Analysis of Series and Parallel Hybrid Bus Fuel Consumption on Different Edmonton Transit System

Routes

X. Liang, C. Wang, C. Chapelsky, D. Koval, A.M. Knight Department of Electrical and Computer Engineering

University of Alberta Edmonton, AB, T6G 2V4 Canada

Abstract—Hybrid vehicles are widely used in many cities because of their lower rates of fuel consumption and for environmental initiatives. This paper presents the fuel consumption rate analysis of multiple parallel and series drive train type hybrid buses operating in the City of Edmonton, Alberta, Canada, on different urban routes. Based on an analysis of real time daily bus data, the optimal choice between the different types of hybrid buses for Edmonton Transit System (ETS) urban routes is presented and discussed.

Keywords - hybrid electric vehicles (HEV); transit bus; fuel rate; drive train

I. INTRODUCTION Hybrid electric transit buses are emerging as a method of

lowering fuel consumption and tailpipe emissions. Compared to standard diesel buses, they attract a considerable public interest to improve urban environments because of their higher fuel efficiency, lower emissions, lower noise levels, and better overall well-to-wheel efficiency than trolleybuses [1]. Hybrid vehicles can be defined as vehicles having a powertrain that is supplied from two power sources. Typically, these are comprised of an internal combustion engine (ICE) and a battery pack. The battery pack can be charged from either the ICE or energy recovered during regenerative braking in deceleration. Three main types of hybrid electric drive train configurations are commonly used: series hybrid, parallel hybrid and series-parallel hybrid types. Both series and parallel hybrid drive train buses are operated in the City of Edmonton, and the main goal of this paper is to present results from a detailed study comparing the fuel consumption rates of these buses.

In this paper, two main types of hybrid drive trains are introduced, then, the configurations of hybrid buses in consideration are presented. Due to their specific bus designs, series and parallel hybrid buses have different fuel consumption rates when operating on the same route, since different route operating conditions affect the fuel consumption of hybrid buses. This paper presents and compares the fuel consumption rates of different hybrid bus structures from real time collected data and provides an optimized choice between two types of hybrid buses in use for the urban Edmonton Transit System (ETS) [2] routes.

II. HYBRID DRIVE TRAIN CONFIGURATIONS

A. Series Hybrid Drive Train In the series hybrid drive train, the engine is mechanically

decoupled from the driveline and is only used to generate electrical power. Wheels of the vehicle can only obtain power from the electrical motor, which also operates as an electrical generator to recharge the battery during regenerative braking. As shown in Fig. 1, the generator, battery, and motor/generator are connected only by electrical cables. During braking, kinetic energy of the bus is converted to electrical energy by the motor/generator, which operates as a generator. The electrical energy is delivered through power electronics, which act as a rectifier and charge controller to the battery. However, usually the braking mode to recharge the battery has a certain velocity limitation of 16-19 km/h [3], after which conventional pneumatic brakes must be activated by further pushing the brake pedal.

Figure 1: Components of series hybrid vehicles.

The primary advantage of the series hybrid drive train is that it allows the engine to work independently from the traction system. Thus, the internal combustion engine (ICE) in a series hybrid can theoretically operate at its peak efficiency region [4] at all times. In addition, the speed control strategy is relatively simple due to the single torque source (electric motor) [5]. However, series hybrid vehicles also have some disadvantages. They convert energy twice (i.e., mechanical to electrical, and electrical to mechanical again) and can encounter significant energy losses from this process. As well, a large space for a large engine and generator system is necessary. Therefore, series hybrids are usually used in large vehicles that have enough space to house the drive train.

978-1-4244-2601-0/09/$25.00 ©2009 IEEE 1470

B. Parallel Hybrid Drive Train In the parallel hybrid drive train, as shown in Fig. 2, the

engine and motor/generator have their own paths to directly supply their torque to wheels through a mechanical coupling such as gearbox or pulley belt [5].

Figure 2: Components of parallel hybrid vehicles.

