A Comparison of Emissions and Fuel Economy from Hybrid-Electric

Energy & Fuels 2004, 18, 257-270

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A Comparison of Emissions and Fuel Economy from Hybrid-Electric and Conventional-Drive Transit Buses W. Scott Wayne,* Nigel N. Clark, Ralph D. Nine, and Dennis Elefante† West Virginia University, P.O. Box 6106, Morgantown, West Virginia 26506-6106 Received April 24, 2003. Revised Manuscript Received September 15, 2003

Hybrid-electric transit buses offer potential benefits over conventional transit buses of comparable capacity, including reduced fuel consumption, reduced emissions, and the utilization of smaller engines. Emissions measurements were performed on a 1998 New Flyer 40-foot transit bus equipped with a Cummins ISB 5.9-L diesel engine, an Engelhard DPX catalyzed particulate filter, and an Allison series-drive system. Results were compared to a conventional-drive, dieselpowered bus that was equipped with an oxidation catalyst, and to a liquefied natural gas (LNG)powered bus. Tests were performed according to the guidelines of SAE Recommended Practice J2711. On average, the oxides of nitrogen (NOx) emissions from the hybrid bus were reduced by 50%, compared to the conventional-drive diesel bus, and 10%, compared to the LNG bus. Particulate matter (PM) emissions from the catalyzed filter-equipped hybrid bus were reduced by 90%, relative to those of the conventional diesel bus, and were comparable to those of the LNG bus.

Introduction

Hybrid-Electric Vehicles

Growing concerns over energy conservation and the reduction of air pollution have led to the development of hybrid-electric power plants for heavy-duty transit buses. Hybrid-electric transit buses offer potential benefits over conventional transit buses of comparable capacity, including reduced fuel consumption, reduced emissions, and the utilization of smaller engines. These benefits are made possible through the use of regenerative braking and reductions in engine transient operation through sophisticated power management systems. Characterization of emissions from hybrid-electric buses represents new territory, because standardized procedures have only recently emerged in the form of SAE J2711, the Recommended Practice for Measuring Fuel Economy and Emissions of Hybrid-Electric and Conventional Heavy-Duty Vehicles.1 The West Virginia University Transportable Heavy Vehicle Emissions Testing Laboratory (TransLab) was used to characterize emissions from a diesel hybridelectric-powered transit bus, a conventional-drive dieselpowered transit bus, and liquefied natural gas (LNG)powered transit buses. The Orange County (California) Transit Authority (OCTA) supplied the buses that have been included in this study. The goal of this research program was to evaluate the emissions and fuel economy from a hybrid-electric transit bus and compare its performance and emissions with that of conventional transit buses.

A hybrid-electric is defined as a vehicle that can draw propulsion energy from two sources of stored energy: (i) a consumable fuel and (ii) a rechargeable energy storage system that is recharged by an on-board electric generating system or an off-board charging system or power supply.1 Hybrid-electric power plants have the potential to reduce emissions and fuel consumption. These benefits are realized by allowing the vehicle’s engine to operate more often within its optimum performance band, which is a compromise between fuel economy and emissions. Also, a substantial portion of the energy required to accelerate the vehicle can be recaptured during deceleration through the use of regenerative braking.2 Hybrid-electric drive systems consist of three components: (i) a propulsion system that consists of an electric motor coupled through a drivetrain to the wheels; (ii) a rechargeable energy storage system (RESS), such as batteries, capacitors, and electromechanical flywheels; and (iii) a power unit that consists of an internal combustion engine coupled to a generator.2 Parallel and series drive configurations are the most commonly encountered systems. In a series drive system, the power unit supplies electrical energy directly to the electric motor, which, in turn, drives the wheels. In a parallel configuration, the engine is directly coupled to the vehicle drivetrain so that power to drive the wheels can be supplied by the engine, the electric motor, or a combination of both. Most existing hybrid vehicle literature3,4 has addressed light-duty vehicle performance.

* Author to whom correspondence should be addressed. E-mail: [email protected]. † Currently with Orange County Transit Authority, Orange, CA 92863-1584. (1) Recommended Practice for Measuring Fuel Economy and Emissions of Hybrid-Electric and Conventional Heavy-Duty Vehicles, SAE Standard J2711; Society of Automotive Engineers: Warrendale, PA, 2002.

(2) McKain, D.; Clark, N.; Balon, T.; Moynihan, P.; Lynch, S.; Webb, T. Characterization of Emissions from Hybrid-Electric and Conventional Transit Buses. Presented at CEC/SAE Spring Fuels and Lubricants Meeting and Exposition, Paris, June 2000, SAE Paper No. 2000-01-2011.

10.1021/ef030096t CCC: $27.50 © 2004 American Chemical Society Published on Web 12/19/2003

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Figure 1. Vehicle under test in position on the dynamometer rollers.

