To force the development and use of the best available emission technologies needed to significantly reduce Diesel particulate matter (DPM) mass, the 2007 United States Environmental Protection Agency DPM standards for on-road trucks were reduced by 90% to 0.01 g/hp-hr. On-road Diesel engines manufactured after 2007 emit low levels of DPM. The gravimetric method used for certification differentiates between compliant and noncompliant engines at the 0.01 g/hp-hr level. At concentrations below ~10 µg/m3 the method lacks sensitivity, making it difficult to evaluate alternative engine designs, emission control devices, alternative fuels, and modified lubricants that reduce DPM even further. Alternative methods and metrics like the particle size and number concentration measurements extend this lower limit of detection and may enable engine manufacturers and others to make better decisions on what future technologies are required for meeting a zero emission goal. The objectives of this research are to improve the understanding of variables like dilution and sampling conditions that contribute to particle-based emission measurements, to identify and improve current and emerging methods, and to use alternative methods to make measurements of engine exhaust to further elucidate the impact of fuels, emission control and engine state-of-maintenance on emissions.
Additional background information is found in Chapter 1.
Chapter 2 is a synthesis and evaluation of ideas and perspectives that were presented at a
series of workshops sponsored by the Coordinating Research Council that aimed to
evaluate the current and future status of DPM measurement. Measurement of DPM is a
complex issue with many stakeholders, including air quality management and
enforcement agencies, engine manufacturers, health experts, and climatologists.
Adoption of the U.S. Environmental Protection Agency 2007 heavy-duty engine DPM
standards posed a unique challenge to engine manufacturers. The new standards reduced
DPM emissions to the point that improvements to the gravimetric method were required
to increase the accuracy and the sensitivity of the measurement. Despite these
improvements, the method still has shortcomings. The objectives of this chapter are to review the physical and chemical properties of DPM that make gravimetric measurement
difficult at very low concentrations and to review alternative metrics and methods that are
potentially more accurate, sensitive, and specific. Particle volatility, size, surface area,
and number metrics and methods to quantify them are considered. Although an
alternative method is required to meet the needs of engine manufacturers, the methods
reviewed are applicable to other areas where the gravimetric method detection limit is
approached and greater accuracy and sensitivity are required. The review suggests that a
method to measure active surface area, combined with a method to separate semi-volatile
and solid fractions to further increase the specificity of the measurement, has potential for
reducing the lower detection limit of DPM and enabling engine manufacturers to reduce
DPM emissions in the future.
Chapter 3 improves the understanding of variables like dilution and sampling conditions
that contribute to particle-based emission measurements by assessing and comparing the
nucleation tendency of Diesel aerosols when diluted with a porous wall dilutor or an air
ejector in a laboratory setting. A de facto standard air-ejector dilutor and typical dilution
conditions were used to establish the baseline sensitivity to dilution conditions for the given engine operating condition. A porous tube dilutor was designed and special
attention was given to integrating the dilutor with the exhaust pipe and residence time
chamber. Results from this system were compared with the ejector dilutor. Exhaust
aerosols were generated by a Deere 4045 Diesel engine running at low speed (1400 rpm)
and low load (50 Nm, ~10% of rated). Primary dilution parameters that were varied
included dilution air temperature (25 and 47 °C) and dilution ratio (5, 14, and 55).
Particle measurements were made at 0.3, 0.75, and 1.0 s to evaluate particle growth in the
residence time chamber.
Exhaust size distribution measurements made using the ejector dilutor were bimodal with
high concentrations of nucleation mode particles. Varying the dilution ratio from 5 to
55:1 (with a dilution air temperature of 25 ºC and residence time of 1 s) caused the
greatest change in the particle number concentration (4 x 108 to 4 x 1010particles/cm3) compared to changes in the other variables. Particle concentration was lower with higher
dilution air temperatures and particles were larger in size. Size distributions downstream
of the porous tube and ejector dilutor were qualitatively similar in shape. Using a simple
dilution model and equations for particle growth in the free molecular regime, particle
growth in the two residence time chambers was compared. Model results suggest that
dilution in the porous tube dilution system occurs more slowly than in the ejector dilutor.
This is consistent with the findings that the particle number concentrations were
consistently higher and the geometric mean diameter was generally 1 to 5 nm larger
downstream of the porous tube dilutor.
