Lightweight flywheel: how an easy action can result in a disaster



Of all the parts that can be replaced or modified, as allowed by the technical rules, when tuning a road car engine into a racing unit, the flywheel is among the first ones on which attention is turned on.

To better understand the reason of that, let's briefly recall the main functions of this very simple device.
The flywheel gets the excess of the engine cycle work versus the lost work, in the form of kinetic energy, by minimizing the angular speed fluctuation.

This is essential in an internal combustion engine in which the motion irregularity is the physical effect of the operating principle of the engine itself.
The angular speed fluctuation of course can be reduced by designing the engine with a larger number of cylinders for the same size, so that the engine moment can be distributed over them and be closer to a sinusoidal waveform, with a higher average value.
The two pictures below show the engine moment of a twin engine vs. a six cylinder engine.

Comparison of a twin cylinder and six cylinder engine

The number of cylinders is mainly dictated by requirements related to weight, dimensions, costs, the need to install the same powertrain on different types of cars.

In recent years all car manufacturers have gone towards a "downsizing"of the engines that, disregarding all the economic implications of the design and production costs of multi-cylinder engines and engines above two litres, have offered the possibility to reduce the dimension and the weight of the cars with advantages from the point of view of consumption and emissions, especially of CO2 for petrol engines.

Unfortunately, the displacement reduction and the lower number of cylinders without sacrificing the performance has led to more severe engines in terms of stress and vibrations.
This forced the engine designers to make use of devices that could guarantee the ever increasing requirements of running comfort and noise with engines that, for their own characteristics, make these aspects worse.

Among them the double mass flywheel can be mentioned. Its main function is to filter the torsional vibrations coming from the crankshaft that are transmitted to the driveline, from the gearbox, to the differential, up to the driveshafts and wheels.
This device, now commonly used on almost all the diesel engines and most of the petrol engines for passenger cars, performs its function at the expense of axial dimensions and especially weight, far higher than those ones of a single-mass flywheel that are not consistent with weight, overall dimensions and engine torques of a race application.

Of course, car manufacturers, when designing the crankshaft and the flywheel, carry out a detailed analysis and study before freezing the characteristics of both accessories and devices connected to them.

Now let's go back to our topic.

When turning a serial production engine into a race powertrain, one of the first steps is the mass reduction of the flywheel.
This allows the engine to be more responsive to the throttle input and accelerate faster.
This comes from the equation below, that describes the engine acceleration as a function of the engine moments and the resistant moment

Mmot - Mres = I x Aang

It is easy to point out how the acceleration of the engine depends on the moment of inertia of the flywheel: the smaller the flywheel the higher the acceleration.

By reducing the moment of inertia of the flywheel the speed fluctuation will increase, especially at low rpms, but this is not a concern for a race engine that is not required to fullfill special features in terms of flexibility and comfort but mainly performance characteristics at much higher revs than the idle speed of a road car engine.

Unfortunately (that's not the case of the factory race departments or professional engine tuners), too often the flywheel lightening and size reduction, with the aim of reducing the moment of inertia, is carried out without fully analysing the possible results.

To better understand what is happening we need to introduce the concept of torsional elastic line of the crankshaft and its vibrating modes.
Let's consider a four-stroke, in line four-cylinder engine as shown below.

Scheme of a four-stroke, four-cylinder engine

As a result of the loads generated by the combustion in the different cylinders, the shaft will have torsional deformations at different frequencies.
Let's focus on the first vibrating mode, whose amplitudes and frequency can be calculated by using complex algorithms since the design stage.

The elastic line will have, in terms of rotational amplitudes, a node on the axis of the crankshaft very close to the flywheel that is the the mass with the highest moment of inertia.

Torsional vibrating mode

What happens when you lighten the flywheel?
A reduction of the moment of inertia of the flywheel, in addition to changing the eigen frequency of the crankshaft for the different vibrating modes (this subject will be discussed in another article), will make the node to move to the left and the vibration amplitude on the flywheel side to increase (see figure below).

Torsional vibrating mode with light flywheel

Such a change of the elastic line can result in different effects:

  1. The node moves to an area of the crankshaft where sections are not able to withstand the new torsion stress, with possible reduction of the safety factor and the level of reliability of the crankshaft itself
  2. The higher torsional amplitudes on the flywheel side are transmitted to the driveline with possible negative effects on the gearbox, differential, driveshafts if not properly countered
  3. The higher torsional vibration amplitudes also increase the bending vibrations of the crankshaft that are transferred to the crankcase and then to the fixing points of the engine to the chassis.

Just to tell a short story, a few years ago we were asked to identify the cause of strange failures of the engine supports and the gearbox on a touring car.

After a detailed analysis of the changes made to the engine and the help of a simulation model, it was quite clear how simply lightening the flywheel, for performance purposes without proper counteractions, resulted in the breakage.

Of course, there are analysis and calculation techniques available to predict the effects of such changes, as well as possible countermeasures, to guarantee the reliability of the components without jeopardising the expected performance.

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