Two important considerations must be addressed when measuring emissions from a hybrid-electric vehicle. First, a correction of performance data based on the state of charge of the RESS must be performed to ensure test-to-test consistency. Second, hybrid-electric vehicles are recognized to be sensitive to decelerations prescribed in the test cycle, because of characteristics of their regenerative braking systems. During severe braking, energy will still be lost to heat through the use of the service brakes of the vehicle. West Virginia University Transportable Laboratories The West Virginia University Transportable HeavyDuty Vehicle Emissions Testing Laboratories were constructed to gather emissions data from in-use heavyduty vehicles. Detailed information pertaining to the design and operation of the laboratories can be found in previous technical papers.5-8 Each laboratory is based around two trailers; one incorporates rollers, flywheels, and power absorbers for the dynamometer function, whereas the second trailer houses control and emissions measurement equipment and data acquisition systems. The vehicle to be tested is driven onto the chassis dynamometer and positioned on two sets of rollers (Figure 1). Usually, the outer wheels of the dual wheel set on each side of the vehicle are removed and replaced with hub adapters that couple (3) Glenn, B.; Washington, G.; Rizzoni, G. Operation and Control Strategies for Hybrid Electric Automobiles. Presented at Future Car Congress, Crystal City, VA, April 2000, SAE Paper No. 2000-01-1537. (4) Duoba, M.; Ng, H.; Larsen, R. Characterization and Comparison of Two Hybrid Electric Vehicles (Hevs)sHonda Insight and Toyota Prius. Presented at the SAE 2001 World Congress, Detroit, MI, March 2001, SAE Paper No. 2001-01-1335. (5) Wang, W.; Gautam, M.; Sun, X.; Bata, R.; Clark, N.; Palmer, G. M.; Lyons, D. Emissions Comparisons of Twenty-Six Heavy Duty Vehicles Operated on Conventional and Alternative Fuels. SAE Tech. Pap. Ser. 1993, 932952. (6) Clark, N.; Gautam, M.; Bata, R.; Lyons, D. Design and Operation of a New Transportable Laboratory for Emissions Testing of HeavyDuty Trucks and Buses. Int. J. Vehicle Des.: Heavy Vehicle Syst. 1995, 2, (3/4), 285-299. (7) Clark, N.; Gautam, M.; Rapp, B.; Lyons, D.; Grabosky, M.; McCormick, R.; Alleman, T.; Norton, P. Transit Bus Emissions Characterization by Two Chassis Dynamometer Laboratories: Results and Issues. In SAE 1999 TransactionssJournal of Fuels and Lubricants. Society of Automotive Engineers: Warrendale, PA, 1999; Vol. 108, pp 801-812. (8) Wang, W. G; Bata, R. M.; Lyons, D. W.; Clark, N. N.; Palmer, G. M.; Gautam, M.; Howell, A. D.; Rapp, B. L. Transient Response in a Dynamometer Power Absorption System. SAE Tech. Pap. Ser. 1992, 920252.

Wayne et al.

Figure 2. Hub adapters connect the vehicle’s drive axle to the power absorber unit.

the drive axle directly to the dynamometer units on each side of the vehicle (Figure 2). Each dynamometer unit consists of a power absorber and a set of selectable flywheels, which consist of a series of disks to allow simulation of an inertial load equivalent to a gross vehicle weight of up to 60 000 lbs in 250-lb increments. During the test cycle, torque cells and speed transducers in the power absorber drive train measure the vehicle load and speed and the power absorber is managed in closed-loop mode to mimic tire losses and wind drag. Prompted by a video screen, a human driver operates the vehicle through a wide range of available computerized test schedules to simulate either transient or steady-state driving conditions. The full exhaust from the tail pipe of the test vehicle is ducted to a full-scale dilution tunnel 18 in. (45 cm) in diameter and 20 ft (6.1 m) in length (see Figure 1). The exhaust is mixed with air in the dilution tunnel, and the flow rate of diluted exhaust is metered and measured precisely by a critical-flow venturi system. The diluted exhaust is analyzed using nondispersive infrared (NDIR) analysis for carbon monoxide (CO) and carbon dioxide (CO2), and chemiluminescent detection for NOx. Hydrocarbons (HCs) are analyzed using flame ionization detection (FID). The nonmethane hydrocarbon (NMHC) level is determined on a schedule-averaged basis, using gas chromatography (GC). The gaseous data are available as continuous concentrations throughout the test, and the product of concentration and dilution tunnel flow are integrated to yield emissions in units of grams per mile (g/mi). Particulate matter (PM) is collected using 70-mm fluorocarbon-coated glass-fiber filter media, and PM emissions are determined gravimetrically. Fuel efficiencies are determined using a carbon balance and exhaust emissions data. Test Vehicles Three vehicles were examined in this program; a series-drive diesel hybrid-electric transit bus, a conventional-drive diesel-powered transit bus, and a conventional-drive LNG-powered transit bus. The hybrid-electric vehicle examined in this program was a 1998 New Flyer 40-foot transit bus equipped with a 275-hp Cummins ISB275 5.9-L diesel engine, an Engelhard DPX catalyzed particulate filter, and an Allison series-drive system. The Cummins ISB275

