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Theory Of Testing
Volume 2, 9/10/04
Section 8 - Page 5
8.1 Overview
8.1.1 Inertia Testing
Mechanical power is defined as the ability to accomplish a certain amount of work in a certain
amount of time. On an inertia chassis dynamometer, the work is accomplished by accelerating a
roll of known mass and inertia. If time and rate of acceleration are recorded, it is possible to derive
power. Inertia dynamometers provide a fast, cost effective, and reasonably accurate means to
determine the maximum power output of an engine or vehicle.
There is frequent confusion over the terminology and interpretation of the
specifications of inertia dynamometers. Inertia is the resistance of a mass
to linear or angular acceleration and is normally expressed in
lbs.in
2
[ kg.m
2
] or lbs.in.sec
2
. “Roll inertia” on SuperFlow dynamometers is
provided in “lbs. [kg] of equivalent vehicle weight” which is easier to relate
to real-world situations. This number actually means that the roll
simulates the inertial resistance to acceleration of a vehicle of this weight.
For convenience, the documentation generally uses the term “XXX lbs.
roll”. Once again, this should not be interpreted as the actual mass of the
roll but as the equivalent vehicle weight. For example: the inertial
resistance of the roll to acceleration of a 450 lbs. (205 kg) roll as used in
our inertia-only CycleDyn dynamometer models is similar to that of a
motorcycle of 450 lbs. (205 kg) total weight (= including the rider).
Ideally, the inertia (equivalent vehicle weight) of the roll should match the actual weight of the
vehicle under test for perfect simulation of on-road performance. Because the weight of the vehicle
under test varies considerably, this is only possible with variable inertia dynamometers. Some
variable inertia dynamometers use a series of flywheels which can be individually engaged or
disengaged to achieve close approximation of equivalent vehicle weight (Mechanical Inertia
Simulation or MIS dynamometers). Other dynamometers use electric motors capable of
electrically simulating inertia (Electrical Inertia Simulation or EIS dynamometers). These variable
inertia dynamometers are considerably more complex and expensive to build and install and are
typically only required for accurate emissions drive cycle simulation. SuperFlow has developed
and installed several hundred of such MIS and EIS dynamometers for government-mandated
emissions test programs throughout the world.
For performance testing and diagnostic purposes, the inertia (equivalent vehicle weight) of the
dynamometer must not necessarily simulate vehicle weight accurately. Differences will show up as
slower (if the actual vehicle weight is lower than the dyno simulates) or faster (if the vehicle weight
is higher than the dyno simulates) rates of acceleration than would occur on the road. The
difference in the rate of acceleration between the dynamometer and the road will not have any
influence on the actual power measurement as long as the air and fuel flow are not significantly
affected. Air and fuel flow will be affected only if the rate of acceleration differs substantially. This
is an important consideration when tuning A/F ratios on an Inertia only dyno.
Developing a good inertia chassis dynamometer with a fixed inertia thus requires a carefully
determined compromise between the extremes of vehicle weights to be expected on the system.