Wind tunnel measurements and direct tailpipe particulate matter (PM) sampling are utilized to examine how the combination of oxidation catalyst and fuel sulfur content affects the nature and quantity of PM emissions from the exhaust of a light duty diesel truck. When low sulfur fuel (4 ppm) is used, or when high sulfur (350 ppm)fuel is employed without an active catalyst present, a single log-normal distribution of exhaust particles is observed with a number mean diameter in the range of 70-83 nm. In the absence of the oxidation catalyst, the high sulfur level has at most a modest effect on particle emissions (<50%) and a minor effect on particle size (<5%). In combination with the active oxidation catalyst tested, high sulfur fuel can lead to a second, nanoparticle, mode, which appears at approximately 20 nm during high speed operation (70 mph), but is not present at low speed (40 mph). A thermodenuder significantly reduces the nanoparticle mode when set to temperatures above approximately 200 degrees C, suggesting that these particles are semivolatile in nature. Because they are observed only when the catalyst is present and the sulfur level is high, this mode likely originates from the nucleation of sulfates formed over the catalyst, although the composition may also include hydrocarbons.
Stringent North American emission regulations present several challenges for lean-burn diesel vehicles that
offer significant fuel economy advantages over gasoline vehicles. This article presents a useful systems approach
to developing robust models and combining them to analyze diesel aftertreatment (AT) technologies: (1) the
diesel oxidation catalyst (DOC) that oxidizes carbon monoxide (CO), unburned hydrocarbons (HCs), and
nitric oxide (NO), and stores HCs and (2) the urea-based selective catalytic reduction (SCR) catalyst that
hydrolyzes aqueous urea to ammonia (NH3), which, in turn, reduces nitrogen oxides (NO
x
). The DOC and
SCR models integrate information from multiple sourcesdetailed thermal balances, laboratory performance
data, phenomenological descriptions of adsorption and desorption in the catalyst, and experience-based
correlationsusing optimization and statistical tools. The DOC model predicts cumulative HC and CO tailpipe
vehicle emissions as well as DOC NO
x
outlet composition (NO vs NO2). The SCR model uses the exotherm
and NO2 information from the DOC model to predict NO
x
conversion and NH3 slip. System-level analyses
that have resulted in important insights are highlighted using case studies, including (1) extrapolation of the
downstream injection exotherm vehicle data for a fresh DOC to an aged DOC; (2) analysis of the effects of
NO2 produced in the DOC, SCR aging conditions, and NH3 storage capacity on the NO
x
reduction performance
of the SCR on a 6000-lb light-duty truck, and (3) development of urea injection strategies based on a tradeoff
between NO
x
reduction and NH3 slip.
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