assessment of hybrid‐drive bus fuel savings for brazilian urban transit

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Assessment of hybrid‐drive bus fuel savings forBrazilian urban transitSuzana Kahn Ribeiro a & Márcio De Almeida D'agosto aa Federal University of Rio de Janeiro, Cidade Universitária, Centro de Tecnologia , BlocoI, Sala 129, CEP: 21.945‐970, Rio de Janeiro, RJ, BrazilPublished online: 01 Feb 2007.

To cite this article: Suzana Kahn Ribeiro & Márcio De Almeida D'agosto (2004) Assessment of hybrid‐drive bus fuel savingsfor Brazilian urban transit, Transportation Planning and Technology, 27:6, 483-509, DOI: 10.1080/0308106042000316376

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Transportation Planning and Technology, December 2004

Vol. 27, No. 6, pp. 483–509

ASSESSMENT OF HYBRID-DRIVE BUSFUEL SAVINGS FOR BRAZILIAN URBAN

TRANSIT

SUZANA KAHN RIBEIRO and MARCIO DE ALMEIDAD’AGOSTO*

Federal University of Rio de Janeiro, Cidade Universitaria, Centro deTecnologia, Bloco I, Sala 129, CEP: 21.945-970, Rio de Janeiro, RJ, Brazil

(Received 14 January 2004; Revised 24 August 2004; In final form 24 September 2004)

Buses are the main transit mode in Brazil, transporting more than 55 million passengersper day. Most of these vehicles run on diesel oil causing a dependence on oil, extensivegreenhouse gas emissions and increasing air pollution in urban areas. In order toimprove this situation, options for Brazilian cities include the use of alternative fuelsand new propulsion technologies, such as hybrid vehicles. This paper proposes aprocedure for evaluating the performance of a recently developed hybrid-drive technol-ogy. A simple procedure is presented to compare hybrid-drive buses with conventionaldiesel buses in urban operations, particularly with respect to fuel economy. Next thepotential for reducing diesel oil consumption through the use of hybrid-drive buses isassessed. Field tests carried out by the authors indicate that fuel consumption improve-ment through the use of hybrid-drive buses would certainly exceed 20%, resulting inlower fuel costs and carbon dioxide (CO2) emissions.

Keywords: Urban transit; Buses; Fuel economy; Hybrid drives; Brazil

1. INTRODUCTION

In the principal Brazilian state capitals, a fleet of around 55 000 busescarries over 550 million passengers a month, resulting in approxi-mately 250 million km of bus travel each month [1]. In order to satisfy

*Corresponding author. E-mail: [email protected]

ISSN 0308-1060 print: ISSN 1029-0354 online © 2004 Taylor & Francis LtdDOI: 10.1080/0308106042000316376

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484 S. K. RIBEIRO and M. DE A. D’AGOSTO

this market, Brazil is currently the world’s largest bus manufacturer,producing 15 000 to 20 000 vehicles per year [2].

Among other problems, having buses as the main transit moderesults in dependence on oil as a source of energy, extensive green-house gases emissions and increasing local air pollution, as diesel-fu-elled internal combustion engines run the majority of these vehicles.Although currently ranking 18th for oil production worldwide, andwith an average annual growth rate of 8.2% per year, Braziliandependence on imports exceeded 32% of domestic consumption in2001. Transportation is responsible for 80% of Brazilian diesel oilconsumption, with some 90% of this used solely for road-basedtransportation, of both freight and passengers [3].

The implementation of mechanisms designed to reduce diesel oilconsumption, particularly in sectors such as road transportation, isnecessary and desirable. A long-term view indicates the need for heavyinvestment in the implementation of higher-capacity transit modes,such as trains and subways powered by electric traction. However,developing countries such as Brazil do not always have the capitalrequired for these high-cost mass transportation modes.

