Bus Emissions Research Articles

Measuring In-Cabin School Bus Tailpipe and Crankcase PM2.5: A New Dual Tracer Method

ABSTRACT Exposures of occupants in school buses to on-road vehicle emissions, including emissions from the bus itself, can be substantially greater than those in outdoor settings. A dual tracer method was developed and applied to two school buses in Seattle in 2005 to quantify in-cabin fine particulate matter (PM2.5) concentrations attributable to the buses' diesel engine tailpipe (DPMtp) and crankcase vent (PMck) emissions. The new method avoids the problem of differentiating bus emissions from chemically identical emissions of other vehicles by using a fuel-based organometallic iridium tracer for engine exhaust and by adding deuterated hexatriacontane to engine oil. Source testing results showed consistent PM:tracer ratios for the primary tracer for each type of emissions. Comparisons of the PM:tracer ratios indicated that there was a small amount of unburned lubricating oil emitted from the tailpipe; however, virtually no diesel fuel combustion products were found in the crankcase emissions. For the limited testing conducted here, although PMck emission rates (averages of 0.028 and 0.099 g/km for the two buses) were lower than those from the tailpipe (0.18 and 0.14 g/km), in-cabin PMck concentrations averaging 6.8 μg/m3 were higher than DPMtp (0.91 μg/m3 average). In-cabin DPMtp and PMck concentrations were significantly higher with bus windows closed (1.4 and 12 μg/m3, respectively) as compared with open (0.44 and 1.3 μg/m3, respectively). For comparison, average closed- and open-window in-cabin total PM2.5 concentrations were 26 and 12 μg/m3, respectively. Despite the relatively short in-cabin sampling times, very high sensitivities were achieved, with detection limits of 0.002 μg/m3 for DPMtp and 0.05 μg/m3 for PMck. IMPLICATIONS PM2.5 measurements in two Seattle school buses showed average concentrations of 26 and 12 μg/m3 with windows closed and open, respectively. Virtually all PM2.5 was car bonaceous. Tracer measurements showed that bus self-pollution contributed approximately 50% of total PM2.5 concentrations with windows closed and 15% with windows open, with over three-quarters of these contributions attributed to crankcase emissions. Maintaining ventilation in buses clearly reduces total PM2.5 exposures and that from the buses' own emissions. The dual tracer method now offers researchers a new technique for explicit identification of single source contributions in settings with multiple sources of carbonaceous emissions.

On-road pollutant emission and fuel consumption characteristics of buses in Beijing

On-road emission and fuel consumption (FC) levels for Euro III and IV buses fueled on diesel and compressed natural gas (CNG) were compared, and emission and FC characteristics of buses were analyzed based on approximately 28,700 groups of instantaneous data obtained in Beijing using a portable emissions measurement system (PEMS). The experimental results revealed that NO x and PM emissions from CNG buses were decreased by 72.0% and 82.3% respectively, compared with Euro IV diesel buses. Similarly, these emissions were reduced by 75.2% and 96.3% respectively, compared with Euro III diesel buses. In addition, CO 2, CO, HC, NO X, PM emissions and FC of Euro IV diesel buses were reduced by 26.4%, 75.2%, 73.6%, 11.4%, 79.1%, and 26.0%, respectively, relative to Euro III diesel buses. The CO 2, CO, HC, NO x, PM emissions and FC factors all decreased with bus speed increased, while increased as bus acceleration increased. At the same time, the emission/FC rates as well as the emission/FC factors exhibited a strong positive correlation with the vehicle specific power (VSP). They all were the lowest when VSP < 0, and then rapidly increased as VSP increased. Furthermore, both the emission/FC rates and emission/FC factors were the highest at accelerations, higher at cruise speeds, and the lowest at decelerations for non-idling buses. These results can provide a base reference to further estimate bus emission and FC inventories in Beijing.

