Development and application of emerging engine exhaust aerosol measurement technologies.

Abstract

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 nanoparticle formation. 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, 2010. 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.