Comparison of Emissions Measurement Methods based on Chassis Dynamometer Testing 2021-01-0611

Engine and vehicle tailpipe emissions can be measured in laboratories equipped with engine dynamometers and chassis dynamometers, respectively. In addition to laboratory testing, there is an increase in interest to measure on-road vehicle emissions using portable emissions measurement systems in order to determine real-driving emissions. Current methods to quantify engine, vehicle tailpipe, and real-driving emissions include the raw continuous, dilute continuous, and dilute bag measurement methods. Although the dilute bag measurement method is robust, recent improvements to the raw and dilute continuous measurement methods can account for the time delay between the probe tip and analyzer in addition to gas transport dynamics in order to reliably recover the tailpipe concentration signals. These improvements significantly increase the reliability of results using the raw and dilute continuous measurement methods, making them possible alternatives to the bag method. This paper synthesizes factors that can affect the measured emissions results, with the objective of comparing these measurement methods for each of the following emissions species: carbon dioxide, carbon monoxide, oxides of nitrogen, nitric oxide, total hydrocarbons, and methane. The factors considered in this comparison include the expected concentrations of each species, the ratio of the highest and lowest species concentrations measured during a test, and the effect of engine type on the measured species. In addition, the uncertainties in calculating emissions results for each method and species are taken into consideration. Comparisons include measurements from both compression-ignition and spark-ignition engines. Experiments were conducted in a laboratory equipped with a chassis dynamometer, full-scale dilution tunnel, emissions measurement system, and data acquisition software. Data that is simultaneously acquired using the three measurement methods of interest over a simulated transient chassis dynamometer test cycle are used in the analysis. Based on the analysis, optimum measurement methods for tests conducted in the laboratory versus tests conducted on the road using portable emissions measurement systems to determine real-driving emissions are discussed.