Real-world operation conditions and on-road emissions of Beijing diesel buses measured by using portable emission measurement system and electric low-pressure impactor

On-road measurement is an effective method to investigate real-world emissions generated from vehicles and estimate the difference between engine certification cycles and real-world operating conditions. This study presents the results of on-road measurements collected from urban buses which propelled by diesel engine in Beijing city. Two widely used Euro III emission level buses and two Euro IV emission level buses were chosen to perform on-road emission measurements using portable emission measurement system (PEMS) for gaseous pollutant and Electric Low Pressure Impactor (ELPI) for particulate matter (PM) number emissions. The results indicate that considerable discrepancies of engine operating conditions between real-world driving cycles and engine certification cycles have been observed. Under real-world operating conditions, carbon monoxide (CO) and hydrocarbon (HC) emissions can easily meet their respective regulations limits, while brake specification nitrogen oxide (bsNOx) emissions present a significant deviation from its corresponding limit. Compared with standard limits, the real-world bsNOx emission of the two Euro III emission level buses approximately increased by 60% and 120% respectively, and bsNOx of two Euro IV buses nearly twice standard limits because Selective Catalytic Reduction (SCR) system not active under low exhaust temperature. Particle mass were estimated via particle size distribution with the assumption that particle density and diameter is liner. The results demonstrate that nanometer size particulate matter make significant contribution to total particle number but play a minor role to total particle mass. It is suggested that specific certified cycle should be developed to regulate bus engines emissions on the test bench or use PEMS to control the bus emissions under real-world operating conditions.

Powertrain system optimization for a heavy-duty hybrid electric bus

This research concerns the design of a powertrain system for a plug-in parallel diesel hybrid electric bus equipped with a continuously variable transmission (CVT) and presents a new design paradigm for the plug-in hybrid electric bus (HEB). The criteria and method for selecting and sizing powertrain components equipped in the plug-in HEB are presented. The plug-in HEB is designed to overcome the vulnerable limitations of driving range and performance of a purely electric vehicle (EV), and it is also designed to improve the fuel economy and exhaust emissions of conventional buses and conventional HEBs. Optimization of the control strategy for the complicated and interconnected propulsion system in the plug-in parallel HEB is one of the most significant factors for achieving higher fuel economy and lower exhaust emissions in the hybrid electric vehicle (HEV). In this research, the proposed control strategy was simulated to prove its validity using the ADVISOR (advanced vehicle simulator) analysis simulation tool.

Integrated Multimodel Evaluation of Transit Bus Emissions in Toronto, Canada

This paper investigates transit bus emissions in the city of Toronto, Ontario, Canada, by linking the results of a microsimulation transit assignment model, MILATRAS (microsimulation learning-based approach to transit assignment), with emission factors derived from Mobile6.2C. Emissions were estimated at the level of individual buses during idling conditions at bus stops and on roadway links between stops during the morning peak period. The busiest routes were associated with the highest total emissions as a result of a combination of high ridership and lower speeds; this association confirmed the common wisdom that newer, low-emitting buses should be first allocated to these routes. The highest dwell emissions occurred at intermodal transfer stations (bus to subway and vice versa). On a passenger kilometer basis, the highest-emitting routes were not the busiest, but rather were those with the lowest ridership. In fact, the highest emissions per passenger kilometer were associated with the Airport Rocket, a route that provided service to the airport and was characterized by low ridership in the morning peak period. On average, bus trips in Toronto were about three times more fuel efficient than were private car trips and created 20 times less carbon monoxide pollution. The effects of changing fuel types and fleet age on transit bus emissions were assessed. Implications for bus operations are discussed relative to fleet allocation to minimize total emissions.

