Understanding the factors that influence testing precision for modern diesel vehicles

Modern diesel vehicles employ many technologies to reduce emissions whilst minimising fuel consumption. Many of these technologies have a time-dependent behaviour which results in unexplained test-to-test variation when conducting short tests on chassis dynamometers. This vehicle-borne imprecision increases the effort required to demonstrate small but significant differences between automotive technologies. The aim of this paper is to identify and quantify variability inherent to the vehicle and to propose methodologies that improve testing precision.
Two vehicles with DPF and SCR technology were characterised on-road and on chassis dynamometer. The on-road analysis gave an understanding of the long-term vehicle behaviour and was used to inform the chassis dynamometer tests. The chassis dynamometer tests were conducted in a twin-axle facility, located in a climatic chamber, and driven by a robot driver. Data was recorded through the vehicle OBD port and via dedicated emissions and fuel flow measurement instruments.
The duty cycle was seen to affect the precision with higher speed cycles offering higher precision: the RMSE of the WLTC Low phase was 1.6% compared with 0.6 % for the Very High phase. Interestingly, the WLTC had better repeatability than the NEDC cycle. The vehicle start/stop system affected precision by 0.2-0.6 %-points and the battery smart charging strategy caused a 1 %-point decrease in precision. Understanding and managing each of these factors is key to achieving high precision vehicle testing.