Comparison of Hybrid and Conventional Transit Buses

engine had a power rating of 275 hp at 2500 rpm. The hybrid drive configuration was a series drive, consisting of a diesel engine that drove a generator, an electric motor that drove the rear axle, a battery pack, and an associated power electronics/control system. The control strategy was to capture the braking energy via regeneration from the traction motor to storage batteries and maintain a reasonable rate of retardation, regardless of battery state of charge (SOC). Retardation initially was a result of charging the batteries, and as the battery charge increased (approaching the threshold), the surplus energy from the traction motor was redirected to the engine generator in an effort to maintain negative torque at the traction motor. Motoring the generator and driving the engine against an exhaust brake as a “compressor” dissipated the surplus energy. The energy strategy was for the engine generator to maintain a lesser/reasonable battery SOC, allowing for a higher state of charge via brake regeneration. The acceleration strategy was to provide the bulk of the initial acceleration energy via the storage batteries, and as the speed increased, the engine generator power was blended together. The New Flyer bus had a curb weight of 30 510 lbs and a gross vehicle weight rating of 37 930 lbs, and the bus was tested at a simulated weight of 35 085 lbs. The simulated test weight was determined by adding the vehicle curb weight and half of the passenger load (1/2 × passenger capacity × 150 lbs, plus 150 lbs for the driver). The diesel-powered transit bus was a 1997 New Flyer 40-foot transit bus that was equipped with a Detroit Diesel Corporation (DDC) Series 50 engine, a Nelson diesel oxidation catalyst, and an Allison B400R automatic transmission. The 8.5-L DDC Series 50 engine had a power rating of 275 hp at 2100 rpm. The dieselpowered New Flyer bus had a curb weight of 29 625 lbs and a gross vehicle weight rating of 37 930 lbs, and the bus was tested at a simulated weight of 32 775 lbs. The LNG-powered transit bus was a 2001 North American Bus Industries (NABI) 40-foot transit bus equipped with a DDC Series 50G engine and an Allison B400R automatic transmission. The LNG bus was not equipped with an exhaust aftertreatment device; however, it did use DDC low-NOx R-27 software. The NABI bus had a curb weight of 30 200 lbs, and a gross vehicle weight rating of 40 600 lbs. It was tested at a simulated weight of 34 775 lbs. Driving Schedules Driving cycles were selected based on the guidelines contained in SAE J2711.1 Although the SAE J2711 Recommended Practice can be applied using any driving cycle, the Manhattan cycle (representing low-speed operation), the Orange County Transit Authority (OCTA) cycle (representing intermediate-speed operation), and the Urban Dynamometer Driving Schedule (UDDS or Test D, representing high-speed operation) are recommended for transit buses. In addition, a limited number of tests were conducted using the Central Business District (CBD) cycle, in the interests of comparison with historical data. Charge-sustaining hybrid-electric vehicles require longer test runs, because a single drive cycle is unlikely to affect the SOC at a level sufficient to cause the engine management system to provide additional power to the

Energy & Fuels, Vol. 18, No. 1, 2004 259

Figure 3. Manhattan cycle.

RESS. The use of a longer test cycle increases the probability of a smaller net energy change (NEC) between the initial and final values on a percentage basis.1 The use of a longer test cycle also facilitates improved accuracy in the detection of emissions produced from low-emitting vehicles, such as those equipped with particulate filters that approach the threshold detection limits of the laboratory. The SAE J2711 procedure recommends that back-to-back drive cycles be combined to produce a test run of ∼30 min. The Manhattan cycle (Figure 3) is representative of transit-bus operation in city service. The Manhattan cycle was developed by West Virginia University to reflect driving conditions in the New York City metropolitan area.2 The cycle was developed by utilizing actual, in-use route segments of data logged from New York City Metropolitan Transit Authority (NYC MTA) buses that were operating in Manhattan. Data were logged from both conventional-drive and hybrid-electric buses over several different routes. The complete data set consisted of 237 microtrips from conventional buses and 112 microtrips from hybrid-electric buses. Microtrips are defined as vehicle operation (at speeds of >0.5 mph) from a starting point until the vehicle has arrived at a destination (when the vehicle speed is 2 mph/s), medium (1-2 mph/s), and light (0.3-1 mph/s). Histograms were plotted for each acceleration and deceleration bin and for cruise operation, showing the percentage of time spent in each acceleration/speed bin. The express route was not included as part of the database, because it was not used to construct the cycle. The histogram for the medium acceleration bin is shown in Figure 5. The remaining histograms are provided in Appendix A.


Figure 6. Heavy-duty urban dynamometer driving schedule (UDDS).

The United States Environmental Protection Agency (USEPA) developed the heavy-duty UDDS,9 which is also called “Test D”, which is used to precondition vehicles for evaporative emissions testing but is not federally recognized for emissions testing. This is a speed-time schedule, as shown in Figure 6, and was originally constructed using a Monte Carlo (MC) simulation that was based on a statistically binned speed and acceleration matrix. This matrix was derived from a range of instrumented vehicles, which included 44 trucks and 4 buses in New York and 44 trucks and 3 buses in Los Angeles. The vehicles were gasoline- and diesel-fueled and included two- and three-axle straightframe trucks and tractor-trailer configurations. The subcycles, namely MC simulations of New York City nonfreeway driving conditions and Los Angeles freeway and nonfreeway driving conditions were combined to develop complete cycles that were fairly representative of real-life driving patterns. The UDDS may be criticized, in that (i) it is not necessarily representative of a present-day fleet and (ii) it represents average vehicle characteristics. In consequence, heavy-duty vehicles with low power-to-weight ratios and unsynchronized transmissions have difficulty in following the trace, whereas lighter automatic transmission vehicles follow the schedule easily. It is understood that vehicles that fall below the required speed must maintain full power until reaching the required speed, but then a lesser distance than the scheduled distance is covered and emissions, in regard to mass/ distance, are effectively based on a different schedule. However, hybrid or automatic-transmission buses usually follow the UDDS closely. In accordance the SAE J2711 recommendation that test cycles have duration of 30 min, the hybrid-electric bus was exercised over two back-to-back UDDS cycles. The CBD cycle, shown in Figure 7, was originally configured to determine the fuel consumption of a bus driven on an oval track. Although it was subsequently adopted for determining emissions on chassis dynamometers, it does not mimic true bus operation sufficiently for emissions characterization.1 There are several issues with using the CBD cycle for emissions characterization. A major concern is the fact that the (9) Code of Federal Regulations, Title 40, Part 86, Subpart N, Protection of Environment; US Government Printing Office: Washington, DC, 1998.


Figure 7. Central Business District (CBD) cycle.