Analysis of Global and Local Environmental Impacts of Bus Transport by LCA Methodologies

The mobility of the globalized world is supported by internal-combustion engines, which generally use fossil fuels. City people have to move daily from place A to B. For that reason cities have public transport systems, including local bus as an important element. As the bulk of the population is concentrated in the city, the atmospherical emissions of local buses have a considerable impact on human health. This study analyses these impacts using the methodology of LCA and the database of the ARTEMIS project. The whole study includes the emissions models of pre-euro and euro 1-5 buses, CNG, biodiesel and hydrogen buses, and the fuel production. Furtermore seven scenarios for modeling traffic situations are included, too. For the impact assessment three CML2001 indicators are used. Global warming potential (GWP) is for assessing the global impacts, carbon footprint; human toxicity potencial (HTP) and photochemical ozone creation potential (POCP) are used for the estimation of impacts in urban environments.

Spatial Distribution of Diesel Transit Bus Emissions and Urban Populations: Implications of Coincidence and Scale on Exposure

Macroscale emissions modeling approaches have been widely applied in impact assessments of mobile source emissions. However, these approaches poorly characterize the spatial distribution of emissions and have been shown to underestimate emissions of some pollutants. To quantify the implications of these limitations on exposure assessments, CO, NO(X), and HC emissions from diesel transit buses were estimated at 50 m intervals along a bus rapid transit route using a microscale emissions modeling approach. The impacted population around the route was estimated using census, pedestrian count and transit ridership data. Emissions exhibited significant spatial variability. In intervals near major intersections and bus stops, emissions were 1.6-3.0 times higher than average. The coincidence of these emission hot spots and peaks in pedestrian populations resulted in a 20-40% increase in exposure compared to estimates that assumed homogeneous spatial distributions of emissions and/or populations along the route. An additional 19-30% increase in exposure resulted from the underestimate of CO and NO(X) emissions by macroscale modeling approaches. The results of this study indicate that macroscale modeling approaches underestimate exposure due to poor characterization of the influence of vehicle activity on the spatial distribution of emissions and total emissions.

Vehicle Emissions during Children’s School Commuting: Impacts of Education Policy

We explore how school policies influence the environmental impacts of school commutes. Our research is motivated by increased interest in school choice policies (in part because of the U.S. "No Child Left Behind" Act) and in reducing bus service to address recent budget shortfalls. Our analysis employs two samples of elementary-age children, age 5-12: a travel survey (n = 1246 respondents) and a school enrollment data set (n = 19,655 students). Multinomial logistic regression modeled the determinants of travel mode (automobile, school bus, and walking; n = 803 students meeting selection criteria). Travel distance has the single greatest effect on travel mode, though school choice, trip direction (to- or from-school), and grade play a role. Several policies were investigated quantitatively to predict the impact on school travel, vehicle emissions, and costs. We find that eliminating district-wide school choice (i.e., returning to a system with neighborhood schools only) would have significant impacts on transport modes and emissions, whereas in many cases proposed shifts in school choice and bus-provision policies would have only modest impacts. Policies such as school choice and school siting may conflict with the goal of increasing rates of active (i.e., nonmotorized) school commuting. Policies that curtail bus usage may reduce bus emissions but yield even larger increases in private-vehicle emissions. Our findings underscore the need to critically evaluate transportation-related environmental and health impacts of currently proposed changes in school policy.

Mitigating Excessive Idling of Transit Buses

In recent years much media attention has been paid to research developments related to three types of diesel vehicles: school buses, long-haul trucks, and transit buses. Transit bus emissions studies often gain attention because the public wants to know whether transit agencies are spending public dollars efficiently and with concern for public health. The public health and environmental effects of transit bus idling and strategies to mitigate idling practices need to be understood. Most transit bus idling is considered nondiscretionary and occurs while a bus is stuck in traffic or making a stop along its route. However, in some scenarios transit buses idle unnecessarily. Although many solutions exist to limit idling, these solutions have not garnered much interest from transit agencies, researchers, or lawmakers. This paper will review the causes and effects of transit bus idling and examine the potential costs and benefits of using idling reduction technologies and policies on the bus fleet of a large metropolitan transit agency in the Midwest to limit fuel consumption and emissions. The paper is organized as follows: Economic, environmental, and health effects of diesel emissions are discussed in the introductory section, as is current anti-idling public policy. The research methodology is described, and causes of excessive bus idling at the Chicago Transit Authority in Illinois are discussed, as are potential solutions. Idling reduction technologies are examined to determine which provides the greatest economic and environmental benefit for the lowest cost. The paper concludes with specific recommendations for the Chicago Transit Authority.

