Seungju Yoon

Criteria pollutant and greenhouse gas emissions from CNG transit buses equipped with three-way catalysts compared to lean-burn engines and oxidation catalyst technologies

Engine and exhaust control technologies applied to compressed natural gas (CNG) transit buses have advanced from lean-burn, to lean-burn with oxidation catalyst (OxC), to stoichiometric combustion with three-way catalyst (TWC). With this technology advancement, regulated gaseous and particulate matter emissions have been significantly reduced. Two CNG transit buses equipped with stoichiometric combustion engines and TWCs were tested on a chassis dynamometer, and their emissions were measured. Emissions from the stoichiometric engines with TWCs were then compared to the emissions from lean-burn CNG transit buses tested in previous studies. Stoichiometric combustion with TWC was effective in reducing emissions of oxides of nitrogen (NOX), particulate matter (PM), and nonmethane hydrocarbon (NMHC) by 87% to 98% depending on pollutants and test cycles, compared to lean combustion. The high removal efficiencies exceeded the emission reduction required from the certification standards, especially for NOX and PM. While the certification standards require 95% and 90% reductions for NOX and PM, respectively, from the engine model years 1998–2003 to the engine model year 2007, the measured NOX and PM emissions show 96% and 95% reductions, respectively, from the lean-burn engines to the stoichiometric engines with TWC over the transient Urban Dynamometer Driving Schedule (UDDS) cycle. One drawback of stoichiometric combustion with TWC is that this technology produces higher carbon monoxide (CO) emissions than lean combustion. In regard to controlling CO emissions, lean combustion with OxC is more effective than stoichiometric combustion. Stoichiometric combustion with TWC produced higher greenhouse gas (GHG) emissions including carbon dioxide (CO2) and methane (CH4) than lean combustion during the UDDS cycle, but lower GHG emissions during the steady-state cruise cycle. Implications: Stoichiometric combustion with three-way catalyst is currently the best emission control technology available for compressed natural gas (CNG) transit buses to meet the stringent U.S. Environmental Protection Agency (EPA) 2010 heavy-duty engine NOX emissions standard. For existing lean-burn CNG transit buses in the fleet, oxidation catalyst would be the most effective retrofit technology for the control of NMHC and CO emissions.

Characterization of Particulate Matter Emissions from a Current Technology Natural Gas Engine

Experiments were conducted to characterize the particulate matter (PM)-size distribution, number concentration, and chemical composition emitted from transit buses powered by a USEPA 2010 compliant, stoichiometric heavy-duty natural gas engine equipped with a three-way catalyst (TWC). Results of the particle-size distribution showed a predominant nucleation mode centered close to 10 nm. PM mass in the size range of 6.04 to 25.5 nm correlated strongly with mass of lubrication-oil-derived elemental species detected in the gravimetric PM sample. Results from oil analysis indicated an elemental composition that was similar to that detected in the PM samples. The source of elemental species in the oil sample can be attributed to additives and engine wear. Chemical speciation of particulate matter (PM) showed that lubrication-oil-based additives and wear metals were a major fraction of the PM mass emitted from the buses. The results of the study indicate the possible existence of nanoparticles below 25 nm formed as a result of lubrication oil passage through the combustion chamber. Furthermore, the results of oxidative stress (OS) analysis on the PM samples indicated strong correlations with both the PM mass calculated in the nanoparticle-size bin and the mass of elemental species that can be linked to lubrication oil as the source.

Real-world exhaust temperature profiles of on-road heavy-duty diesel vehicles equipped with selective catalytic reduction

On-road heavy-duty diesel vehicles are a major contributor of oxides of nitrogen (NOx) emissions. In the US, many heavy-duty diesel vehicles employ selective catalytic reduction (SCR) technology to meet the 2010 emission standard for NOx. Typically, SCR needs to be at least 200°C before a significant level of NOx reduction is achieved. However, this SCR temperature requirement may not be met under some real-world operating conditions, such as during cold starts, long idling, or low speed/low engine load driving activities. The frequency of vehicle operation with low SCR temperature varies partly by the vehicles vocational use. In this study, detailed vehicle and engine activity data were collected from 90 heavy-duty vehicles involved in a range of vocations, including line haul, drayage, construction, agricultural, food distribution, beverage distribution, refuse, public work, and utility repair. The data were used to create real-world SCR temperature and engine load profiles and identify the fraction of vehicle operating time that SCR may not be as effective for NOx control. It is found that the vehicles participated in this study operate with SCR temperature lower than 200°C for 11-70% of the time depending on their vocation type. This implies that real-world NOx control efficiency could deviate from the control efficiency observed during engine certification.

