2-4 AC Motoring Chassis Dynamometer
A dynamometer can also be used to determine the
power required to “drive” an engine or vehicle. In
that case, a motoring or driving dynamometer is used.
A dynamometer that is designed to only be driven by
the machine under test (MUT) is called an absorption
or passive dynamometer. These are typically water
brake or eddy current systems. A dynamometer that
can either drive or absorb is called a universal or
active dynamometer. Universal dynamometers can
not only absorb the power of the MUT but can also
drive the MUT for measuring friction, drive train
losses and other factors.
Figure 2-2. AC Motor Module
The SuperFlow motoring dynamometer uses an AC electric motor with a specialized type of
adjustable-speed drive. This setup classifies the system as a “universal” dynamometer where it
can operate:
• As a generator that is driven by the MUT and applies a load as commanded by the system
controller. In this mode the dyno functions as a typical water brake or eddy current system.
However, the power “absorbed” by the AC motor must be transferred to a resistive load
(load bank) or other outlet. In some systems, regenerative control units can transfer power
from the unit under test to the electric utility where owner of the dynamometer can receive
payment (or credit) from the utility for the returned power.
• As a motor that spins the dynamometer roll in controlled conditions, and therefore can
“drive” the MUT at a desired speed or torque setpoint.
A motoring dynamometer provides the features of a brake dynamometer system, but can also can
measure the frictional losses in an engine and allow testing of very small power small outputs. On
chassis dynamometers it can duplicate speeds and loads that are experienced when operating a
vehicle traveling downhill or on/off throttle operations. Many tests can even be run without the
test vehicle’s engine running.
In addition to being used to determine the torque or power characteristics of a machine under test,
motoring dynamometers are employed in a number of other roles. In standard emissions testing
cycles such as those defined by the US Environmental Protection Agency (US EPA),
dynamometers are used to provide simulated road loading of either the engine (using an engine
dynamometer) or the full power train (using a chassis dynamometer). The hardware and software
system can then “motor” the test vehicle to perform emissions drive cycles and inertia
simulations, evaluate frictional losses, map Engine Control Units (ECUs) and numerous other
engineering-grade test procedures. Using the AC Motoring option, you can test brakes,
drivetrains, and other chassis components.
In fact, beyond simple power and torque measurements, motoring dynamometers can be used as
part of a test bed for a variety of engine development activities such as the calibration of engine
management controllers or detailed investigations into combustion behavior.