The advantage of the parallel hybrid vehicle is the direct application of both engine torque and motor torque to the wheels. Although parallel hybrid vehicles can be considered as mechanically driven, the acceleration of the vehicle is assisted by the power from the battery. During regenerative braking, the engine clutch is open and the motor/generator clutch is closed to transmit the energy from wheels to the battery [6]. However due to the direct mechanical coupling between the ICE and the transmission, the ICE in a parallel drive train cannot operate in its optimal region [7]. This also serves to complicate the control strategy of parallel hybrid vehicles. However, because of the smaller traction motor and generator, parallel hybrid drive trains can be used in small vehicles with compact space requirements such as passenger cars.

III. BUS SPECIFICATIONS AND DATA COLLECTION

A. Series Hybrid Bus Specifications The series hybrid buses operated in Edmonton are diesel/

electric vehicles with approximately 15,250 kg net weight with a length of 12.4 m. The CUMMINS ISL C8.3G engine is used as the generator/power plant, and the Allison B 400 R, ZF HP592, Voith D86 is used for the transmission system. The energy storage system (ESS) consists of a bank of lead acid batteries with a nominal voltage of 600V.

Figure 3: ETS series hybrid bus #1.

B. Parallel Hybrid Bus Specification The parallel hybrid buses operated in Edmonton are

diesel/electric vehicles with approximate 14,000 kg net weight. The main components related to the electrical side of the bus are the electric variable drive unit, a dual power inverter module (DPIM), and the energy storage system. The electric drive unit has two motor/generators that work together. Each motor/generator is a 3-phase, 10-pole asynchronous machine, which can produce up to 75 kW continuous electrical power. The DPIM consists of two identical inverters, one for each motor/generator. The DPIM module is capable of producing 160 kW continuous 3-phase AC output with current range from -350 to +350 A. The ESS contains 240 nickel metal hydride (NiMH) modules (batteries) to store electrical energy with a nominal voltage of 600V.

Figure 4: ETS parallel hybrid bus #1.

C. Data Collection Method In order to analyze the performance of hybrid buses in

Edmonton, data loggers and global positioning system (GPS) antennas are installed on the hybrid buses in question. The data loggers are programmed to read controller messages on the vehicle’s heavy-duty J1939 CAN bus, to collect information such as vehicle speed, engine speed, engine torque, fuel consumption rate, and other parameters. etc. The GPS antenna is integrated with the CAN module of the logger to obtain synchronous time-stamped positional information such as latitude, longitude, and altitude. The data collecting equipment is shown in Fig. 5.

Figure 5: DataTaker DT80 (left) and Garmin GPS 18 LVC

(Right) [8].

The data loggers were configured to power-on and log all relevant CAN messages when the bus accessory power is turned-on, and set to log each parameter once per second. This provides a data-set that can be used to accurately analyze the steady-state performance of the buses with enough resolution to observe moment to moment changes in bus parameters during normal driving patterns.

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Figure 6: Logger installation on parallel #1.

IV. HYBRID BUS FUEL CONSUMPTION ANALYSIS

A. General Comparsion of Hybrid Bus Fuel Consumption Data was collected from Edmonton hybrid buses dates

since August of 2007. The data collected for two parallel hybrid buses and one series bus operating between August 2007 and February 2008 are analyzed in this paper. The geography of the Edmonton region is typical of a number of prairie cities: much of the region is predominantly flat, with a river valley running through the city. Operation on inclines is generally limited to routes running across the river valley. In addition, the city is at an elevation of 668m above sea level.

Usually, parallel hybrid buses start around 5:00 a.m. in the morning, and end at almost 1 a.m. in the next day with about 19 hours work during their operating days. The series hybrid bus often starts around 5:00 a.m. and experiences an average of 15 hours work during their operating days. One of the reasons for the different working schedule is the fact that the series buses were delivered before the parallel hybrid buses, and hence it was desirable for the buses to have driven similar distances at the end of the evaluation period. Several ETS routes are investigated in this paper:

1. Route 4. This route crosses the downtown core and river valley via some of the steeper routes, extending out to suburban areas.

2. Route 4. This route has the same start and end points as route 1, but combines freeway driving with urban driving. The route crosses the river valley via freeway and does not enter the downtown core.

