Particulate Matter Sampling and Volatile Organic Compound Removal for Characterization of Spark Ignited Direct Injection Engine Emissions

摘要

More stringent emissions regulations are continually being proposed to mitigate adverse human health and environmental impacts of internal combustion engines. With that in mind, it has been proposed that vehicular particulate matter (PM) emissions should be regulated based on particle number in addition to particle mass. One aspect of this project is to study different sample handling methods for number based aerosol measurements, specifically, two different methods for removing volatile organic compounds (VOCs). One method is a thermodenuder (TD) and the other is an evaporative chamber/diluter (EvCh). These sample handling methods have been implemented in an engine test cell with a spark ignited direct injection (SIDI) engine. The engine was designed for stoichiometric, homogeneous combustion. SIDI is of particular interest for improved fuel efficiency compared to other SI engines, however, the efficiency benefit comes with greater PM emissions and may therefore be subject to the proposed number based PM regulation. Another aspect of this project is to characterize PM from this engine in terms of particle number and composition. PM number distributions were acquired using a TSI Scanning Mobility Particle Sizer (SMPS). For the operating conditions considered, PM number distributions have a large accumulation mode (30-294 nm) but a very small nucleation mode (8-30 nm). This small nucleation mode is understood to represent a lack of condensation particles meaning that neither the exhaust conditions nor the sample handling conditions are conducive to condensation. This lack of nucleation mode does not, however, represent a lack of VOCs in the sample. It was found, using mass spectral analysis (limited to PM>50 nm), that PM from the SIDI engine has approximately 40% organic content through varying operating conditions. This raises the question of how effective different sample handling methods are at removing these VOCs. For one specific operating condition, called Cold Start, the un-treated PM was 40% organic. The TD reduced the organic content by 7% while the EvCh reduced it by 13%. For other operating conditions, PM treated for volatile removal actually exhibited an increase in organic fraction on the order of 5%. This addition with the TD appears to be sensitive to the gaseous hydrocarbon concentrations in the exhaust. Based on the remaining organic content while using a VPR, it has been concluded that PM emissions from the SIDI engine contain VOCs which are tightly bound to the solid carbon PM structure.