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Background and Motivation

Ever since the introduction of diesel engines, many research and development activities have focused on improving their performance and minimizing their emissions. To control diesel particulate matter (PM) emissions, diesel particulate filter (DPF) systems have been developed to trap and oxidize the solid PM from diesel engine exhaust gases. Models for the transient PM loading and the pressure drop in the DPF channels during loading can be coupled with PM regeneration models to lead to a more efficient and safe regeneration in the DPF.

The goals of this project are to understand the fundamental transport processes and reaction kinetics in the DPF through modeling efforts and then to utilize the models to help diesel engines meet EPA or California PM emission standards, which can lead to improved environmental quality and human health.

Summary of Annual Progress and Impact of Research

In the initial stages of this project, parametric and sensitivity analyses were carried out using two different DPF regeneration models. This led to the completion of a MS Thesis by graduate student Di Huang in September 2008. Then, this student developed models for particulate filtration (trapping) in the DPF and also for calculation of the pressure drop across a loaded DPF. The MS Thesis work served as a basis for a journal publication in 2009.

During the third year of this project, research has focused on the application of the regeneration model to real world driving conditions (using the urban dynamometer driving schedule). The model is based upon the Urban Dynamometer Driving Schedule (UDDS) which is a standard vehicle test cycle for diesel engines. The cycle lasts 1340 s and has a maximum vehicle speed of 55 miles per hour and as it has many starts and stops, the average speed is 19.6 miles per hour. Data was obtained from the Society of Automotive Engineers literature for studies on a Chevrolet Silverado 2500 series pickup truck. This data allows for us to estimate the engine speed (in rpm), the engine torque (in ft-lb), diesel particulate concentration (in milligrams per standard cubic meter), and the exhaust temperature (in degrees Kelvin) as a function of time during the UDDS cycle. These all serve as inputs to the regeneration model that we have used in the past.

To begin, the existing regeneration model was adjusted to allow for the changes in feed conditions with time. An electric heater was then used to increase the exhaust temperature to a set value. Based upon our prior research, a DPF inlet temperature of 710 K was high enough to lead to rapid regeneration at the leading edge of the DPF. The leading edge ignition is important to make sure that all of the particulate burns off of the filter. We note that the DPF inlet temperature is on average 275 K higher than the engine exhaust temperature, so a significant amount of external power is required. Also, due to the fluctuating nature of the exhaust gas (due to sudden starts and stops), the heater power will vary during the course of the regeneration. We stress that it is important to maintain the feed temperature at or above 710 K or the ignition may occur downstream within the DPF, and leave some particulate near the entrance of the DPF, which could lead to problems in future regenerations.

The duration of one single UDDS driving cycle is 1340 seconds. External heat is supplied to the DPF for the duration of a regeneration simulation, which is about 180 s. Since we do not know a priori when the regeneration should be initiated, we perform a series of regeneration simulations at various points within the UDDS cycle. During each simulation the regeneration occurs near the leading edge of the DPF. However, the amount that is burned off varies with different conditions. , Since the average exhaust gas flowrate during different each of these regenerations is different, the regeneration efficiency varies between cases.  The regeneration efficiency varies between 93.6% during highway driving simulations and 99.9% during city driving simulations.  Future efforts will investigate ways to increase the regeneration efficiency and determine the optimum conditions when the regeneration should occur.

Impact on Career of PI and Students

Funding from the American Chemical Society has allowed the PI to pursue this important project in emissions abatement and to support a graduate student that otherwise could not be supported. The student has benefited from the ability to study a research problem and completed his MS in Chemical Engineering. A presentation was given at the 2009 AIChE Annual Meeting and the student and the PI have also published a journal article in 2009 based upon the MS thesis research. The article was titled “Parametric and Sensitivity Analysis of Diesel Particulate Filter Regeneration” and is published in volume 7 of the International Journal of Chemical Reactor Engineering as article A56, pages 1-24.