@article{frey_choi_kim_2012, title={Portable Emission Measurement System for Emissions of Passenger Rail Locomotives}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-84869852000&partnerID=MN8TOARS}, DOI={10.3141/2289-08}, abstractNote={ The purpose of this study was to demonstrate a method for measuring passenger railroad locomotive emissions with the use of a portable emission measurement system (PEMS) based on rail yard load tests of three locomotives, including one GP40 and two F59PHIs. These locomotives have mechanically governed diesel prime mover engines (PMEs) with an approximately 3,000-hp output. Each locomotive has a head end power (HEP) engine that produces approximately 600 hp for generating electricity used in the passenger cars. The engine measurements were based on ultralow sulfur diesel fuel. Each engine was instrumented to measure manifold absolute pressure, engine revolutions per minute, intake air temperature, and exhaust concentrations of selected gases and particles. These data were used to quantify exhaust and fuel flow. The exhaust concentrations of nitric oxide, carbon monoxide (CO), carbon dioxide, hydrocarbons, and particulate matter were measured. The PMEs are operated at each of many throttle notch settings. For the HEP engines, three electrical loads were applied on the basis of power usage for one, two, and four passenger cars, respectively. More than 97% of the raw data survived a multistep quality assurance process. The data obtained from the PEMS for the main engines were found to be comparable on a fuel basis to data reported by others, particularly for oxides of nitrogen and CO. The key results from this work are the establishment of a simplified methodology for future tests and the development of baseline data. }, number={2289}, journal={TRANSPORTATION RESEARCH RECORD}, author={Frey, H. Christopher and Choi, Hyung-Wook and Kim, Kangwook}, year={2012}, pages={56–63} }
 @article{graver_frey_choi_2011, title={In-Use Measurement of Activity, Energy Use, and Emissions of a Plug-in Hybrid Electric Vehicle}, volume={45}, ISSN={["0013-936X"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-80054685657&partnerID=MN8TOARS}, DOI={10.1021/es201165d}, abstractNote={Plug-in hybrid electric vehicles (PHEVs) could reduce transportation air emissions and energy use. However, a method is needed for estimating on-road emissions of PHEVs. To develop a framework for quantifying microscale energy use and emissions (EU&E), measurements were conducted on a Toyota Prius retrofitted with a plug-in battery system on eight routes. Measurements were made using the following: (1) a data logger for the hybrid control system; (2) a portable emissions measurement system; and (3) a global positioning system with barometric altimeter. Trends in EU&E are estimated based on vehicle specific power. Energy economy is quantified based on gasoline consumed by the engine and grid energy consumed by the plug-in battery. Emissions from electricity consumption are estimated based on the power generation mix. Fuel use is approximately 30% lower during plug-in battery use. Grid emissions were higher for CO₂, NO(x), SO₂, and PM compared to tailpipe emissions but lower for CO and hydrocarbons. EU&E depends on engine and plug-in battery operation. The use of two energy sources must be addressed in characterizing fuel economy; overall energy economy is 11% lower if including grid energy use than accounting only for fuel consumption.}, number={20}, journal={ENVIRONMENTAL SCIENCE & TECHNOLOGY}, author={Graver, Brandon M. and Frey, H. Christopher and Choi, Hyung-Wook}, year={2011}, month={Oct}, pages={9044–9051} }
 @article{choi_frey_2010, title={Estimating Diesel Vehicle Emission Factors at Constant and High Speeds for Short Road Segments}, ISSN={["2169-4052"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-78651279535&partnerID=MN8TOARS}, DOI={10.3141/2158-03}, abstractNote={ Vehicle emission estimates with high spatial and temporal resolution are needed to estimate near-roadway air quality and human exposure to emissions. The U.S. Environmental Protection Agency's (EPA) MOBILE6 emission factor model is based on test cycles with transient speeds and durations on the order of 10 min. MOBILE6 does not report emission factors for average speeds higher than 65 mph. However, for near-roadway studies, emission factors are needed for short highway segments that represent