Flywheels

The flywheel is a large and heavy carefully balanced disc, bolted onto the rear of the crankshaft to act as a reservoir for mechanical energy created by the power strokes of the engine.

The large mass of the flywheel assists in maintaining the crankshaft turning at a steady rate in between power strokes in order to assist smooth running of the engine. This is especially important at low running speeds when the time between strokes is longer. The mechanical energy stored by the flywheel is also used to assist each piston back up the bore and during induction, compression and exhaust strokes, when additional forces are imposed onto the crankshaft.

Road cars typically feature flywheels made from cast iron however those converted to racers commonly use a lighter billet steel alternative with aluminium generally being reserved for out-and-out racers.

Pictured below is a standard flywheel and alternative lightened version both from the BMC A-series engine;



The flywheel must also assist in carrying drive from the engine to the gearbox, via the clutch, which on a manual transmission vehicle is bolted to it and provide a flat, true surface on which the clutch friction plate can engage and disengage. Provision is also given to carry the ring gear around its circumference upon which the pinion of the starter motor engages in order to turn and start the engine.

Larger engines with a higher number of cylinders are not as dependent on the action of the flywheel owing to the increased number of power strokes per crankshaft revolution. Without the flywheel, the engine would have to reach a considerable amount of RPM in order to pull away without stalling.

Provision is given so that the flywheel may only be fitted in one position and therefore will not run out of true with the crankshaft, which would set up a heavy vibration placing components under much undue stress. However, a very small amount of imbalance is tolerable. The flywheel and crankshaft are ideally balanced as one, but this is not always possible. 

A more recent feature on today's modern engines, with their higher torque and power outputs, especially diesel engines is the dual-mass flywheel (DMF). The DMF features two flywheels, the input, bolted onto the rear of the crank as per a standard flywheel, and an output driven by and linked to the input flywheel by means of very strong springs, rubber, or a combination of the two in order to further dampen any vibrations from the engine and thus provide a smoother take-off of power upon moving off.

However, failure rates of dual-mass flywheels are much more common than the single-mass flywheel, which provided is not abused, is virtually indestructible!. For this reason, several owners of vehicles featuring a dual mass flywheel have where possible, converted to a single-mass flywheel.

Assembly of a typical dual-mass flywheel

Flywheels may be lightened by either turning material off the back face or by drilling holes around the periphery. Alternatively, a lightweight alternative to the standard article may be fitted.

A lightweight flywheel is a definite advantage for competition use as it improves the acceleration and rapidity of the gear changes, however, this is hardly a necessary modification unless competition work is envisaged. If it is carried out it should not be overdone or the engine will lose some of its low-speed flexibility and smoothness, and if properly done,this can reduce stresses in the crank.

Careful dynamic balancing of the crankshaft and flywheel is undoubtedly the most valuable modification that can be carried out on engines that are perpetually driven and high rev/min.,

A lightened flywheel must of course be rebalanced afterwards and the work is best left to experts as haphazard lightening and any resultant severe weakening of flywheels can be extremely dangerous, especially given that on some cars, the driver is sitting directly in line with it!