A web-based support system for estimating and visualizing the emissions of diesel transit buses

Diesel transit buses are heavy-duty vehicles that are major sources of greenhouse gases and toxic pollutants. Although various models have been used to estimate their emissions, it has been difficult to effectively apply these estimation models due to the need for user-friendly interfaces, the large amounts of underlying data, and the potential data inaccuracy. In this paper, we present a web-based support system developed for transit operators who need to estimate and visualize the emissions of diesel transit buses where a micro-scale Vehicle Specific Power approach is used to estimate emissions based on global positioning system data. Case studies show that the web-based support system provides a user-friendly environment that makes it easier to apply emission estimation methodologies and visualize emissions.

The design and aerodynamics of commercial vehicles

Improving transport efficiency is an ongoing challenge for bus and truck manufacturers. Especially aerodynamics offer a high potential in terms of reducing fuel consumption and emissions of trucks and buses. MAN reaches with a aerodynamic-optimised semitrailer tractor a CW value of 0.30.

Modal Analysis of Vehicle Operation and Particulate Emissions from Connecticut Transit Buses

Transit buses represent a significant source of particulate exhaust emissions, especially in urban areas, but few previous studies have quantified these emissions by using real-world, onboard sampling while the vehicles operate in the transportation network. In this study, real-world particle number emissions for hybrid diesel–electric (HDE) and conventional diesel (CD) buses are examined for various vehicle operating conditions and road types in the Hartford, Connecticut, region. The results presented in this paper are based on analysis of the unique second-by-second Connecticut Transit on-road transit bus emissions and operations data set collected between January and November 2004. The modal analysis results indicate that hybrid buses operate differently than do conventional diesel buses. Although the distributions of vehicle specific power (VSP) values (in kw) were similar for the two bus types, the distributions of engine operation parameters (load and speed) were different. More important, VSP alone cannot be used to distinguish between vehicle types for modeling engine operation of, and possibly emissions from hybrid and conventional vehicles. Furthermore, modal analysis of ultrafine particle emissions indicates that in some situations the HDE buses do not outperform the CDs and may even produce higher emission rates than do the CD buses tested. Thus, there are routes and conditions in which transit authorities should avoid the use of HDE buses similar to those tested here when particle emissions are of concern. The observed high particle emissions on steep uphill grades for hybrids indicate that careful route selection for hybrid buses is warranted to optimize the environmental benefits of the hybrid vehicles.

Sustainability provisions in the bus-scheduling problem

The traditional vehicle scheduling problem attempts to minimize capital and operating costs. However, the carbon footprint and toxic air pollutants have become an increasingly important consideration. This paper studies the bus-scheduling problem and evaluates new types of buses that use alternative energy sources to reduce emissions, including some toxic air pollutants and carbon dioxide. A time-space network based approach is applied to formulate the problem to reduce the numbers of arcs in the underlying network; CPLEX is used to solve the problem. The results show that the bus-scheduling model can significantly reduce the bus emissions – hence reducing the carbon footprint of the transit operation – while only slightly increasing operating costs.

A Vehicle-Specific Power Approach to Speed- and Facility-Specific Emissions Estimates for Diesel Transit Buses

Emissions during a trip often depend on transient vehicle dynamics that influence the instantaneous engine load. Vehicle specific power (VSP) is a proxy variable for engine load that has been shown to be highly correlated with emissions. This study estimates roadway link average emission rates for diesel-fueled transit buses based on link mean speeds, using newly defined VSP modes from data gathered by a portable emissions monitoring system. Speed profiles were categorized by facility type and mean travel speed, and stratified into discrete VSP modes. VSP modal average emission rates and the time spent in the corresponding VSP modes were then used to make aggregate estimates of total and average emission rates for a road link. The average emission rates were sensitive to link mean speed, but not to facility type. A recommendation is made regarding the implementation of link average emission rates in conjunction with transportation models for the purpose of estimating regional emissions for diesel transit buses.

