the engine will run poorly or at reduced power. The timing of the spark in the cylinder is determined by a groove in the engine crankshaft and another in the flywheel where it mounts on the crankshaft. A small half-moon shaped piece of steel, called a key, fits snugly in these two grooves, aligning the flywheel position with the top of the piston stroke.
This method forces the magnet to pass by the coil just prior to the piston's reaching top dead center (TDC) of its stroke, producing a current at that moment, which travels down the sparkplug cable to fire the sparkplug at TDC, forcing the piston down on its power stroke. In multi-cylinder use, the coil fires two sparkplugs at the same time, one on the power stroke and the other at the bottom of its travel. This spark is essentially wasted as it does nothing to produce power at this point. Small multi-cylinder engines are designed this way to eliminate the need for an ignition coil for each cylinder.