Within this context, a short-term alternative for reducing both dieseloil consumption and air pollutant emissions is the ongoing use ofurban buses and the replacement of diesel oil by compressed naturalgas (CNG) or renewable fuels (ethanol, biogas and biodiesel), or eventhe introduction of new propulsion technologies, such as hybrid-drivevehicles [4].

Considering that hybrid-drive buses offer operational advantagesover conventional diesel ones, such as smoother and quicker acceler-ation, more efficient braking, improved fuel economy and reducedemissions [4–7], the main objective of this paper is to verify thepotential for reducing diesel oil consumption through the use ofBrazilian technology hybrid-drive buses.

In the first part of the paper, a procedure is proposed for evaluatingthe performance of a recently developed Brazilian hybrid-drive tech-nology and for comparing it to diesel-drive technology in order toverify the specific operational advantages of hybrid-drive in urbanoperation, such as fuel economy. As part of the procedure, a set ofdynamic performance trials, consisting of acceleration, decelerationand fuel consumption tests, were carried out on a prototype hybrid-drive bus in September 2002 by the Graduate School of Engineeringat the Federal University of Rio de Janeiro.

In the second part of the paper, the potential for reducing diesel oil

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485ASSESSMENT OF HYBRID-DRIVE BUS FUEL SAVINGS

consumption with the use of hybrid-drive buses in urban transit isanalysed considering two alternatives. Besides reduction on fossil fueldependence and also operating costs, lower diesel oil consumptionoffers the direct advantage of having a proportional drop in carbondioxide (CO2) emissions, which is the main greenhouse gas, and theindirect advantage of lower emissions of local atmospheric pollutants,such as carbon monoxide (CO), nitrogen oxides (NOx), sulphur oxides(SOx), particulate matter (PM) and hydrocarbons (HC), helping toreduce environmental impacts in urban areas [8–10].

Following this introduction, Section 2 provides a brief description ofthe hybrid-drive concept and the Brazilian experience of this technol-ogy. The performance evaluation procedure, specification and planningof the field trials, the establishment of data processing methods anddiscussion of findings are presented in Section 3. In Section 4, thepotential fuel savings are calculated on the basis of two alternatives:(1) replacing part of the bus fleet operating on the Sao Mateus –Jabaquara Metropolitan Corridor; and (2) replacing part of the urbanbus fleet in the Sao Paulo Metropolitan Region. Finally, the conclu-sions and some recommendations are presented.

2. BRAZILIAN EXPERIENCE OF HYBRID-DRIVE INURBAN BUSES

In the broadest sense, the term ‘hybrid-drive’ applies to vehicle driveswith more than one drive source, with their basic components in oneof two arrangements: series or parallel. For the series arrangement,vehicle traction is handled by a single component of the drive system,usually an electric motor. For the parallel arrangement, the traction isprovided alternately by two components in the drive system, usually anelectric motor or an internal combustion engine. Fig. 1 illustrates themost usual form of these arrangements. For both cases, the drivesystem consists of an energy conversion unit (ECU), an energy storageunit (ESU) and a traction unit (TU) [6,7,11].

Brazil has developed hybrid drives for buses, and the models shownin Table I are already in operation [12]. From the options presented inTable I, the Padron model meets the specifications established duringthe 1980s by the Ministry of Transportation as the best suited to urbantransit. Padron models are large buses of high passenger capacity, witha simple internal layout and wide doors that streamline passengerboarding and alighting procedures. It offers better on-board accommo-

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S. K. RIBEIRO and M. DE A. D’AGOSTO486

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FIGURE 1 Components of hybrid drive

dation, is equipped with two or three double doors and has a weight/capacity ratio optimized for urban traffic conditions [13,14].

Only 16% of the urban bus fleet in Brazilian state capitals arePadron vehicles, with restricted use, because most of the fleet consistsof smaller, lower capacity buses (75 passengers), known as conven-tional urban buses (CUB) [15].

