Browse Publications Technical Papers 2007-01-0600
2007-04-16

Reliability Analysis of Dynamometer Loading Parameters during Vehicle Cell Testing 2007-01-0600

In automotive testing, a chassis dynamometer is typically used, during cell testing, to evaluate vehicle performance by simulating actual driving conditions. The use of indoor cell testing has the advantage of running controlled tests where the cell temperature and humidity and solar loads can be well controlled. Driving conditions such as vehicle speed, wind speed and grade can be also controlled. Thus, repeated tests can be conducted with minimum test variations.
The tractive effort required at the wheels of a vehicle for a given set of operating parameters is determined by taking into account a set of variables which affect vehicle performance. The forces considered in determination of the tractive effort include the constant friction force, variable friction force due to mechanical and tire friction, forces due to inertia and forces due to aerodynamic and wind effects. In addition, forces due to gravity are considered when road grades are simulated. When towing a trailer, similar parameters for a trailer should be considered. A set of coefficients is used to determine the above forces. The vehicle weight and trailer weight are included in the determination of gravity and inertia forces.
Therefore, the dynamometer tractive effort, for a given vehicle speed and load, affects the exhaust gas temperature, engine speed and heat rejection. For a specific vehicle configuration and design, component temperatures are a strong function of the exhaust surface temperature, air flow and air temperature. Uncertainties in the coefficients used to determine the dynamometer tractive effort will ultimately affect the measurements for component temperatures as well as measurements for the vehicle system performance such as the cooling and climate control systems.
In this paper, the reliability of the tractive effort calculation is determined in terms of the uncertainty in the tractive effort loading coefficients. Understanding the significance of accuracy of these coefficients on the tractive effort, is essential to evaluate and improve the reliability of test data.

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