CBD cycle contains only one (repeated) accelerationcruise-deceleration event. All of the cruise operation occurs at a speed of 20 mph. This limited modal operationsin particular, the cruisescan affect one bus configuration more than another, relative to real use.1 In some cases, the 20-mph cruise speed may correspond to a natural shift point of some automatic transmissions, such that the behavior of the transmission can profoundly affect the engine operation and emissions from two fairly similar buses with slightly different transmission shift points. Another concern with the CBD cycle is that the aggressive deceleration rates limit the effectiveness of the regenerative braking system and may not fully characterize the advantage of hybrid-electric drive systems. Research has shown that actual bus use often has less aggressive deceleration. Although the CBD cycle has traditionally been used to assess emissions from transit buses and a large database of results exists, it is not recommended for hybrid-electric buses. Therefore, only a limited number of CBD cycles were performed, in the interest of comparison with historical data. Three back-to-back repeats of the CBD cycle were performed to obtain a cycle with a duration of 30 min. Statistics of each of the test cycles are listed in Table 2. Vehicle Preparation The emissions tests were conducted on location in Riverside, CA, during May 2002. Prior to beginning the test program, the laboratory analytical systems were checked and calibrated according to the procedures outlined in the calibration methodology outlined in CFR Title 40, Part 86 Subpart N (CFR40),9 SAE J2711,1 and WVU Transportable Laboratory standard operating procedures (SOPs). SAE J2711 protocol was followed as closely as possible. Upon receipt of each vehicle, a safety inspection was conducted and pretest information was documented. Fuel samples were collected from each test vehicle for subsequent analysis. The outer rear wheels were removed, the hub adapters were fitted and the vehicle was installed onto the dynamometer. Hydraulic jacks positioned on scales were used to support ∼50% of the rear axle curb weight (the weight that would normally be carried by the outer set of tires). No modifications to the vehicles were required to disable antilock braking systems or traction control systems.

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Wayne et al. Table 2: Test Cycle Statistics

Speed (mph) test cycle Manhattan ×2 OCTA UDDS ×2 CBD ×3

standard maximum maximum total idle total distance number of average deviation maximum acceleration (mph/s) deceleration (mph/s) time (s) time (s) (miles) idle periods 6.83 12.33 18.84 12.58

7.34 10.3 19.84 8.36

25.3 40.63 58 20

3.98 4.05 4.19 2.40

Figure 8. On-road coast-down profile of diesel-powered bus number 5347.

-5.73 -5.13 -4.51 -4.50

2178 1909 2121 1722

786 406 706 345

4.13 6.54 11.1 6.2

41 30 27 43

Figure 9. On-road coast-down profile of LNG-powered bus number 2201.

Road-Load Simulation During chassis dynamometer testing, nonrecoverable road-load and wind losses encountered in real-world onroad operation must be duplicated. The aerodynamics of the vehicle body shape and the rolling resistance of the tires, as a function of vehicle speed, must be determined and mimicked on the dynamometer. There are two basic methods of determining the road-load profile of a vehicle: experimentally and analytically. The profile can be determined experimentally by recording the deceleration of the vehicle on the road as it coasts from a speed of 55 mph to a stop. The procedure is repeated multiple times in both directions to minimize bias caused by terrain, wind, and other factors. Alternatively, physical constants for the vehicles can be estimated and entered into a physical model to produce the profile. In this study, the road-load profile was experimentally determined by performing an on-road coast down of each bus. The measured coast-down curves are shown in Figures 8-10. Each bus was then allowed to coast down on the dynamometer at the appropriate simulated test weight and a dynamometer loading profile was derived to match the on-road coast-down profile so that the dynamometer provided the appropriate retarding force as a function of vehicle speed throughout the test speed range. The absorbed road power profile includes the inherent dynamometer friction as well as the power absorbed by the power absorber units.

Figure 10. On-road coast-down profile of hybrid-electric bus number 5419.

Allison Electric Drive Systems recorded battery current and voltage data from the hybrid-electric bus for the purpose of determining the SOC correction for the hybrid-electric RESS. WVU also recorded the battery current with an inductive Fluke current meter that had an input range of 0-1000 A (dc) and an output of 1 mV/A. WVU recorded battery voltage using an adjustable resistor network. To assess the effect of regenerative braking, the service-air-brake actuation chamber pressure at one rear wheel of each bus was also measured.

Test Instrumentation

Air Conditioning Loads

Regulated emissions of HC, NOx, CO, and PM, as well as CO2 emissions and fuel economy, were measured according to CFR40 Part 86 Subpart N procedures.9 Samples for subsequent methane and NMHC analysis, using GC, were collected from the LNG-powered bus. In addition to total particulates, PM10 and PM2.5 measurements were also performed. A representative from

Air conditioning (AC) loads in buses can be a significant consumer of on-board vehicle power and, as a result, can have a significant effect on the emissions and fuel economy. A holistic view of bus efficiency and emissions should consider the typical air conditioning load. Unfortunately, the typical air conditioning load is not readily described. The degree to which the bus is


