What is a Chassis Dynamometer?


This article is the second in a series of three articles highlighting the basics of dynamometer testing. In this first article, “What is an Engine Dynamometer,” we reviewed the principle components of the water brake engine dynamometer and how they work. In this article we will look at the principle components of the chassis dynamometer (also known as a dyno).

A chassis dyno is a tool that lets the operator safely place a controlled load on a vehicle. With the use of a dyno, a vehicle’s primary power train components, including the engine, transmission, and differential as well as vehicle components such as the braking, cooling, and electrical systems, can be properly operated throughout a vehicle’s power and speed range. Deficiencies or errors in the assembly of the engine may be discovered before the vehicle is driven and a thorough assessment of the operating condition of the engine can be carried out. Basically the dynamometer is the last test of quality before a vehicle is put into service.

Chassis Dynamometer Construction

A chassis dynamometer has three major components: the roll set, the absorption unit, and the torque indication system.

Chassis dyno roll sets come in a variety of diameters depending on the application. An automotive chassis dynamometer will typically have smaller roll sets, while a large truck dyno will have a larger set. These dyno roll sets are placed in a specially designed frame and are either coupled directly to the dynamometer absorption unit or to a belt drive system. Dyno roll sets are available in both fixed and adjustable width versions which can accommodate a variety of wheelbases for testing multiple vehicles on a single machine.

The characteristics of a water brake absorber when used in a chassis dynamometer are very similar to an engine dynamometer. In addition to water brake absorbers, chassis dynos can also be equipped with eddy current and AC regenerative absorption units. An eddy current absorber uses electrical current to produce a load. Eddy current dynamometers require an electrically conductive core, shaft or disc, moving across a magnetic field to produce resistance to movement. Available in both air cooled and liquid cooled applications, eddy current chassis dynamometers provide quick response rates. Most eddy current absorbers use cast iron discs, similar to vehicle disc brake rotors, and use variable electromagnets to change the magnetic field strength to control the amount of braking.

Much like a water brake absorber, the housing of an eddy current absorber is restrained by a torque arm within the chassis dyno assembly that is connected to a load cell. AC regenerative chassis dynamometers use an electric motor/generator that is not only capable of assisting with bringing the vehicle’s drive wheels up to speed and act as a motoring chassis dyno, but is able to put power absorbed by the generator function back onto the electrical grid that powers the dynamometer.

In all cases, the absorption unit is restrained using a torque arm that is connected to a load cell. The force with which the stators are trying to rotate is measured by the load cell. By measuring the distance from the torque arm to the axis of the absorber, the amount of torque can be measured.

Torque = force x distance

So if we measure the speed, the amount of horsepower can be found with this formula:

HP = (torque x rpm)/5252

Chassis Dynamometer Operation

A vehicle or chassis is driven onto a chassis dynamometer and the vehicle is secured using straps or chains typically provided with the dynamometer system. The vehicle then performs a series of tests that mimic the operating conditions the vehicle would face during its intended use. These tests can be performed by an operator either inside or outside of the vehicle, or through an automated test depending on the control system supplied with the chassis dyno.

What makes up a Chassis Dynamometer System?

The chassis dynamometer itself is only one element of the overall chassis dyno system. In general, a dyno system consists of a chassis dynamometer plus a fuel measurement system, a room exhaust system, and a data acquisition and control system.

Fuel Measurement System

Fuel measurement systems are designed to monitor engine efficiency during a power run. A typical chassis dynamometer fuel measurement system works by initially measuring the total volume of fuel inside the storage tank prior to a test run. As a test run is performed, the system automatically calculates the amount of fuel being used and displays the monitored information on the dynamometer’s data acquisition system.

Room Exhaust System

The two most common types of room exhaust systems are hood and pipe systems. Hood systems are preferred because direct connection to the engine or exhaust system is not required. Hood fans pull make-up air into the room containing the dynamometer and removes both engine and room exhaust. Pipe systems can be powered by a fan to remove engine exhaust. When using a pipe system, a separate fan-powered room air exhaust system is necessary to pull make-up air into the room and to discharge heat, smoke and fumes.

Data Acquisition and Control Systems

Generally a dyno controller contains the dynamometer system’s temperature and pressure sensors. These sensors are contained within an industrial cabinet and are provided with quick disconnects. Data is gathered from the temperature and pressure sensors, and in many cases an ECM, and are merged with speed, torque and power measurements from the dyno and sent to the dynamometer system’s computer.

The computer in a data acquisition and dynamometer control system interfaces with the controller and the dyno and executes all of the embedded control operations. It’s also where new tests are run and reports are created, printed, and stored.

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