Comparison of Parallel and Series Hybrid Powertrains for Transit Bus Applications

1 The fuel economy and emissions of both conventional and hybrid buses equipped with emissions 2 aftertreatment were evaluated via computational simulation for six representative city bus drive cycles. 3 Both series and parallel configurations for the hybrid case were studied. The simulation results indicate 4 that series hybrid buses have the greatest overall advantage in fuel economy. The series and parallel 5 hybrid buses were predicted to produce similar CO and HC tailpipe emissions but were also predicted to 6 have reduced NOx tailpipe emissions compared to the conventional bus in higher speed cycles. For the 7 New York bus cycle (NYBC), which has the lowest average speed among the cycles evaluated, the series 8 bus tailpipe emissions were somewhat higher than they were for the conventional bus, while the parallel 9 hybrid bus had significantly lower tailpipe emissions. All three bus powertrains were found to require 10 periodic active DPF regeneration to maintain PM control. Plug-in operation of series hybrid buses appears 11 to offer significant fuel economy benefits and is easily employed due to the relatively large battery 12 capacity that is typical of the series hybrid configuration. 13 14 Z. Gao, C.S. Daw, D.E. Smith, P.T. Jones, T.J. LaClair, J.E. Parks II 3 INTRODUCTION 15 The city transit bus is an important mode of public transportation that is undergoing rapid change in terms 16 of the vehicles employed in active service. There are nearly 70,000 buses operating in the U.S. today. In 17 1996, more than 95% were powered by conventional diesel powertrains, but by the end of 2014 about 18 40% had been replaced with buses using alternative emerging power sources [1-2]. By 2008, hybrid buses 19 already represented 18% of the city transit bus market [2], and this market share has increased 20 significantly since then. A recent report released by the business intelligence survey firm IDTechEx 21 projects that the emerging global market for hybrid and pure electric buses will be worth $100 billion by 22 2025 [3]. This is mainly attributed to the excellent fuel savings and emissions reductions offered by 23 hybridization for the city transit bus application, due to their high frequency of stop-and-go and idle 24 operation [4]. 25 Hybrid powertrains used in city bus applications are generally either series or parallel 26 configurations [5]. In a series configuration, the wheels are driven directly by an electric motor, while the 27 engine drives a generator that converts its mechanical power into the electricity that powers the electric 28 motor or is stored in a battery. The primary electrical components the motor, generator and the battery 29 of a series hybrid must provide or accept power levels approaching or even exceeding what is required to 30 propel the vehicle, resulting in rather significant costs. A primary advantage of the series hybrid is that the 31 battery is able to accommodate most of the transient power fluctuations associated with stop-and-go 32 driving. Thus, the engine for a series hybrid can be downsized to provide only the average power needed 33 by the vehicle, and the engine is able to operate near its peak efficiency most of the time. A downside to 34 this scenario is the efficiency penalty associated with the dual-step conversion of energy (i.e., mechanical 35 to electric to mechanical). In parallel configurations the bus can be propelled separately by the engine or 36 electric motor, or by both at the same time. The option of having two power sources makes it possible to 37 keep the electric motor and battery smaller, thus reducing costs [5]. Although favorable results have been 38 reported in the literature for the fuel consumption and emissions of hybrid transit buses [6-13], many 39 issues related to the combined fuel efficiency and emissions control of series and parallel hybrid 40 powertrain configurations, particularly when integrated with emerging aftertreatment systems required to 41 meet new emissions regulations, remain unresolved. This is particularly true when the effects of different 42 urban drive cycles are considered [14]. 43 The objective of the present paper is to develop a consistent simulation methodology that can 44 account for rapidly evolving diesel hybrid and aftertreatment technologies and the increasing constraints 45 imposed by the current emissions regulations. By evaluating and comparing the fuel economy and 46 emissions of conventional and hybrid city transit buses equipped with aftertreatment devices, the authors 47 aim to identify potential advantages and technical barriers that can be expected when applying hybrid 48 technologies in city bus applications. The software platform used in this study is Autonomie, an open 49 architecture powertrain and vehicle systems simulation tool developed by Argonne National Laboratory 50 [15]. In addition to components included in the default bus configuration specified in Autonomie, the 51 authors added engine, aftertreatment, and battery models developed at Oak Ridge National Laboratory 52 (ORNL) that account for the transient fuel consumption, battery energy, engine-out emissions, and 53 aftertreatment component performance of comparable conventional and hybrid buses operating over 54 representative city bus drive cycles. 