Page 56 - Automotive Engineering

Measurement of torque, power, speed and fuel consumption CHAPTER 2.1

M ¼ mass (kg) is only used to accelerate the chosen flywheel combina-
R ¼ radius of flywheel mass tions up to the rotational speed required to simulate the
In the case of a flywheel taking the form of a uniform vehicle axle speed at the chosen vehicle speed. Flywheel
disc, which is the common form found within dyna- brake rigs have been made up to the size that can provide
mometer cells and chassis dynamometer designs: the same kinetic energy as fully loaded high speed trains.
Flywheels are also used on rigs used to test automatic
1 2 automotive gearboxes.
I ¼ MR
2 Test rig flywheel sets need to be rigidly and securely
mounted and balanced to the highest practical standard.
The engine or vehicle test engineer would normally Multiples of flywheels forming a common system that
expect to deal with flywheels in two roles: can be engaged in different combinations and in any
1. As part of the test object, as in the common case of an radial relationship require particular care in the design of
engine flywheel where it forms part of the engine/ both their base frame and individual bearing supports.
dynamometer shaft system and contributes to the Such systems can produce virtually infinite combinations
system’s inertial masses taken into account during of shaft balance and require each individual mass to be as
a torsional analysis. well balanced and aligned on as rigid a base as possible.
2. As part of the test equipment where one or more
flywheels may be used to provide actual inertia that 2.1a.12.1 Simulation of inertia* versus
would, in ‘real life’, be that of the vehicle or some iron inertia
part of the engine driven system.
Modern a.c. dynamometer systems and control software
No mention of flywheels should be made without consid- have significantly replaced the use of flywheels in chassis
eration of the safety of the application. The uncontrolled and engine dynamometer systems in the automotive in-
discharge of energy from any storage device is hazardous. dustry. Any perceived shortcoming in the speed of re-
The classic case of a flywheel failing by bursting is now sponse or accuracy of the simulation is usually considered
exceptionally rare and invariably due to incompetent
modification rather than the nineteenth century problems to be of less concern than the mechanical simplicity of
of poor materials, poor design or overspeeding. the electric dynamometer system and the reduction in
In the case of engine flywheels, the potential danger in required cell space.
the test cell is the shaft system attached to it. This may Finally, it should be remembered that, unless engine
be quite different in mass and fixing detail from its final rig flywheels are able to be engaged through a clutch, the
application connection, and can cause overload leading to engine starting/cranking system will have to be capable of
failure. Cases are on record where shock loading caused accelerating engine, dynamometer and flywheel mass up
by connecting shafts touching the guard system due to to engine start speed.
excessive engine movement has created shock loads that
have led to the cast engine flywheel fracturing, with 2.1a.13 Notation
severe consequential damage.
The most common hazard of test rig mounted fly- Frequency of torsional oscillation n cycles/min
wheels is caused by bearing or clutch failure where Critical frequency of
consequential damage is exacerbated by the considerable torsional oscillation n c cycles/min
energy available to fracture connected devices or because Stiffness of coupling shaft C s N m/rad
of the time that the flywheel and connected devices will 2
rotate before the stored energy is dissipated and move- Rotational inertia of engine I e kg m 2
ment is stopped. Rotational inertia of dynamometer I b kg m
It is vital that flywheels are guarded in such a manner Amplitude of exciting torque T x Nm
as to prevent absolutely accidental entrainment of Amplitude of torsional oscillation q rad
clothing or cables, etc. Static deflection of shaft q 0 rad
A common and easy to comprehend use of flywheels is Dynamic magnifier M
as part of a vehicle brake testing rig. In these devices, Dynamic magnifier at
flywheels supply the energy that has to be absorbed and critical frequency M c
dissipated by the brake system under test. The rig motor Order of harmonic component N o

* Some readers may object to the phrase ‘simulation of inertia’ since one is simulating the effects rather than the attribute, but the concept has
wide industrial acceptance.

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