only a few seconds of vehicle travel time at approximately constant speed and speeds greater than 65 mph. Constant-speed and high-speed correction factors for nitrogen oxides, hydrocarbons, carbon dioxide (CO2), and carbon monoxide were developed on the basis of 59,286 s of EPA dynamometer emissions data for diesel vehicles from which speed–acceleration modal models were derived. The constant speed correction factor is the ratio of the emission factor at constant speed to the emissions factor during transient cycles with an equivalent average speed. For example, the constant speed correction factor for CO2 increases from 0.71 to 1 as speed increases from 31 to 78 mph. High-speed correction factors are based on the ratio of emission factors at a constant speed greater than 65 mph to emission factors at a constant speed of 65 mph. For example, at 80 mph, the high-speed correction factor is approximately 1.5 for CO2. Sensitivity analysis of emission estimates to these correction factors was conducted for speeds of 40 to 80 mph. }, number={2158}, journal={TRANSPORTATION RESEARCH RECORD}, author={Choi, Hyung-Wook and Frey, H. Christopher}, year={2010}, pages={19–27} }
 @article{choi_frey_2010, title={Method for in-use measurement and evaluation of the activity, fuel use, electricity use, and emissions of a plug-in hybrid diesel-electric school bus}, volume={44}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-77951823059&partnerID=MN8TOARS}, DOI={10.1021/es903330k}, abstractNote={The purpose of this study is to demonstrate a methodology for characterizing at high resolution the energy use and emissions of a plug-in parallel-hybrid diesel-electric school bus (PHSB) to support assessments of sensitivity to driving cycles and comparisons to a conventional diesel school bus (CDSB). Data were collected using onboard instruments for a first-of-a-kind prototype PHSB and a CDSB of the same chassis and engine, operated on actual school bus routes. The engine load was estimated on the basis of vehicle specific power (VSP) and an empirically derived relationship between VSP and engine manifold absolute pressure (MAP). VSP depends on speed, acceleration, and road grade. For the PHSB, the observed electrical discharge or recharge to the traction motor battery was characterized on the basis of VSP. The energy use and emission rates of the PHSB from tailpipe and electricity use were estimated for five real-world driving cycles and compared to the engine fuel use and emissions of the CDSB. The PHSB had the greatest advantage on arterial routes and less advantage on highway or local routes. The coupled VSP-MAP modeling approach enables assessment of a wide variety of driving conditions and comparisons of vehicles with different propulsion technologies.}, number={9}, journal={Environmental Science and Technology}, author={Choi, H.-W. and Frey, H. Christopher}, year={2010}, pages={3601–3607} }
 @article{choi_frey_2009, title={Light duty gasoline vehicle emission factors at high transient and constant speeds for short road segments}, volume={14}, ISSN={["1361-9209"]}, url={http://www.scopus.com/inward/record.url?eid=2-s2.0-71249122701&partnerID=MN8TOARS}, DOI={10.1016/j.trd.2009.09.001}, abstractNote={Vehicle emissions estimates are needed at high spatial and temporal resolution to estimate near-roadway air quality and human exposures. The MOBILE6 emission factor model is based on transient test cycles of less than 65 mph. Correction factors for high speed and constant speed are developed based on vehicle-specific power-based modal models for light duty gasoline vehicles, using data from portable emission measurement systems. At 80 mph versus 65 mph, the estimated average emission rates are greater by 30%, 20%, 80%, and 10% for NOx, HC, CO, and CO2. The ratio of constant to average of transient speed emission rates range from 0.49 to 0.94 for NOx at speeds of 20 mph and 80 mph. The high speed and constant speed correction factors are applied to estimate vehicle emissions for a freeway segment that includes vehicle cruising speeds between 65 and 80 mph. The potential error for not accounting for constant speed operation on a short segment of highway could be 49% at moderate speed and 24% at high speed.}, number={8}, journal={TRANSPORTATION RESEARCH PART D-TRANSPORT AND ENVIRONMENT}, author={Choi, Hyung-Wook and Frey, H. Christopher}, year={2009}, month={Dec}, pages={610–614} }