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 rule elements of the water brake engine dynamometer and how they work. In this article we will look at the rule elements 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 means. With the use of a dyno, a means’s dominant strength aim elements, including the engine, transmission, and differential in addition as means elements such as the braking, cooling, and electrical systems, can be properly operated throughout a means’s strength and speed range. Deficiencies or errors in the assembly of the engine may be discovered before the means is pushed 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 means is put into service.

Chassis Dynamometer Construction

A chassis dynamometer has three major elements: 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 excursion system. Dyno roll sets are obtainable in both fixed and adjustable width versions which can adjust to 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 chief, shaft or disc, moving across a magnetic field to produce resistance to movement. obtainable 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 means 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 means’s excursion wheels up to speed and act as a motoring chassis dyno, but is able to put strength absorbed by the generator function back onto the electrical grid that powers the dynamometer.

In all situations, 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 means or chassis is pushed onto a chassis dynamometer and the means is secured using straps or chains typically provided with the dynamometer system. The means then performs a series of tests that mimic the operating conditions the means would confront during its intended use. These tests can be performed by an operator either inside or outside of the means, or by 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 component 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 strength run. A typical chassis dynamometer fuel measurement system works by initially measuring the total quantity 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 observed information on the dynamometer’s data acquisition system.

Room Exhaust System

The two most shared 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 release 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 situations an ECM, and are merged with speed, torque and strength 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 encased control operations. It’s also where new tests are run and reports are produced, printed, and stored.

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