Comparison of on-road emissions for hybrid and regular transit buses

Abstract

Hybrid technology offers an attractive option for transit buses, since it has the potential to significantly reduce operating costs for transit agencies. The main impetus behind use of hybrid transit vehicles is fuel savings and reduced emissions. Laboratory tests have indicated that hybrid transit buses can have significantly lower emissions compared with conventional transit buses. However, the number of studies is limited and laboratory tests may not represent actual driving conditions, since in-use vehicle operation differs from laboratory test cycles. This paper describes an on-road evaluation of in-use emission differences between hybrid-electric and conventional transit buses for the Ames, Iowa transit authority, CyRide. Emissions were collected on-road using a portable emissions monitoring system (PEMS) for three hybrid and two control buses. Emissions were collected for at least one operating bus day. Each bus was evaluated over the same route pattern, which utilizes the same driver. The number of passengers embarking or disembarking at each stop was collected by an on-board data collector so that passenger load could be included. Vehicle emissions are correlated to engine load demand, which is a function of factors such as vehicle load, speed, and acceleration. PEMS data are provided second by second and vehicle-specific power (VSP) was calculated for each row of data. Instantaneous data were stratified into the defined VSP bins and then average modal emission rates and standard errors were calculated for each bus for each pollutant. Pollutants were then compared by bus type. Carbon dioxide, carbon monoxide, and hydrocarbon emissions were higher for the regular buses across most VSP bins than for the hybrid buses. Nitrogen oxide emissions were unexpectedly higher for the hybrid buses than for the control buses.Implications: The main reason agencies consider hybrid transit vehicles is fuel savings and reduced emissions. Hybrid electric buses offer an attractive option and have the potential to significantly reduce operating costs and emissions for transit agencies. However, purchase of hybrid buses cost 50-70% more than regular buses. As a result, agencies require more quantitative information about the on-road costs and benefits of hybrid buses. This study assessed on-road emissions for in-use transit buses. The resulting information can be used by transit agencies in assessing the likely emission reduction benefits for hybrid buses.

Real-World Measurement of Hybrid Buses’ Fuel Consumption and Pollutant Emissions in a Metropolitan Urban Road Network

“…The hybrid bus engines are equipped with EGR and, presumably, a more efficient SCR system to comply with Euro VI limits. The current results show that aftertreatment and powertrain tuning of hybrid buses may offer substantial reductions in NO x , a finding that comes in contrast to some earlier studies [8,15,17]. As an example of emission control specific tuning, Euro VI bus over route #1 exhibits a higher exhaust gas temperature than Euro V, presumably as a result of engine thermal management to retain a high NO x conversion efficiency in the SCR.…”

“…To explain the difference, Figure 5 shows the distribution of fuel consumption over different fuel rate classes. This is equivalent to expressing emissions over vehicle specific power (VSP) bins, as many studies suggest (e.g., Guo et al [17]; Hallmark et al [8]; Zhang et al [29]), but the chosen representation is more reliable, being directly based on measured signals. In route #1, conducted at low speeds and high temperatures, most of the fuel consumption is at low flow rates and the peak in the concentration is shifted to higher fuel rates for the hybrid than the conventional bus.…”

“…Fuel consumption is the main direct cost element of urban transit fleet operation [8]. Options that can reduce direct and external costs of road transport are therefore of importance [9,10].…”

“…Merkisz and Pielecha [15] found that the FC of a hybrid-diesel bus was 15-18% lower than a corresponding conventional one, depending on traffic conditions in the city of Poznan, Poland. Hallmark et al [8] reported an average 11.8% higher fuel economy of three hybrid buses than two conventional ones in Iowa, USA. Semercioglu et al [16] observed a 30% fuel savings potential, after examining the results of one hybrid bus over a conventional one in Sakarya, Turkey.…”

Evaluating Nitrogen Oxides and Ultrafine Particulate Matter Emission Features of Urban Bus Based on Real-World Driving Conditions in the Yangtze River Delta Area, China

“…To study the bus emission characteristics in urban driving conditions, on-road Portable Emission Measurement System (PEMS) has been used predominantly in different cities to link the vehicle driving condition and the emissions [4][5][6][7][8][9][10]. Previous studies can be divided into two parts.…”