Considerations for Achieving Equivalence between Hub- and Roller-Type Dynamometers for Vehicle Exhaust Emissions

Abstract

Emissions from vehicles can be measured on the road or in laboratories using dynamometers that simulate the forces that a vehicle is subject to while driving on the road. In the light-duty vehicle regulations, only roller-type dynamometers are allowed. For hub-type dynamometers, due to the direct connection of the dynamometers to the wheel hubs, additional parameters that are used are rotational mass, dynamic wheel radius, and the tire force–slip relationship. Following up on an experimental study which showed that equivalent emission results can be achieved between roller- and hub-type dynamometers, this work presents and evaluates methods to determine parameters used by a hub-type dynamometer for mimicking roller-type dynamometer behavior. It also discusses methods to determine the parameters to simulate specific road conditions or when using only a hub-type dynamometer. The results show that using a constant dynamic radius for each wheel and a linear tire force–slip relationship is sufficient for emission measurement because typical errors in these parameters are practically negligible. A typical error in rotational mass results in a minor error in the determined forces during coast down, but the typical accuracy of this parameter is in parity with the difference allowed in the regulation. The final conclusion is that using the information already stated in the certificate of conformity (CoC) of the vehicle (for the coast down), and reasonably set parameters for wheel dynamic radius and the tire slip–force relationship, hub-type dynamometers should yield equivalent results to roller-type dynamometers.