Development of a methodology for the quantification of particle number and gaseous concentrations in a bidirectional bus tunnel and the derivation of emission factors

Particle number, NO x and CO concentrations were measured simultaneously at the air entry portal and at the mid-point of a 511 m bidirectional road tunnel, used entirely by urban public transport buses. The aim of this study was to provide information on concentrations of these pollutants inside a unique bus tunnel, and to develop a viable methodology for determining emission factors for on-road vehicles. Measurements were made continuously over a period of five days that included a complete weekend. Traffic flow rate and air flow rate were also monitored. The mean particle number concentration at mid-tunnel was 4.1 × 10 4 cm −3, which was over four times higher than the urban background concentration. The mean concentrations of NO x and CO at mid-tunnel were 464 ppb and 802 ppb, respectively. All these values were between 2 and 4 times higher than at the air entry portal. Median concentrations during selected time segments coinciding with the morning and evening rush hours, mid-day during weekdays and full day during the weekends were determined and the corresponding bus emission factors of each of the three parameters was calculated. Mean emission factors found for particle number, NO x and CO were 7.1 × 10 14 particles km −1, 8.1 g km −1 and 15.9 g km −1, respectively. These values compared well with previous studies, showing that the methodology adopted was sound and viable.

Experimental Analysis of Vehicle Operation Parameters Affecting Emission Behavior of Public Transport Buses with Alternative Diesel Fuels

Key results are presented from an exhaust emission study conducted on more than 105 buses in idling condition and selected sample buses from seven fleets in real-world operating conditions in Toledo, Ohio, of the effects of varying vehicular operation parameters on public transport bus emissions. It was observed that for the same amount of time in operation, vehicles in idling mode produced higher average concentrations of carbon monoxide, nitric oxide, nitrogen dioxide, and sulfur dioxide than those in on-road operation. Operating the vehicles in normal engine idling mode and with higher engine temperatures was found to reduce vehicular emissions by up to 30% to 42% as compared with fast idling and lower or colder engine temperatures. Performing regular preventive maintenance reduced emission concentrations by 15% to 20%. The results also highlight that a combination of multiple influencing factors including engine technology, vehicle age, vehicular maintenance, fuel, and past and present operating conditions should be evaluated to better understand and predict vehicular emission behavior.

A case study of real-world tailpipe emissions for school buses using a 20% biodiesel blend

Numerous laboratory studies report carbon monoxide, hydrocarbon, and particulate matter emission reductions with a slight nitrogen oxides emission increase from engines operating with biodiesel and biodiesel blends as compared to using petroleum diesel. We conducted a field study on a fleet of school buses to evaluate the effects of biodiesel use on gaseous and particulate matter fuel-based emission factors under real-world conditions. The field experiment was carried out in two phases during winter 2004. In January (phase I), emissions from approximately 200 school buses operating on petroleum diesel were measured. Immediately after the end of the first phase measurement period, the buses were switched to a 20% biodiesel blend. Emission factors were measured again in March 2004 (phase II) and compared with the January emission factors. To measure gaseous emission factors we used a commercial gaseous remote sensor. Particulate matter emission factors were determined with a combination of the gaseous remote sensor, a Lidar (light detection and ranging), and transmissometer system developed at the Desert Research Institute of Reno, NV, U.S.A. Particulate matter emissions from school buses significantly increased (up to a factor of 1.8) after the switch from petroleum diesel to a 20% biodiesel blend. The fuel used during this campaign was provided by a local distributor and was independently analyzed at the end of the on-road experiment. The analysis found high concentrations of free glycerin and reduced flash points in the B 100 parent fuel. Both measures indicate improper separation and processing of the biodiesel product during production. The biodiesel fuels used in the school buses were not in compliance with the U.S.A. ASTM D6751 biodiesel standard that was finalized in December of 2001. The U.S.A. National Biodiesel Board has formed a voluntary National Biodiesel Accreditation Program for producers and marketers of biodiesel to ensure product quality and compliance with the ASTM standard. The results of our study underline the importance of the program since potential emission benefits from biodiesel may be reduced or even reversed without appropriate fuel quality control on real-world fuels.

