Q:

What does Steady State mean, and what type of Power Absorber is right for my application?

  A:

Steady state describes a condition wherein there is no change in speed. 

The means for measuring power on dynamic loading dynamometers, or steady state dynamometers, is to absorb the power being generated while a strain gauge, or load cell, provides a signal proportional to force being applied.  This is achieved by attaching a moment arm equipped with a load cell to the braking device – usually perpendicular to the drive shaft.  This process is utilized by nearly every engine dynamometer in operation today for tuning and breaking-in of engines – just the same as it is with any steady state chassis dynamometer.

Earlier, more rudimentary systems offered open-loop control systems that allow the user to increase or decrease load, or resistance, via a dial-based potentiometer – but without the means to base load off of another variable (i.e. speed).  In order to hold the system at any one speed independent of throttle, the user had to keep adjusting load in an attempt to damper fluctuations in throttle position. 

Closed-loop controllers, on the other hand, allow for hassle-free steady state tuning.  In the case of the chassis dynamometer, the computer-controller holds the vehicle at the designated speed independent of throttle position, save those circumstances where the vehicle’s output exceeds the rating of the dynamometer.

To further enhance utility, more advanced controllers allow the user to preprogram a series of speed-points over several time intervals, typically referred to as step-tests or programmed-load tests.  While this type of testing is typical of engine dynamometers, it has not been as closely associated with chassis dynamometers among high performance professionals, primarily due to the explosion of the inertial-exclusive chassis dynamometer in the mid-nineties.

More recent technological advances have led to much more sophisticated closed-loop control systems, which incorporate algorithms to compensate for the inertial and aerodynamic characteristics of particular vehicles.  Simulation testing is a combination of both acceleration power and static power.  This technology has led to new breakthroughs in performance testing - allowing users to simulate actual road-load conditions with the push of a button.  Elapsed Time Testing (ET Testing) can be achieved, including distance tests (quarter-mile sprint), rolling acceleration tests (60 – 90-mph), and standing start acceleration tests (0-60-mph).  If run properly, these tests will correlate very closely with data collected under real-world track conditions.  The real advantage of these machines is that they can actually reproduce the competitive playing field; certainly a benefit for those of us whose success can be ascertained by counting the number of trophies we’ve won.

The Air-cooled Eddy Current Power Absorption Unit has proven to be, dollar for dollar, the best means for controlling load on a chassis dynamometer. As a result, it is estimated that more than sixty percent of the loading chassis dynamometers currently in operation employ an air-cooled Eddy Current PAU. The Air-cooled Eddy Current Power Absorption Unit is essentially an Electromagnetic Brake. The load applied is frictionless, and positively related to the amount of DC Current applied to the coils of the brake.  As the brake absorbs the kinetic energy transmitted by the test vehicle, it transforms this energy into heat, and dissipates this energy into the ambient air. 

The first chassis dynamometer used a hydrokinetic brake, or water brake. Controlling a valve upstream from the device regulates load applied. Recycling of the water requires local circuitry, consisting primarily of a water source, the dynamometer, a cooling tower, with plumbing and pumps between the three. For applications that demanded “brute force” over extended periods of time fluid based brakes remained a very viable solution. 

DC and AC Motors offer the user the ability to actively load and drive the system; ideal for applications requiring transient load capabilities - generative power and regenerative power.  The term transient describes a test whose main purpose is to simulate dynamic road-load (changes in load over time) under varying levels of acceleration and deceleration.  The two most well known transient test cycles, or driving schedules, are the Federal Test Procedure (FTP) and the I/M 240; both of which were developed by engineers at the Environmental Protection Agency (EPA) in cooperation with OEMs and Test Equipment Manufacturers. These tests mirror driving habits encountered in suburban areas.    The I/M 240 is essentially a small 240-second section of the FTP Test.  Today, I/M 240 Testing can be found in the states of Missouri, Illinois, Ohio, Arizona, and Washington.  In addition the these well-known tests, virtually any driving schedule that can be defined in terms of speed over time over percent grade can be duplicated on a properly sized transient dynamometer

AC and DC based chassis dynamometers are relatively expensive, and therefore not practical for some applications.  Laboratory-grade test systems are typically very useful and extremely accurate when used to perform the test they are designed for.  However, these applications are usually very purpose-specific; with the equipment being designed from the ground-up to accommodate only one prescribed test.  In many cases, these prescribed test applications can be traced back to the Environmental Protection Agency (EPA), the National Highway Traffic Safety Administration (NHTSA), or some other agency responsible for policing the transportation industries.  In other cases, test equipment may be engineered around a customer’s unique needs and requirements.