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.
