The Effect of Flywheel Mass
It is easy to accept the idea that lightening the flywheel of a car will provide improved acceleration. The questions are, by how much and what are the drawbacks?
The first point to understand is that the mass of the flywheel makes no difference at all to power output from the engine in any steady state condition. Where it does make a difference is when the engine is accelerating from one speed to another, because energy expended in accelerating the flywheel (and other moving parts of the engine) must be deducted from that available to accelerate anything else driven by the engine.
The term ‘radius of gyration’ is used to describe the radial path through which the mass of a flywheel is said to act – rather like the centre of gravity of a beam. For the purpose of discussion, when we refer now to the mass of the flywheel we will be referring to the mass acting at the radius of gyration. We will also disregard friction and other peripheral losses that might apply in reality.
When an engine with a flywheel of a certain mass accelerates from 1000 r.p.m. to 5000 r.p.m. the energy required to accelerate the flywheel will be constant regardless of the rate of acceleration.
If the engine accelerates at full throttle against a load so that it take 10 seconds to increase its speed over that range then it will expend a certain amount of energy in order to do so.
If now the engine accelerates at full throttle against a different load so that it takes only 5 seconds to reach the higher speed then it will have expended half the energy it required the previous time.
Now in both cases the energy required to accelerate the flywheel will be the same so in the latter case the final energy output will be depleted more than for the former.
It follows that the mass of the flywheel becomes more of a deficit the faster the engine is accelerating. It therefore also follows that the potential advantage from reducing the flywheel mass is more significant as the power/weight ratio of a vehicle increases. Hence a light powerful vehicle (such as a racing car or high performance motor-bike) will be affected by a change of flywheel mass much more obviously than a large heavy car. The difference will be most marked in low gears when acceleration is at its most vigorous and becomes less important at high speeds where acceleration tails off for a variety of reasons.
The unfortunate point is that acceleration in low gears is often limited by tyre adhesion so the effect of lightening the flywheel might then be more dramatic than truly physical.
Of course an engine out of gear with no load on it will appear to be much more lively if the flywheel is lightened because that is all it has to drive.
The other less obvious effect of lightening the flywheel is the reduction in overall vehicle weight which in many cases will not be significant enough to notice.
Practical Factors Affecting Flywheel Mass.
The first thing that should be obvious is that there is no reason at all why any manufacturer would want to use an excessively heavy flywheel. It would add cost, and in some very small way could reduce performance and worsen fuel consumption and exhaust emissions. The mass of the flywheel is determined mainly for its effect on engine and drivetrain refinement.
It is worth remembering the primary reason for having a flywheel at all. Its main purpose is simply to smooth the delivery of power between individual power strokes of the engine. In effect the flywheel stores energy as each firing impulse applies acceleration and releases it during the period until the next impulse. The rotation is therefore always slightly erratic even when the engine has a lot of cylinders. That this is so is illustrated by the OBD2 diagnostics on the final version of the Jaguar V12 engine used in the X300 XJ12 saloon. By monitoring the rate of rotation of a 12 toothed rotor mounted next to the flywheel the system could identify any particular cylinder if it were to suffer even a partial misfire and take appropriate action to prevent catalyst damage.
Like most things in life the mass of the flywheel is a compromise. Lighten it and there will be a loss of refinement, particularly of idle quality. It will increase accelerative performance but the benefits will be more pronounced in low gears and where the car has a high power/weight ratio and good traction. The least advantage will be observed with a heavy car having indifferent traction.
In essence the more accelerative performance a vehicle already has the more there is to be gained from using a lighter flywheel. Money is probably better spent elsewhere if the vehicle in question is of relatively moderate performance.