Case Study of Water Condensation and Evaporation Effects on Tailpipe Continuous Mass Emission Calculations in a Gasoline Powertrain

Emissions development work for gasoline aftertreatment systems is often conducted in a laboratory on a chassis dynamometer. In this situation, extractive sample lines are frequently connected to the aftertreatment system before and after various components, such as a three-way catalyst, selective catalytic reduction substrate, and the like. This is done to measure the conversion efficiency of the aftertreatment system components as a function of time. The time series exhaust component concentration data, also referred to as continuous data, are combined with a measure of exhaust volumetric flowrate and used to calculate mass-based emissions. As gasoline powertrains become cleaner and produce lower levels of criteria emissions, the proximity (i.e., colocated or not colocated) of the volumetric flowrate and concentration measurements may affect the accuracy of the overall mass emission calculation. In this work, we investigate the effect of water condensation and evaporation on continuous mass emission calculations in a gasoline powertrain. A dual-pitot tube flowmeter system is implemented to measure differences in volumetric flowrate between two tailpipe locations in the same vehicle. It was found that carbon monoxide, ammonia, and formaldehyde are especially susceptible to impacts from water condensation and evaporation effects due to the timing of their emission within a drive cycle and/or their water solubility. The mass emission of ammonia in the case study vehicle, for example, was underpredicted by up to ~15% if the tailpipe measurements of its concentration and the exhaust gas volumetric flowrate were not colocated.