3. Route 5. This route crosses the downtown core, does not include highway driving and does not cross the river valley.

4. Route 8. This route links urban and suburban areas with downtown, crossing the river valley.

5. Route 106. This route is similar to Route 4

Analysis of the collected data for fuel usage and speed is carried out and average fuel consumption rate and vehicle speed for the 7 month period for three vehicles, identified as parallel #1, parallel #2, and series #1 are shown in Table I.

As shown in Table I, the series hybrid bus has the lowest average fuel consumption rate. However, it has the largest standard deviation, because it operates on routes with high

speed variations, and so the average speed of these buses is lower, because all the buses experience a lot of stops during their operating hours. In general, buses in Edmonton urban area have 45% zero-speed time during their daily operations. This 45% includes all types of stops such as for passengers, traffic lights, traffic jam, and breaks at a transit centre. In non zero-speed operating times, buses experience a range of different acceleration and deceleration levels. If the acceleration “a” is grouped into three ranges, from 0 to 0.5 m/s2, 0.5 to 1 m/s2, and larger than 1 m/s2, the acceleration map is built up as shown in Fig. 7.

Table I: Statistical Results for Comparative Speed and Fuel Consumption Rates

Type of Hybrid-

Electric Bus

Fuel Rate (L/100km) Speed (km/hr)

Mean Value Standard Deviation Mean Value Standard

Deviation Parallel#1 58.41 4.01 17.31 1.96

Parallel#2 60.75 3.41 16.92 1.81

Series#1 53.37 8.83 14.33 5.79

Figure 7: Acceleration map of buses in Edmonton.

B. Detailed analysis of Hybrid Bus Fuel Consumption Average daily fuel usage rates for buses operating on Route

5 in February 2008 are shown in Fig. 8.

Daily Fuel Rate on Route 5

01020304050607080

2008

-2-11

2008

-2-12

2008

-2-13

2008

-2-14

2008

-2-15

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-2-16

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-2-17

2008

-2-18

2008

-2-19

2008

-2-20

2008

-2-21

2008

-2-22

L/10

0km Parallel#1

Parallel#2

Series#1

Figure 8: Daily fuel rate (L/100km) on Route 5.

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Viewing the fuel data in Fig. 8, it is clear that the series hybrid bus operated with lower fuel usage than the parallel hybrid buses when operating on the same routes at the same times. However, buses on route 5 require a large amount of time to travel via downtown and experience many stop-and-go situations during an expected 50 minutes one-way trip. In comparison, buses on route 8 also travel through some stops in the downtown area, although route 8 is twice as long as route 5 with an expected time for a one-way trip of 90 minutes. On route 8 parallel bus #1 runs with an average speed of 17.9 km/hr and an average fuel rate of 57.0 L/100km. On route 5, parallel bus #1 has an average speed of 14.2 km/hr and a higher average fuel rate of 64.0 L/100km. A similar difference in fuel rates and average speed can be observed with the series hybrid bus on the these routes, but the difference is not as significant as with the parallel hybrid buses. Series bus #1 operates on route 8, with an average speed of 14.2 km/hr and an average fuel rate of 59.3 L/100km, while on route 5 the average speed is 11.5 km/hr and a average fuel rate is 58.4 L/100km. However, the series hybrid has a better performance on fuel consumption in general. Detailed data of the hybrid buses in different speed ranges and routes are given in Table II. Note that the data in Table II gives fuel rates as average of the instantaneous fuel flow in L/hr

Table II. Fuel Consumption Rate in Different Speed Levels

Speed (km/hr)

Parallel (L/hr)

Series (L/hr)

Time

% Parallel (L/hr)

Series (L/hr)