Chapter 4 describes the comparison of two methods that are used to separate the solid
and volatile components of an aerosol: the thermal denuder (TD) and catalytic stripper
(CS). The TD and CS were challenged with atmospheric and laboratory generated
aerosols. Laboratory generated particles were composed of tetracosane, tetracosane and
sulfuric acid, and dioctyl sebacate and sulfuric acid. These compositions were chosen
because they roughly simulate the composition of nanoparticles found in Diesel exhaust The TD method produced semi-volatile particle artifacts due to the incomplete removal
of evaporated compounds that nucleated and formed particles and solid particle artifacts
that formed during treatment of the aerosol by the TD. Fundamental differences in the
performance of the two methods lead to different conclusions regarding the presence or
absence, size, and concentration of solid particles in Diesel exhaust.
In Chapter 5 the physical and chemical nature of the engine exhaust from a Formula
SAE spark ignition engine was evaluated using two competition fuels, 100 octane race
fuel and E85. Three engine conditions were evaluated: 6000 RPM 75% throttle, 8000
RPM 50% throttle, and 8000 RPM 100% throttle. Diluted emissions were characterized
using a Scanning Mobility Particle Sizer (SMPS) and a Condensation Particle Counter
(CPC). E85 fuel produced more power and produced less particulate matter emissions at
all test conditions, but more fuel was consumed.Chapter 6 demonstrates how exhaust aerosol measurements can be used to diagnose an
engine fault in a Diesel engine. A cyclic variation in total particle number concentration
was observed while making routine exhaust emission measurements. Many dilution and
engine operating conditions were examined and by sequentially shutting down individual
cylinders the problem was traced to cylinder 2. The engine was disassembled and piston
2’s oil control ring was found to be fractured. Replacement of the ring eliminated the
particle concentration fluctuation. This chapter presents the results of experimental
measurements made to determine the cause of the irregular emissions.
Chapter 7 describes the results of three experiments performed with Continuously
Regenerating Traps (CRTs) in a controlled laboratory setting to elucidate the effects of
fuel sulfur content, filter age, and storage and release effects on particle concentration. In
the first experiment, a new CRT was evaluated using near zero sulfur Fischer Tropsch
fuel and low sulfur lubricating oil (420 ppm). The objective was to measure particle
emissions from an emission control device that had not previously been exposed to sulfur
under a variety of operating and dilution conditions. Next, a used CRT was evaluated
using the same fuel and lubricating oil. Finally, the used uncatalyzed Diesel particulate
filter (DPF) from the used CRT was replaced with a new, uncatalyzed DPF. The
emissions from the used Diesel oxidation catalyst (DOC) + new DPF configuration were evaluated and compared to those of the used CRT.
Results show that particle number emissions from the new CRTs are 99.9% lower than
equivalent used CRT data collected on-road at an exhaust temperature of 370°C. Even as
the new CRT temperature was increased to almost 400°C, emissions levels were still at
background levels for roadway aerosol and no nucleation mode was observed. At an
exhaust temperature of about 380°C, the nucleation mode particle number concentration
increased sharply and remained high for the duration of the used CRT test. Mass
emissions were estimated and found to exceed U.S. EPA on-road standards. The particle
number concentration at the start of the used DOC + new DPF evaluation was equal to
that measured at the end of the used CRT experiment, suggesting that only sulfates released from the DOC and not the uncatalyzed DPF significantly contribute to
Contents of this thesis have been or will be published in the following peer-reviewed
journals. Reprint copyright permissions are found in Appendix A.
Chapter 2: Swanson, J., Kittelson, D., Pui, D., Watts, W. “Alternatives to the
Gravimetric Method for Quantification of Diesel Particulate Matter Near the Lower
Level Of Detection.” In press, J. Air Waste Management Association, 2010.
Chapter 3: Swanson, J., Watts, W., Kittelson, D. “Diesel Exhaust Aerosol
Measurements Using Air-Ejector and Porous Wall Dilution Techniques.” SAE Tech.
Pap. Ser. 111PFL-0658.
Chapter 4: Swanson, J., Kittelson, D. “Evaluation of Thermal Denuder and Catalytic
Stripper Methods for Solid Particle Measurements.” In review, J. Aerosol Science,
Chapter 5: Ragatz, A., Swanson, J., Watts, W., Kittelson, D. “Particle and Gaseous
Emissions Characteristics of a Formula SAE Race Car Engine.” SAE Tech. Pap. Ser. 2009, 2009-01-1400.
University of Minnesota Ph.D. dissertation. August 2010. Major: Mechanical Engineering. Advisors: Dr. David B. Kittelson, Dr. David Y.H. Pui. 1 computer file (PDF); xiv, 201 pages, appendix p. 198-201.
Swanson, Jacob John.
Development and application of emerging engine exhaust aerosol measurement technologies..
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