insulated or reflective and the relative heat and humidity loads imparted by passengers and environmental conditions make the simulation of typical air conditioning loads a complicated task.1 Factors that affect the AC load include target temperature and humidity inside the bus, temperature and humidity outside of the bus, heat-transfer characteristics of the bus, internal component mass, and air infiltration rates. There is a temptation simply to test with the AC activated and attempt to correct for ambient conditions; however, this approach would be fraught with inaccuracies.1 Although evaluation of the emissions with the AC activated is possible, results cannot be equitably compared from one bus to another or from one test to another unless all operating variables are similar and reasonable. For this reason, SAE J2711 protocol does not recommend that testing be conducted with the AC in operation. It was recognized during the course of this study that AC and other hotel loads might be responsible for substantial differences in fuel economy data between the dynamometer results and in-service fuel economy data. Therefore, one test was performed using the LNG-powered bus with the AC activated. Test Cycles and Sequencing The hybrid-electric bus (vehicle 5419) was tested May 22, 2002. The hybrid bus was tested at a simulated weight of 35 085 lbs. As recommended by the SAE J2711 procedures, the hybrid-electric bus was exercised over extended test cycles. Two runs each of the double UDDS (2XTestD), double Manhattan (2XManhat), single OCTA, and triple CBD (3XCBD) cycles were performed using the ultralow sulfur fuel supplied in the vehicle’s fuel tank. In addition, one triple CBD cycle and one OCTA cycle were performed using a Fischer Tropsch fuel. This fuel was highly paraffinic, with an almost-zero sulfur level. The diesel-powered bus (vehicle 5347) was tested on May 28, 2002. The simulated test weight was 32 775 lbs. Because of budgetary constraints, the diesel bus was exercised over single-length test cycles. Two test runs of the UDDS, Manhattan, and OCTA cycles and one test run of the CBD cycle were performed using in-tank fuel. The LNG-powered bus (vehicle 2201) was tested on June 6, 2002 at a simulated test weight of 34 775 lbs. The LNG-powered bus was also exercised over singlelength test cycles. One additional OCTA cycle was performed on the LNG-powered bus with the AC operating, in a brief attempt to evaluate the potential effect of AC on fuel economy. Variability of Data Emissions data will usually vary from test to test, because of variation in driver performance,7 as well as variation in engine and aftertreatment temperature. Although analyzers are subject to regular calibration, analyzer drift will also have a role in this variability. Customarily, WVU manages data quality by ensuring that the distance traveled during the test cycle is within 5% of the target distance, and that the coefficients of variation on NOx and CO2 emissions between runs are 5% of the total cycle energy are considered invalid. Below 1%, no correction is required. A summary of the SOC data for the hybrid bus is presented in Table 3. With the exception of test run 2239-2, no SOC corrections of the emissions results were required. Test run 2239-2 was just above the threshold at which emissions must be corrected to reflect a zero NEC. To compute a SOC correction for each emissions species and for fuel economy, the measured value was plotted against the NEC for each run. A linear interpolation (in some cases, extrapolation may be allowed) was performed to establish the corrected value at an NEC of zero (i.e., the data was corrected to reflect a net zero change in SOC). The SOC correction factor for NOx emission is shown in Figure 15. Similar corrections were derived for the remaining emissions species and for fuel economy. The SOC-corrected emissions values are listed as run 22392C in Table B-2 in Appendix B. Note that the SOC correction shown in Figure 15 follows the spirit of the SAE J2711 procedure; however, the points are insufficient for true compliance with the SAE J2711 recommendation. NOx Emissions NOx emissions are shown in Figure 16. Each bar represents the average of two or more test runs, with the exception of the CBD cycle, in which case only one test run was performed. The error bars represent the maximum and minimum of the test runs. In all cases, the conventional-drive diesel bus had the highest NOx emissions. NOx emissions from the conventional LNG-powered bus were 44% lower in the Manhattan cycle, 55% lower in the UDDS cycle, 37% lower in the OCTA cycle, and 36% lower in the CBD cycle, compared to that of the conventional diesel bus. The hybrid bus showed the lowest NOx emissions. NOx emissions from the hybrid bus were lower than those of the conventional diesel bus by 52%, 60%, 42%, and 45%, relative to the Manhattan, UDDS, OCTA, and CBD cycles, respectively. Compared to the LNG-powered bus, the hybrid bus showed NOx reductions on the order of

Figure 12. NEC over a double UDDS cycle (test run 22392).

Figure 13. NEC over an OCTA cycle (test run 2238-1).

10% for the Manhattan, UDDS, and OCTA cycles and on the order of 20% for the CBD cycle. Operating the hybrid bus on synthetic Fischer Tropsch diesel fuel produced further reductions in NOx: ∼20%, compared to those of the hybrid bus for conventional diesel fuel. PM Emissions Total particulate emissions are shown in Figure 17. Both the LNG-powered and hybrid-electric bus produced substantial PM reductions, relative to that of the conventional diesel bus. Note that the hybrid bus was equipped with an Engelhard catalyzed particulate filter, whereas the conventional diesel bus was equipped with


Figure 14. NEC over a triple CBD cycle (test run 2237-4).


Figure 16. NOx emissions.

Table 3: Summary of State of Charge (SOC) Data test run

cycle

NEC (kJ)

total cycle energy (kJ)

tolerance (%)

2240-1 2240-2 2239-1 2239-2 2238-2 2238-2 2242-1 2237-3 2237-4 2241-1

Manhattan Manhattan UDDS UDDS OCTA OCTA OCTA-FT CBD CBD CBD-FT

998 -101 -1115 -2154 864 -1537 -903 288 1263 -659

125191 126692 189123 188824 169549 169021 165056 160508 154683 155766

0.80 0.08 0.59 1.14 0.51 0.91 0.55 0.18 0.81 0.42

Figure 17. Total particulate emissions.

