Vehicle Level Brake Drag Target Setting for EPA Fuel Economy Certification

The strong focus on reducing brake drag, driven by a historic ramp-up in global fuel economy and carbon emissions standards, has led to renewed research on brake caliper drag behaviors and how to measure them. However, with the increased knowledge of the range of drag behaviors that a caliper can exhibit comes a particularly vexing problem - how should this complex range of behaviors be represented in the overall road load of the vehicle? What conditions are encountered during coastdown and fuel economy testing, and how should brake drag be measured and represented in these conditions? With the Environmental Protection Agency (amongst other regulating agencies around the world) conducting audit testing, and the requirement that published road load values be repeatable within a specified range during these audits, the importance of answering these questions accurately is elevated. This paper studies these questions, and even offers methodology for addressing them. It includes a review of how variation in brake drag can affect fuel economy and carbon emissions certification, a review of the many transient and driver-dependent behaviors and operating conditions that can affect drag at a vehicle level (and means of measuring them) and then offers a methodology (based on probabilistic modeling) for predicting the range of drag that can be encountered in fuel economy testing. In the course of developing the methodology, a significant database of vehicle level brake drag measurements is analyzed, and a case study vehicle is used to show correlation in a “walk” from component level to vehicle level caliper drag behavior.