Factors Affecting Test Precision in Latest Vehicle Technologies

Demonstrating the cost/benefits of technologies in the automotive sector is becoming very challenging because the benefits from technologies are sometimes of similar magnitude to testing precision. This paper aims to understand vehicle-borne imprecision and the effect of this on the quality of chassis dynamometer (CD) testing. Fuel consumption and NOx emissions precision is analyzed for two diesel vehicles with particulate filter and SCR systems. The two vehicles were tested on a high precision CD facility over the NEDC (New European Drive Cycle) and WLTC (World harmonized Light-duty Test Cycle) cycles. The CD base precision of testing was characterized between 0.6-3% depending on the cycle phase. A novel application of multi-variate statistical analysis was used to identify the factors that affected testing precision, allowing isolation of small differences that were not obvious when conducting cycle-averaged or cycle-phase-averaged analysis. One of the vehicles was particularly sensitive to the vehicle warm-up rate which caused significant variations in catalyst light-off strategy within the engine management system (EMS). This was seen to cause up to 2.3% variability in fuel consumption. Both vehicles were affected by inconsistencies in the stop-start system and the intelligent alternator system, resulting in 0.5% and 1% imprecision respectively. Tailpipe NOx emissions were seen to vary significantly during the WLTC depending on the nature of the preceding test, however this behavior appeared to be limited to the SCR system and not engine combustion. This work has presented new insights which can be leveraged to improve vehicle testing precision to aid the development and evaluation of new hardware, fuel and lubricant technologies designed to bring about fuel consumption and emissions benefits.