Figure 15. NOx emission SOC correction for test run 22392.

a diesel oxidation catalyst. The presence of the catalyzed particulate filter is primarily responsible for the large PM reductions of the hybrid-electric bus, compared to that of the conventional diesel bus. The LNG-powered and hybrid-electric buses showed PM reductions of >90%, compared to the conventional-drive diesel bus. The use of Fischer Tropsch fuel in the hybrid-electric bus produced an-order-of-magnitude reduction in PM emissions, compared to conventional diesel fuel. In addition to total PM, PM10 and PM2.5 were also sampled and are shown in Figures 18 and 19. Note that, in the case of the LNG-powered and hybrid-electric buses, the mass of sample collected on the filter media was extremely low and on the same magnitude as the accuracy of the gravimetric measurement techniques. As a result, these masses are subject to large measurement inaccuracy and large test-to-test variability. As with total particulate emissions, the LNG-powered and

Figure 18. PM10 emissions.

hybrid-electric buses showed PM10 and PM2.5 reductions of g95%, compared to that of the conventional-drive diesel bus. Carbon Monoxide Emissions Carbon monoxide (CO) emissions are shown in Figure 20. As expected, the LNG-powered bus had the highest CO emissions. On average, the CO emissions from the LNG-powered bus were approximately double those of the diesel-powered bus. The hybrid-electric bus showed CO reductions of 73%, 47%, 82%, and 72% over the Manhattan, UDDS, OCTA, and CBD cycles, respectively, compared to the conventional-drive diesel bus. Both the catalyzed particulate filter on the hybrid-

266


Figure 19. PM2.5 emissions.

Figure 20. Carbon monoxide (CO) emissions.

Figure 21. Hydrocarbon (HC) emissions.

electric bus and the diesel oxidation catalyst on the conventional-drive bus reduce CO emissions. The difference in aftertreatment systems may be responsible, in part, for the reduction in CO emissions. It is difficult to distinguish exactly what fraction of the reduction is due to the hybrid drive system and what fraction, if any, is due to the difference in aftertreatment system. Hydrocarbon and Nonmethane Hydrocarbon Emissions Hydrocarbon (HC) emissions are shown in Figure 21. In the case of the LNG-powered bus, only the nonmethane portion of the total HC emissions are plotted. A large percentages95% or moresof the HC in the

Wayne et al.

Figure 22. Nonmethane hydrocarbon (NMHC) emissions.

exhaust of a natural gas fuel vehicle is unburned fuel (in this case, methane). Methane is not generally considered to be a volatile organic compound, because it is nonreactive, with respect to the formation of ground-level ozone. Therefore, it is common practice to consider only the nonmethane component of HC emissions from natural gas vehicles. Methane is a greenhouse gas, however, with a global warming potential that is 21 times more powerful than that of CO2.11 The fractions of methane and nonmethane hydrocarbons (NMHCs) in the exhaust of the LNG-fueled bus are shown in Figure 22. The NMHC emissions from the LNG-fueled bus were substantially higher than the total HC emissions from the diesel-powered bus. Note that the LNG-powered bus was not equipped with an oxidation catalyst. In all cases, the total HC emissions from the hybrid-electric bus were below the detection capability of the laboratory, which represents a reduction of >98%, compared to the conventional diesel bus. As with CO emissions, the catalyzed particulate filter on the hybrid-electric bus and the oxidation catalyst on the conventional diesel bus will reduce the HC emissions and the difference in aftertreatment systems on the two buses may be at least partially responsible for the difference in HC emissions between the two buses. CO2 Emissions and Fuel Economy Carbon dioxide (CO2) emissions are not regulated, although CO2 is considered a greenhouse gas. CO2 emissions are generally reported as an indication of the fuel consumption. CO2 emissions are shown in Figure 23. Fuel economy was calculated by a carbon-balance method. The method assumes that the mass of carbon in a quantity of fuel is equal to the mass of carbon found in the exhaust produced when that fuel is combusted. In executing this method, contributions to exhaust carbon from lubricating oil and loss of fuel carbon by mass loss past the piston rings (blow-by) are neglected: both of these factors are minor. The mass of carbon measured in the exhaust is given by

GS ) R2HCmass + 0.429COmass + 0.273(CO2)mass where R2 is the carbon weight fraction of the fuel determined from analysis of the test fuel, HCmass the (11) USEPA Inventory of U.S. Greenhouse Gas Emissions and Sinks, 1990-1997; US Government Printing Office: Washington, DC, April 1999.



Figure 23. Carbon dioxide (CO2) emissions.

Figure 24. Fuel economy.

Table 4: Fuel Properties Value parameter

ULSD

LNG

mass of carbon/mass of fuel, R2 mass of carbon (grams/equ gal D2) lower heating value (BTU/gal)

0.866 2743 129882

0.698 2139 128155

mass of hydrocarbons emitted in the exhaust, COmass the mass of CO emitted in the exhaust, and (CO2)mass the mass of CO2 emitted in the exhaust.9 The mass of carbon emitted in the exhaust is then converted to a fuel volume and then further to a fuel economy by the relation

MPG )

[

]

gC/gal of fuel (distance traveled) GS

where gC/gal of fuel is the carbon weight fraction of the test fuel. Fuel property data used for the fuel economy calculation are listed in Table 4. Analysis of the LNG fuel used in this study was performed by Core Laboratories and provided by OCTA. In the case of the LNGfueled bus, a diesel-equivalent fuel economy is reported. To accomplish this conversion, the mass of carbon contained in a mass of LNG that has an equivalent energy content of a gallon of diesel fuel is calculated as

gC 0.698 kg C kg LNG 129882 BTU ) equ gal D2 kg LNG 42.383 BTU gal D2

(

)(

)(

)

Fuel economy results are shown in Figure 24. The LNG-powered bus showed a fuel economy penalty of 25%, 27%, 29%, and 35% on the Manhattan, UDDS, OCTA, and CBD cycles, respectively, compared to the conventional-drive diesel bus. Natural-gas-powered buses suffer a fuel economy penalty that is primarily due to throttling losses under partial-load operation and greater vehicle weight. The fuel economy results from the hybrid-electric bus are somewhat controversial, in that it was expected that the hybrid bus would realize a fuel economy benefit over the conventional-drive diesel bus. This did not seem to be the case, based on the chassis dynamometer results obtained in this study. In fact, in most cases, the diesel bus exhibited greater fuel economy than the hybridelectric bus. These results are contrary to the fuel economy data collected by Orange County from fueling records of the buses during normal in-service operation.

