Characterizing the Variability in Particulate Mass Emissions from Current Model Year Diesel and Natural Gas Engines

Abstract

The objective of this study is to discern and characterize the factors contributing to the variability in Particulate Matter (PM) mass emissions from current model year diesel and natural gas engines observed during in-use chassis dynamometer testing and compare it with an alternate method of mass measurement involving instantaneous particle size distribution and number count. The study involves the analysis of engine and chassis dynamometer data collected from different engine technologies and chassis dynamometer test cycles in order compare the variability in gravimetric PM mass as well as the PM mass estimated by number-concentration. PM and oxides of nitrogen (NOx) have been the two most stringently regulated emissions constituents from heavy-duty diesel engines. Current US-EPA 2010 PM regulations, set at 0.01 g/bhp-hr, have forced manufacturers to implement the use of diesel particulate filters (DPF) in order to comply with the regulations. The use of DPF's has resulted in PM mass decreasing by orders of magnitude since 2004, hence laboratory measurement techniques and instrumentation to accurately quantify the true mass of PM emitted from such engines have been posed with a challenge. Also, the widely gained acceptance of heavy-duty natural gas engines, characterized by their low soot combustion, has inevitably resulted in significant measurement variability due to the high volatile organic content in the exhaust. Particulate matter mass comparisons are performed between PM sampled using the regulated "CFR 1065 Methodology" and PM number count measurements performed with similar exhaust dilution conditions as employed for gravimetric PM sampling. Particle size distribution and number concentration measurements were performed using the Engine Exhaust Particle Sizer ® (EEPS) spectrometer for transient engine operation and Scanning Mobility Particle Sizer ® (SMPS) spectrometer for steady-state modal tests performed on an engine dynamometer. Additionally, PM mass comparisons are extended to an in-use study in order to compare Not-To-Exceed (NTE) PM emission limits. Results from this study showed that an "effective-density" based conversion technique correlated well with gravimetric filter mass for pre-2010 engine technologies without aftertreatment systems, in particular DPFs for diesel fueled vehicles. Gravimetric PM measurements from a natural gas engine resulted in a standard deviation of 3.1 mg/bhp-hr in comparison to mass calculated through particle size distributions, which resulted in a standard deviation of 0.36 mg/bhp-hr. The use of particle size based measurements for in-use PM monitoring resulted in better resolution of PM mass during short, valid NTE windows.

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“…PM mass concentration is calculated using Integral Particle Size Distribution (IPSD) method from the particle number count size distribution measured by the TSI EEPS TM 3090 instrument. A constant density function of 1.26 g/cc was used for the IPSD calculation for this study, based on the inferences made in Thiruvengadam et al [63]. Additionally, the study made used a Kistler High Temperature Pressure Sensor Type 6125C-U20 was used to measure incylinder pressures.…”