Studies of Self-Pollution in Diesel School Buses: Methodological Issues

Considerable interest has focused on levels of exhaust emissions in the cabins of diesel-powered school buses and their possible adverse health effects. Significantly different policy and engineering issues would be raised if compelling evidence found that in-c-cabin contamination was due to self-pollution from bus emissions, rather than ambient pollution, neighboring vehicles, and/or re-entrained road dust. We identified 19 reports from 11 studies that measured diesel exhaust particulate in the cabins of 58 school bus of various type. Studies were evaluated in light of their experimental design, their data quality, and their capacity to quantify self-pollution. Only one study had a true experimental design, comparing the same buses with and without emission controls, while four others used intentional tracers to quantify tailpipe and/or crankcase emissions. Although definitive data are still lacking, these studies suggest that currently available control technologies can nearly eliminate particulate self-pollution inside diesel school buses.

In-vehicle Measurement of Ultrafine Particles on Compressed Natural Gas, Conventional Diesel, and Oxidation-catalyst Diesel Heavy-duty Transit Buses

Many metropolitan transit authorities are considering upgrading transit bus fleets to decrease ambient criteria pollutant levels. Advancements in engine and fuel technology have lead to a generation of lower-emission buses in a variety of fuel types. Dynamometer tests show substantial reductions in particulate mass emissions for younger buses (<10 years) over older models, but particle number reduction has not been verified in the research. Recent studies suggest that particle number is a more important factor than particle mass in determining health effects. In-vehicle particle number concentration measurements on conventional diesel, oxidation-catalyst diesel and compressed natural gas transit buses are compared to estimate relative in-vehicle particulate exposures. Two primary consistencies are observed from the data: the CNG buses have average particle count concentrations near the average concentrations for the oxidation-catalyst diesel buses, and the conventional diesel buses have average particle count concentrations approximately three to four times greater than the CNG buses. Particle number concentrations are also noticeably affected by bus idling behavior and ventilation options, such as, window position and air conditioning.

Commercial Bus Emissions and Fuel Use

This paper summarizes the results of emissions testing conducted on six motor coaches with engines between model years 1997 and 2004. Each bus was tested while idling and also while being driven in simulated low-speed urban traffic. The intent of this program was to evaluate the potential effects of idle restriction policies on coach buses, particularly when coaches are forced to circulate in urban traffic to maintain appropriate cabin temperatures if restricted from idling. All tested buses used more fuel and emitted more nitrogen oxides (NOx) when being driven in simulated urban traffic than when idling. Fuel use at least doubled for all tested buses when driven compared with idling. For older coaches, NOx also doubled, whereas for newer coaches NOx emissions increased by 40%. If a coach bus is forced to circulate in traffic to maintain appropriate cabin temperatures, rather than idling while stationary, it will use up to 375 more gallons of fuel and emit up to 22 lb of excess NOx annually for only 1 h/day of circulating. Both fuel use and NOx emissions generally increased for all buses when the air conditioning was on, compared with when it was not, both while idling and while being driven. In this test program, the increase was less during urban driving than while idling, but given the relatively low ambient temperatures during the testing, the results may not be fully reflective of actual summer results in many parts of the country. For both idling and urban driving, the two newest buses (2004 engines) produced significantly less NOx than the four older buses. This finding is consistent with more stringent U.S. Environmental Protection Agency emissions standards for newer engines.