Emissions During and Real-world Frequency of Heavy-duty Diesel Particulate Filter Regeneration

Recent tightening of particulate matter (PM) emission standards for heavy-duty engines has spurred the widespread adoption of diesel particulate filters (DPFs), which need to be regenerated periodically to remove trapped PM. The total impact of DPFs therefore depends not only on their filtering efficiency during normal operation, but also on the emissions during and the frequency of regeneration events. We performed active (parked and driving) and passive regenerations on two heavy-duty diesel vehicles (HDDVs), and report the chemical composition of emissions during these events, as well as the efficiency with which trapped PM is converted to gas-phase products. We also collected activity data from 85 HDDVs to determine how often regeneration occurs during real-world operation. PM emitted during regeneration ranged from 0.2 to 16.3 g, and the average time and distance between real-world active regenerations was 28.0 h and 599 miles. These results indicate that regeneration of real-world DPFs does not substantially offset the reduction of PM by DPFs during normal operation. The broad ranges of regeneration frequency per truck (3-100 h and 23-4078 miles) underscore the challenges in designing engines and associated aftertreatments that reduce emissions for all real-world duty cycles.

Real-World Exhaust Temperature and Engine Load Distributions of On-Road Heavy-Duty Diesel Vehicles in Various Vocations

Real-world vehicle and engine activity data were collected from 90 heavy-duty vehicles in California, United States, most of which have engine model year 2010 or newer and are equipped with selective catalytic reduction (SCR). The 90 vehicles represent 19 different groups defined by a combination of vocational use and geographic region. The data were collected using advanced data loggers that recorded vehicle speed, position (latitude and longitude), and more than 170 engine and aftertreatment parameters (including engine load and exhaust temperature) at the frequency of one Hz. This article presents plots of real-world exhaust temperature and engine load distributions for the 19 vehicle groups. In each plot, both frequency distribution and cumulative frequency distribution are shown. These distributions are generated using the aggregated data from all vehicle samples in each group.

In-Use Emissions from 2010-Technology Heavy-Duty Trucks

Introduction of a selective catalytic reduction system for heavy-duty diesel trucks (HDDTs) has substantially reduced emissions of oxides of nitrogen (NOx). However, it was found that in-use NOx emissions measured from three 2010-technology HDDTs were higher than the certification standard and higher than the levels measured during engine certification. In-use NOx emissions from three HDDTs tested over chassis dynamometer cycles were 1.7 to 9 times higher than the NOx certification standard of 0.20 grams per brake horsepower-hour, and the emissions measured with a portable emissions measurement system over highway test routes were up to five times higher than the certification standard. Such high in-use NOx emissions occurred primarily during low-speed operations (25 mph or less). This is a concern in California because more than 50% of running-exhaust NOx emissions from HDDTs will occur during low-speed operations that constitute only 11% of total vehicle miles traveled by 2025. This substantial contribution of NOx emissions during low-speed operations should be addressed carefully in the process of developing regulations and strategies to improve air quality in California. For better understanding and control of high in-use NOx emissions, there is a strong need for investigation of NOx control technologies effective at low-speed operation, differences between engine testing and whole vehicle testing procedures, and the roles of both engine certification requirements and in-use compliance requirements in reducing real-world NOx emissions.

Correction to Characterization of Particulate Matter Emissions from a Current Technology Natural Gas Engine

the figure showed a high mass fraction of lubrication oil derived elements and metals. Figure 4 of this erratum shows the corrected mass fraction of PM after the calculation error was addressed. The corrections of the error results in the conclusion that the mass fraction of lubrication oil derived elements and metals are less than 10% of total mass of PM. The comparison of distance-specific emissions of lubricationoil-derived elements from this study with previous SCRequipped diesel work presented by Hu et al. is discussed in Line 2, second column of page 8239 of the original manuscript. The original manuscript suggested that the distance-specific emissions of lubrication-oil-derived elements are an order of magnitude higher from TWC-equipped natural gas vehicles compared to DPF-SCR-equipped diesel over the UDDS driving cycle. As a result of the correction to the calculation, the results now conclude that the distance-specific emissions of lubrication-oil-derived elements from TWC-equipped natural gas engines are up to 2 times higher than the retrofit DPF-SCR equipped diesel engines. The overall conclusions of the study remain unchanged after the correction of the error. The original manuscript suggests the possibility of lubrication oil-derived elements from TWCequipped natural gas engines to contribute more toward particle number count in the 10 nm size range. The manuscript also suggests that renucleation of inorganic lubrication oil additives, passing through the combustion chamber of the engine to be the primary contributor to particle number count in this size range.