55 LITERATURE REVIEW OF HYBRID BUSES 56 A wide range of experimental results have reported that hybrid powertrain technologies increase fuel 57 savings for city transit buses [7-13], as summarized in Table 1. The tested hybrids include both series and 58 parallel buses, and the data were measured for both on-road city driving or chassis dynamometer testing 59 conditions. In general, these results show that the fuel consumption benefits of hybrid buses are most 60 significant for slowand medium-speed drive cycles (i.e. <18 mph) [13]. Transit Authorities have 61 quantified the fuel savings by direct comparison of hybrid and conventional diesel bus fuel consumption 62 [16]. However, the fuel savings reported vary considerably even for the same drive cycle or similar 63 Z. Gao, C.S. Daw, D.E. Smith, P.T. Jones, T.J. LaClair, J.E. Parks II 4 conditions of on-road city testing. This variation is likely the result of multiple factors, including low 64 measurement precision and lack of consistent methodologies for comparing different hybrid concepts 65 used in bus applications [7-8]. For example, in the on-road evaluations, most reported results include only 66 a statistical analysis of the overall fuel economy of a small sample of tested hybrid and conventional 67 buses [7-12]. For the chassis dynamometer measurements, many studies focused on a specific series or 68 parallel hybrid configuration compared to a conventional bus [7-8, 11-13]. Thus the results did not 69 evaluate the difference in fuel saving between series and parallel powertrain configurations. 70 71 TABLE 1 Literature Summary for Fuel Economy and Emissions Reduction. 72 Testing Cycle Fuel Economy CO HC NOx PM Samples & model-year of hybrid & conventional bus CBD -23%~ -59% -97%~-38% -43%~+450% -36%~-49% -93%~-50% 2 S (1999)-: 2 C (1998) [7] -54%~ 59% -94%~ -38% +120%~+450% -49% -93%~-60% 1 S (1999) : 2 C (1999) [12] -48% -48% -75% -27% -97% 1 P (2004) : 1 C (2004) [8] MAN -48% -98% -28% -44% -99% 2S (1999) : 2 C (1998) [7] -75% N/A N/A -39% -93% 1 P (2004) : 1 C (2004) [8] -44%~ -22% -51%~+25% -88%~0% -47%~+56% -78%~0% 1 P & 1 S (2010) : 2 C (2011/2012) [13] NYBC -64% -56% +88% -44% -77% 2 S (1999)-: 2 C (1998) [7] OCTA -51% -32% N/A -29% -51% 1 P (2004) : 1 C (2004) [8] -36%~ -14% -50%~+650% -25%~+60% -31%~-11% -94%~+33% 1 P & 1 S (2010) : 2 C (2011/2012) [13] KCM -30% -60% -56% -18% N/A 1 P (2004) : 1 C (2004) [8] UDDS -14%~ 0% +120%~+300% -70%~0% -7%~+2% 0%~+150% 1 P & 1 S (2010) : 2 C (2011/2012) [13] New York city a -10%~ -22% N/A N/A N/A N/A 10 S (1999) : 10 C (1998) [7] King County a -27% N/A N/A N/A N/A 235 P (2004) : 30 C (2004) [8] 15 cities a , b -21%~-40% N/A N/A N/A N/A 357 P (<2008 ) [9] New York city a -28%~ -38% N/A N/A N/A N/A 20 S (2002/2004) : 10 C (1994/1998) [10] Ames, Iowa a -12% N/A N/A N/A N/A 10 P (2010) : 7 C (2008/2010) [11] C: conventional; P: parallel; S: series; a on-road city testing; b Including DC, New York, Seattle etc.; c Unknown. 73 74 Table 1 also reveals a considerable range of results in the available experimental emissions 75 measurements for hybrid vs. conventional buses in city driving [7-8, 11-13]. The results reported before 76 2007 demonstrate that hybrids achieved significant emissions reductions (with the exception of HC 77 emissions in some cases). However, the data appearing after 2010 show a complex mix of results with 78 less clear emissions differences between hybrid and conventional diesel buses. One possible explanation 79 for this ambiguity could be changes in emission controls that have taken place due to rapidly evolving 80 aftertreatment technologies and regulations [3, 17]. For example, in 2004, the U.S. EPA emissions 81 standard was 2.4 g/bhp-hr for NOx and 0.1 g/bhp-hr for PM emissions; in 2007-2010, a new standard was 82 phased-in that restricts emissions to 0.2 g/bhp-hr for NOx and 0.01 g/bhp-hr for PM, as well as 0.14 83 g/bhp-hr for HC [17]. The new emissions regulation requires buses to be equipped with a full exhaust 84 aftertreatment system that typically consists of a diesel oxidation catalyst (DOC), a diesel particulate filter 85 (DPF), and urea-selective catalytic NOx reduction (urea-SCR) in order to minimize tailpipe emissions [4]. 86 Here, the DPF is generally based on a wall-flow substrate and can be catalyzed or non-catalyzed specific 87 to application and design. The 2002-2004 and earlier bus models were often either equipped with a DOC 88 only or no aftertreatment system was employed at all [7, 12], and SCR was rarely used. 89 As a consequence of the rapidly changing technology and regulatory environment described 90 above, it is very challenging to make consistent, comprehensive comparisons between the various 91 hybridization options for buses utilizing existing published data and reports. Thus, developing a flexible, 92 consistent, and well-defined approach for simulating comprehensive hybrid bus systems under realistic 93 operating conditions will be important for making informed decisions about commercialization and 94 deployment of the next generation of urban public transit systems. 95 Z. Gao, C.S. Daw, D.E. Smith, P.T. Jones, T.J. LaClair, J.E. Parks II 5