In its simplest form a flywheel is a heavy cast iron or aluminum alloy disk or wheel mounted on one end of the crankshaft (see Photo 1). This end of the crankshaft is tapered (see Photo 2) and the flywheel has a matching taper in its center hole (see Photo 3). The tapered fit is held tight with a nut or bolt. Rotating with the crankshaft, the flywheel's mass provides inertia to the system. As the crankshaft and piston finish the power stroke, the flywheel provides the necessary momentum to continue through the exhaust, intake and compression strokes. Without the flywheel's momentum, the piston would travel to the end of the power stroke and stop at bottom dead center. The engine then enters the power stroke again. The flywheel smooths engine speed by resisting both acceleration during the power stroke, and deceleration during the other strokes. Its inertia also smooths engine speed as the external load varies.
A permanent magnet is mounted on the flywheel. As the flywheel rotates, the magnet passes the ignition coil inducing a voltage in it (see Photo 1). The coil provides the spark to the spark plug. In order for the spark to arrive at the correct moment, just before top dead center of the power stroke, the flywheel with its magnet must be assembled to the crankshaft in exactly the right position. To accomplish this, a groove called a keyseat is cut lengthwise into the taper of the crankshaft (see Photo 2) and a corresponding keyway is cut into the flywheel's tapered center hole (see Photo 3). A key (see Photo 4) fits into the keyseat and keyway during assembly, aligning the parts in the correct position. In some two-cycle applications the flywheel key is cast in place as part of the flywheel.
The flywheel may have fins on it that act as a fan when the flywheel spins (see Photo 5), or it may have a separately molded fan that is mounted to it (see Photo 6). A shroud, often called a blower housing, covers the flywheel and much of the engine. The shroud is shaped so as to direct airflow from the flywheel fan across the engine's hot spots, notably the cylinder and head. These parts have fins that aid the cooling effect by increasing the surface area available to the air.
Engines that have the option of an electric starter will have a ring gear around the flywheel's circumference. The ring gear may be cast as an integral part of the flywheel, or it may be a separate piece that is pressed or riveted onto the flywheel (see Photo 7 ).
Engines that have a pull starter may have a starter cup (see Photo 8 ) or starter pawls (see Photo 9 ) mounted to the flywheel. In the former case, the rewind starter's pawls (see Photo 10 ) extend outward from center when the rope is pulled, and engage the cup's indentations . In the latter, the rope pulley has gear-like teeth (see Photo 11 ) that engage the flywheel's pawls. When the engine is running, centrifugal force pushes the pawls outward, away from the pulley. At rest, springs push the pawls inward, ready to be engaged.
Many Briggs and Stratton engines with horizontal crankshafts have a starter clutch (see Photo 12 ) that also acts as the flywheel mounting nut. At rest, steel balls inside the clutch act as a ratchet and engage the starter pulley to the flywheel / crankshaft. At speed, the balls are propelled outward by centrifugal force, disengaging the clutch.
A flywheel may have additional magnets mounted to its underside (see Photo 7) which rotate around a stator coil (see Photo 13) for the purpose of generating electric current. The current may be used to operate lights, an electric clutch, or to charge a battery that powers an electric starter. Engines used in applications where less current is required may have a coil mounted alongside the ignition module (see Photo 14). This coil is energized by the same magnet that energizes the igniton module.
Since 1982 walk-behind lawnmowers for homeowner use are required to have a braking mechanism that will stop the blade within three seconds after the operator releases the control handle. The brake applies friction to the circumference of the flywheel (see Photo 15) helping to stop it quickly. Simultaneously, the ignition coil is grounded so that no spark reaches the spark plug. The engine's compression and internal friction combined with the flywheel brake should stop the engine within three seconds. Mowers that have a blade brake / clutch are not required to have a flywheel brake.
The air movement created by the flywheel fan is used on some engines to operate a pneumatic governor (see Photo 16). The faster the engines runs, the more forcefully air is pushed against the governor's air vane, which in turn moves the carburetor's throttle toward the slow position. Opposing the air vane is a spring which tends to pull the throttle toward the fast position. The engine will run at a steady speed when the spring and the air vane balance each other. As the external load on the engine changes, the governor will open or close the throttle to acheive the correct speed.