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Eddy current dynamometers, while less expensive than full inertia dynamometers, have rarely been considered viable alternatives when it comes to transient simulation. Improvements made to eddy current dynamometers in order to meet California's BAR-97 Specification, however, may reverse that thinking in I/M enviromnents.

The DynoCal system for auditing chassis roll dynamometers has been developed for the California Bureau of Automotive Repair. It evaluates performance of new dynamometer designs and operational dynamometers in the field. The system includes a 50 kW electric motor with torque, thrust, and speed instrumentation. It functions as a vehicle to drive the chassis rolls under computer-controlled test modes. A trailer transports the system and houses the battery power supply, chargers, flux vector power converter, and computer. The controls are user-friendly, with graphic displays and high-speed data acquisition. Among other functions, this unique test instrument measures dynamometer road load and inertia simulation accuracy.

“Specifications for Electric Chassis Dynamometers, Attachment A” was issued by the USEPA in 1991. Although the primary purpose of the document was to serve as a Request for Proposal, it has subsequently been used as a general procedural manual to assess the road-simulation accuracy of electric chassis dynamometers. Significant strides have been made in the development of electric dynamometers since 1991. Procedures are needed to demonstrate that a chassis dynamometer is indeed capable of required accurate road simulation. Procedures for assessing dynamometer calibration, evaluating dynamometer response time, and measuring simulation error are reviewed. Novel methods are proposed for determining dynamometer torque measurement response and overall dynamometer system response to a simulated instantaneous change in vehicle thrust. This is offered to help dynamometer users evaluate the performance of their machines.

Vehicle operation on 8.65″ uncoupled twin-roll hydrokinetic and 48″ single-roll dynamometers was analyzed for speed, road load, and inertia simulation differences. Tire slip measurements were used to determine generic coefficients accounting for differences on the two dynamometers. Control methods were developed to use operational speed and load measurements on the 48″ dynamometer to predict the instantaneous rear roll speed which would have occurred on the twin rolls, and to compute instantaneous load corrections on the 48″ dynamometer to simulate the twin-roll dynamometer. Emission and fuel economy tests were run to evaluate simulation effectiveness.

The vehicle chassis dynamometer recently selected by the EPA for its own research and testing programs is described. The design provides accurate electrical inertia and road-load simulation through a wide power range and eliminates the need for warm-up. The principal innovative features of this dynamometer are the central placement of an AC flux-vector power exchange unit (PEU) between the rolls of a single 48-inch diameter roll-set, the friction-compensated motorized roll bearings, and the unitary cast-iron frame. The dynamometer controller employs a feedforward algorithm that delivers an unusually rapid response to a step-change in load command. It employs two computers to carry out its control functions, to provide the operator with real-time test monitoring, and to perform a variety of automatic functions.

Performance testing was done on a 1.219 m (48 in) roll diameter chassis dynamometer to validate its compliance with EPA specifications. The testing procedure and results are presented. Tests include time, speed, acceleration and torque calibration verification, load control response time, parasitic loss compensation and stability, base mechanical inertia verification, road load curve simulation and repeatability, automatic road coastdown coefficient matching, and overall simulation accuracy during severe acceleration and emission test cycles. These testing methods provide an objective and comprehensive evaluation of chassis roll dynamometer performance.