Time

%

Fuel Rate on Route 5 Fuel Rate on Route 8

≥60 17.55 22.89 0.2 23.95 22.14 3

[50,60) 18.46 19.42 3.1 19.82 20.01 11.8

[40,50) 18.47 16.46 14.6 19.86 18.05 18.1

[30,40) 18.20 14.50 22.3 20.49 17.09 18.3

[20,30) 16.86 11.02 19.1 18.20 13.25 17.4

[10,20) 10.28 7.51 19.9 10.70 9.05 15.2

(0,10) 3.50 4.11 20.8 4.73 5.46 16.2

Fuel Rate on Route 1&4

≥60 24.54 21.57 6.7

[50,60) 20.83 19.51 12.6

[40,50) 21.13 17.90 17.9

[30,40) 20.81 16.37 16.2

[20,30) 18.43 12.47 16.4

[10,20) 11.22 8.62 14.5

(0,10) 5.00 5.10 15.6

As table II indicates, the two types of buses have different fuel consumption rates depending on the route and the speed range considered. Parallel hybrid buses have lower fuel consumption during higher speed operation, but as long as the average speed goes below 50 km/hr, the series hybrid buses start to show better fuel consumption performance. This

demonstrates the effects of operating the series hybrid’s ICE in a relatively high efficiency region for this drive train, even for low average speed operation of the bus. Although the parallel hybrid buses have a lower fuel consumption rate than the series bus in high speed regions and in very low speed regions (i.e., 0 to 10 km/hr), they still operate with a higher fuel consumption rate overall, since the high-speed time for a urban bus in Edmonton is very limited (less than 4%). For the majority of non-zero speed operation, the series hybrid bus has a lower fuel consumption rate than the parallel hybrid bus.

These trends are demonstrated in Figs. 9 to 11. The real time speed of buses with position along Route 8 is plotted using Google Earth in Fig. 9. The real time fuel consumption rates are plotted with position in Fig. 10 and Fig. 11. As shown in Fig. 9, the real time velocity of the parallel hybrid bus is plotted in different colors. With increasing speed from 0 to 100 kilometers per hour, the color scale changes from red to green. The fuel consumption rate plot is similar, except that the unit is liters per hour. The data shown in Fig. 9, Fig.10 and Fig. 11 are represented with three dimensional plots, since the net value of the speed and fuel consumption rate are also shown as color bars from the ground. As shown in the zoom areas in these figures, this allows the speed and fuel rate values to be observed from not just their colors, but also from the height of their bars.

Initial viewing of the fuel usage plots indicates that the fuel consumption rate for the parallel hybrid is considerably higher than the series hybrid. However, it must be taken into account that the plots are fuel flow in L/hr. Table III provides average speeds and fuel usage in L/100km. It can be seen from the data in this table that the average fuel usage in L/100km is lower for bus parallel #1 than bus series #1.

Figure 9: Speed map of parallel hybrid bus on Route 8.

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Figure 10: Fuel consumption rate of parallel #2 on Route 8.

Figure 11: Fuel consumption rate of series #1 on Route 8.

Table III. Statistical Results for Comparative Speed and Fuel Consumption Rate on Different Routes

Route No.

Type of Hybrid-

Electric Bus

Speed (km/hr) Fuel Rate (L/100km)

Mean Value

Standard Deviation

Mean Value

Standard Deviation

8 parallel#1 17.86 0.60 56.94 2.95 parallel#2 17.55 1.04 60.98 3.29 series#1 14.22 6.42 59.29 10.12

5 parallel#1 14.17 2.43 63.99 3.67 parallel#2 13.86 1.63 59.63 3.90 series#1 11.46 3.55 58.38 6.82

1 & 4 parallel#1 18.90 1.47 58.58 2.02 parallel#2 18.66 1.14 62.39 2.72 series#1 16.23 5.75 54.82 9.84

106 series#1 16.43 4.75 51.42 8.71 Fig. 9, Fig. 10 and Fig. 11, demonstrate that high speed

areas correspond to high fuel consumption rate areas. This correspondence can also be observed in Fig. 12, which shows the speed and fuel consumption of a parallel hybrid bus on route 8 and the series hybrid bus on route 106.

0

5

10

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20

25

6:00 AM

7:00 AM

8:00 AM

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12:00 AM

Speed (km/hr) Fuel Consumption (L/hr)

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7:00PM

Speed (km/hr) Fuel Consumption (L/hr)

Figure 12: Hourly speed (km/hr) and fuel consumption (L/hr)

curves on Route 8 (top) with parallel #1 and Route 106 with the series bus (bottom).