Figure 25. Effect of air conditioning (AC) on emissions and fuel economy of the LNG-powered bus.

It was initially speculated that the discrepancy between the chassis dynamometer results and in-service data was related to the simulation of aerodynamic drag and rolling losses. Originally, coast-down data for the conventional-drive diesel and LNG-powered buses were not available to calculate the road-load losses and assumed values were used. However, on-road coast downs were later performed and the buses were retested using the proper road-load profiles. A second factor that could contribute substantially to the fuel economy results is AC loads and other hotel loads such as the alternator, air brake compressor, etc. AC loads in buses can be a significant consumer of onboard vehicle power and, as a result, can have a significant effect on emissions and fuel economy. Most chassis dynamometer emissions tests are performed with the AC systems turned off, because of the complexity of ensuring consistent operation of the AC system from test to test and from bus to bus. Although evaluation of emissions with the AC activated is possible, results cannot be equitably compared from one bus to another or from one test to another, unless all operating variables are similar and reasonable. SAE J27111 procedures recommend that the emissions tests be conducted with the AC system deactivated. To gain some insight into the potential effect of AC loads on the fuel economy and emissions, one test was conducted on the LNG-powered bus with the AC system operating at maximum capacity.

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The emissions and fuel economy from this test are plotted in Figure 25. Fuel economy decreased from 4.1 mpg to 3.5 mpg when the AC system was operated. This corresponds to an increase in CO2 emissions from 1873 g/mile to 2175 g/mile. NOx emissions also increased substantially, from 15.05 g/mile to 24.83 g/mile, whereas PM emissions decreased from 0.064 g/mile to 0.010 g/mile. CO emissions increased from 6.33 g/mile to 7.08 g/mile and HC emissions were unaffected. Conclusions Transit buses have historically received substantial attention, with regard to urban tailpipe emissions reduction. Hybrid-electric transit buses offer potential benefits over conventional transit buses of comparable capacity, including reduced fuel consumption, reduced emissions, and the utilization of smaller engines. Emissions tests were conducted on an Allison series-drive hybrid-electric transit bus, a conventional diesel bus, and a liquefied natural gas (LNG)-powered bus. Results were presented that compared the emissions. The hybrid-electric transit bus showed its ability to reduce oxides of nitrogen (NOx) emissions by 50% on average, compared to that for a conventionally powered diesel bus, and by ∼10%, compared to that for an LNGpowered bus. Particulate (PM) emissions from the hybrid bus were reduced by 90%, on average, over the various test cycles performed, compared to the conventional diesel bus. PM emissions from the hybrid bus were comparable to the PM levels from the LNG-powered bus. It was recognized that the hybrid bus was equipped with a catalyzed particulate filter, whereas the conventional diesel bus was equipped with an oxidation catalyst. The hybrid bus showed average carbon monoxide (CO) reductions of 70% over the four test cycles and hydrocarbon (HC) reductions in excess of 98%, compared to that of the conventional diesel bus. In this study, the hybrid bus did not show the expected improvement in fuel economy. Fuel economy results were comparable to those of the conventional-drive diesel bus.

Figure A2. Data for Orange County Transit Authority (OCTA) Route No. 2.

Figure A3. Data for Orange County Transit Authority (OCTA) Route No. 3 (Express).

Appendix A: OCTA Route Development The Orange County Transit Authority (OCTA) provided data for three bus routes. The speed-time route development plots are shown in Figures A1-A3. Deceleration histograms are given in Figures A4-A6, whereas acceleration histograms are given in Figures A7-A9. Figure A4. Heavy acceleration histogram.

Figure A1. Data for Orange County Transit Authority (OCTA) Route No. 1.

Figure A5. Light acceleration histogram.



Figure A6. Cruise histogram.

Figure A8. Medium deceleration histogram.

Figure A7. Heavy deceleration histogram.

Figure A9. Light deceleration histogram.

Appendix B: Emissions Summary Table B1 shows the emissions summary for the

Manhattan cycle, whereas Table B2 shows the emissions summary for the Urban Dynamometer Driving

Table B-1: Emissions Summary for the Manhattan Cycle Component test ID

CO

NOx1

NOx2

THC

2252-1 2252-2 average

5.86 6.27 6.06

35.1 34.7 34.9

34.1 34.0 34.05

0.48 0.52 0.50

17.9 19.4 18.7

17.6 19.5 18.5

24.6 23.0 23.8

17.1 16.6 16.8

16.9 14.8 15.8

BDLa BDLa BDLa

2296-1 2296-2 average 2240-1 2240-2 average a

10.8 9.8 10.3 2.39 0.83 1.61

CH4

22.40 c 22.40

PM10

PM2.5

CO2

fuel economy (mpg)

BTU/mile

number of miles

Bus 5347, CARB 0.760 0.244 0.810 0.263 0.78 0.243

0.197 0.204 0.200

2485 2611 2548

4.03 3.83 3.93

32243 33890 33066

2.09 2.07 2.08

Bus 2201, LNG 0.034 BDLa 0.180 0.004 0.026 0.004

BDLa BDLa BDLa

2664 2664 2664

2.86 2.86 2.86

46989 46894 46942

2.14 2.09 2.11

Bus 5419, CARB 0.004 BDLa BDLa BDLa 0.004 BDLa

BDLa BDLa BDLa

2286 2281 2283

4.39 4.40 4.40

29578 29484 29531

4.14 4.16 4.15

NMHC

1.01 c 1.01

PM

Below detection limit. Table B-2: Emissions Summary for the Urban Dynamometer Driving Schedule (UDDS) Component

test ID

CO

NOx1

NOx2

THC

2253-1 2251-2 2251-3 average

3.68 1.85 1.99 1.92

24.1 22.2 22.9 22.6

24.5 22.3 23.4 22.8

0.36 BDLa 0.056 0.056

2295-2 2295-3 2295-4 average

4.78 5.16 4.65 4.86

9.4 11.5 9.8 10.2

9.5 11.6 9.8 10.3

11.1 11.0 10.6 10.9

2239-1 2239-2 2239-2C average

0.92 1.12 0.71 1.02

a

9.1 8.7 9.5 8.92

Below detection limit.

8.7 8.8 8.4 8.8

BDLa BDLa BDLa BDLa

CH4

10.22 9.85 9.66 9.84

PM10

PM2.5

CO2

fuel economy (mpg)

BTU/mile

number of miles

Bus 5347, CARB 0.39 0.108 0.28 0.088 0.28 0.090 0.28 0.089

0.087 0.070 0.066 0.068

1690 1256 1256 1256

5.92 7.98 7.98 7.98

21935 16266 16271 16268

2.02 5.56 5.52 5.54

Bus 2201, LNG 0.018 0.006 0.018 0.004 0.015 BDLa 0.017 0.005

BDLa 0.001 BDLa 0.001

1284 1320 1228 1277

5.94 5.78 6.21 5.98

22600 23223 21611 22478

5.59 5.58 5.58 5.58

Bus 5419, CARB 0.017 0.002 0.013 0.001 0.022 0.003 0.015 0.002

BDLa BDLa BDLa BDLa

1280 1266 1283 12.73

7.84 7.93 7.75 7.88

16553 16378 16378 16465

11.08 11.13 11.13 11.11

NMHC

0.49 0.57 0.41 0.49

PM

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Table B-3: Emissions Summary for the Orange County Transit Authority (OCTA) Cycle PM2.5

CO2

fuel economy (mpg)

BTU/mi

number of miles

Bus 5347, CARB 0.46 0.169 0.43 0.141 0.45 0.155

0.140 0.136 0.138

1699 1698 1698

5.89 5.90 5.90

22028 22007 22018

6.55 6.57 6.56

Bus 2201, LNG 0.59 0.016 BDLa 0.58 0.026 0.007 0.59 0.021 0.007

BDLa BDLa BDLa

1842 1829 1835

4.15 4.18 4.16

32316 32094 32205

6.57 6.61 6.59

BDLa BDLa BDLa

Bus 5419, CARB 0.006 BDLa 0.063 BDLa 0.034 BDLa

BDLa BDLa BDLa

1942 1966 1954

5.15 5.12 5.15

25093 25368 25230

6.58 6.54 6.56

BDLa

Bus 5419, FT 0.0067 0.009

0.004

1736

5.32

23225

6.54

CO

NOx1

NOx2

THC

2254-1 2254-2 average

2.94 2.76 2.85

23.1 22.6 22.8

22.3 22.8 22.5

0.38 0.38 0.38

2298-2 2298-3 average

5.90 6.15 6.02

14.5 14.4 14.4

14.3 14.0 14.2

14.2 14.2 14.2

2238-1 2238-2 average

0.52 BDLa 0.52

12.8 13.5 13.1

13.0 13.8 13.4

2242-1

0.37

10.3

10.2

a

Component CH4 NMHC

PM10

test ID

12.92 12.93 12.92

PM

Below detection limit. Table B-4: Emissions Summary for the Central Business District (CBD) Cycle

test ID

CO

NOx1

NOx2

THC

Component CH4 NMHC

PM2.5

CO2

fuel economy (mpg)

BTU/mile

number of miles

Bus 2201, LNG 0.040 BDLa

BDLa

1993

3.84

34906

2.03

0.119 0.004 0.061

2004 1922 1963

5.01 5.22 5.12

25923 24847 25385

6.03 6.09 6.06

0.005

1777

5.20

23766

6.04

PM

PM10

Bus 5397, CARB 2253-1 2297-1

5.66

15.7

15.7

14.4

c

c

2237-3 2237-4 average

1.44 0.58 1.01

12.7 13.9 13.3

12.7 13.9 13.3

BDLa BDLa BDLa

Bus 5419, CARB 0.30 0.011 0.015 0.010 0.023 0.010

2241-1

0.38

10.8

10.4

BDLa

Bus 5419, FT 0.0057 0.009

a

Below detection limit.

Schedule (UDDS) cycle. The emissions summary for the Orange County Transit Authority (OCTA) are given in Table B3, whereas that for the Central Business District (CBD) cycle is given in Table B4. Acknowledgment. The authors wish to acknowledge the Orange County Transit Authority of Orange, CA for funding this research effort and for supplying the transit buses tested in this program. The authors

also wish to acknowledge Edward Bass and Peter Chiang of Allison Transmission, General Motors Corporation, for assistance during the emissions testing of the hybrid-electric transit bus. The authors thank the staff of the West Virginia University Transportable Heavy Vehicle Emissions Testing Laboratory for their hard work in performing the emissions testing and collecting the data reported in this paper. EF030096T

A Comparison of Emissions and Fuel Economy from Hybrid-Electric

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