Some other observations can be drawn from Fig. 12. The parallel hybrid bus has the fastest speed in the early morning between 6:00 AM and 7:00 AM, because of low traffic density. The fuel consumption curve has the same trend with the speed curve for all buses on any ETS routes. The peak fuel consumption time usually happens at 5:00 PM every day while the lowest fuel consumption time usually happens at noon time for all buses.

In general, the parallel hybrid buses experiences more high fuel consumption rates than the series hybrid bus. Because there are no dedicated bus lanes in Edmonton, and the ETS

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urban buses usually have many stop-and-go operations, the series buses are more suitable on-average for the road conditions in Edmonton. This is also supported by the series hybrid drive train theory in part II, in table I, and by the summary of results presented in Table III, which show the total statistical results of speed and fuel rates of each hybrid bus on each route.

C. Future Consideration The data presented in the previous section indicates that,

on typical routes in Edmonton, the series bus topology offers improved performance in terms of total fuel usage. When considering the comparative performance of vehicles for future purchases, additional factors should be taken into account. Other than the drivetrain configuration, one clear difference between the buses is the battery pack. The series hybrid buses in the study use an older technology, with lead-acid batteries, while the parallel hybrid uses the lighter, NiMH battery pack. This difference accounts for some of the 1250kg different in unloaded vehicle weight. Future purchases of the series hybrid are likely to be made with a next-generation battery pack, reducing vehicle weight and improving fuel usage further.

A second drawback of the series topology used for the comparative test is that the buses in the test used mechanically driven hydraulic and cooling systems. As a result, these vehicles were not capable of operation with the ICE off. Moving to electrically pumped fluid systems may offer additional fuel savings, given the fact that on average buses in Edmonton are stationary for 45% of the time.

V. CONCULSION In this paper, the basic parallel and series hybrid drive train

structures are presented. The parallel hybrid vehicles with direct mechanical coupling of engine to wheels are good for constant high speed operation, while the series hybrid vehicles

with electrical connection between engine and wheels are good for stop-and-go operation.

By plotting the HEV data on Google Earth, speed and fuel consumption rate of HEVs can be intuitively observed. According to these bus performance maps and tables, the series hybrid bus under consideration shows lower overall fuel consumption rates on ETS routes. Although the parallel bus has its advantages in some high speed situations, there are many lower speed situations and stop-and-go requirements for the urban bus in Edmonton. Therefore, the series buses provide the best option for low fuel use hybrid transit vehicles in Edmonton.

REFERENCES [1] C. M. Jefferson, R. H. Barnard, “Hybrid Vehicle Propulsion”,

Computational Mechanics Inc. WIT press 2002, pp. 124. [2] The city of Edmonton Transit System,

http://www.edmonton.ca/transportation/edmonton-transit-system-ets.aspx/

[3] J.J.H. Paulides, E.V. Kazmin, B.L.J. Gysen, E.A. Lomonova, “Series Hybrid Vehicle System Aanalysis Using an In-Wheel Motor Design”, IEEE Vehicle Power and Propulsion Conference (VPPC), September 3-5, 2008

[4] Northeast Advanced Vehicle Consortium, M. J. Bradley, “Hybrid-Electric Transit Buses: Status, Issues, and Benefits”, Transit Cooperative Research Program Report 59, Transporation Research Board, National Academy Press, 2000, pp. 5-6.

[5] Y. Gao, M. Ehsani, “Hybrid Electric Vehicle: Overview and State of the Art”, IEEE ISIE 2005, June 20-23, 2005, Dubrovnik, Croatia, pp. 307-315.

[6] A. E. Fuhs, “Hybrid Vehiclesand the Future of Personal Transpprtation”, Taylor & Francis Group, LLC, CRC Press, 2009, pp. 76-81.

[7] K. Chen, A. Bouscayrol, A. Berthon, P. Delarue, D. Hissel, R. Trigui, “Global modeling of different Vehicles Using Energetic Macroscopic Representation”, IEEE Vehicle Power and Propulsion Conference (VPPC), September 3-5, 2008

[8] Datataker Industrial Logger Systems, http://www.datataker.com/

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