Padron bus size, capacity and performance when compared to CUBmodels make this vehicle particularly suitable for operations in busrapid transit (BRT) systems, defined as high-quality and customer-ori-ented transit that delivers fast, comfortable and low-cost urban mo-bility [16]. BRT is an evolution of buslane and busway concepts,proposed to improve bus transportation system performance throughassigning priority to bus traffic by physically separating bus lanes fromregular traffic and considering the use of specially designed buses.

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This type of operation is becoming widespread in Latin America, andis already successfully used in some Brazilian cities [4,15], whererestricted funding for investments in mass transportation systems leadsto the use of buses as an important alternative to railway systems forproviding rapid transit.

The concept of BRT also involves the use of environmental friendlyvehicles [4], which makes the hybrid-drive Padron bus, so faridentified as Padron H, an adequate choice, as it can meet these needswith no loss of flexibility, in contrast to the articulated or bi-articulatedmodels.

2.1. Padron H Hybrid-drive Bus

A brief description of the Padron H hybrid-drive system is presentedhere in order to provide a better understanding of the technology.Although featuring different components, all bus models are equippedwith a series arrangement hybrid-drive configuration, presented in asimplified way in Fig. 2, illustrating the elements constituting theECU, ESU and TU.

The diesel-fuelled internal combustion engine (a mechanical directinjection ICE) works the generator (G) as a motor generator group.The angular speed regulator (ASR) keeps the ICE turning over at aconstant angular speed only in the vicinity of its optimum operatingpoint in terms of fuel efficiency. The triple-phase adjuster (TFA)initially adjusts the voltage produced by the generator. This voltagefeeds the electric traction motor (EM) and its electronic speed controlsystem (SCS) which, together with the control unit (CU) and the TFA,are integral parts of the central control system (CCS).

Whenever the power demanded by the EM is less than that suppliedby the motor generator group, such as when the vehicle is operating atlow speeds or on falling gradients, part of the energy generated is usedto charge the batteries. Part of the energy generated during deceler-ation can feed the batteries. In this case, the EM works as a generatorand is part of the regenerative braking system. The surplus energy isdissipated through a resistor.

Whenever the EM requires power higher than that generated by themotor generator group, the CCS recognizes this and steps up the powerfeed to the EM from energy stored in the batteries. Consequently, thebatteries serve as a reserve, storing energy whenever the vehicleoperating conditions require low energy demand and providing the EM

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FIGURE 2 Hybrid drive configuration diagram for the Padron H bus

with energy whenever the operating conditions are more demanding,such as rapid acceleration or travelling up inclines. Managing theenergy flows and the charging/discharging conditions of the batteriesis also handled by the CCS, ensuring that the correct amounts ofcharging current are sent to the battery, in order to avoid any damageduring this process.

The ancillary equipment of the vehicles can include pneumaticbrakes as well as hydraulic steering, with no air-conditioning system.Table II provides additional information in order to help characterizethe vehicle under study.

3. OPERATIONAL PERFORMANCE EVALUATIONPROCEDURE AND FIELD TRIALS

The evaluation procedure of the operational performance of thePadron H bus may be divided into three steps. The first covers the

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TABLE II Technical characteristics of the Padron H bus

Basic components Type/ValueElements UnitCharacteristics

Fuel –Energy conversion Diesel oilInternalunit (ECU) combustion

engine (ICE)–4Number of cylinders

2.8Volumetric capacity lPotmax 135 hp

m.kgf38Torquemax

75Generator (G) kVA/1800Working rangerpmHz60hpICE/G group 80Potrated

Capacity lEnergy storage 300Fuel tankunit (ESU)

–4Batteries QuantityType –Lead-Acid

Traction unit (TU) Electric motor Potrated 120 kWkW240Potmax

Rotation 1800/3600 rpmCentral control –Electronic control Chopper/unit (CCS) system IGTB

Source: [12]

specification and planning of field trials, the establishment of dataprocessing methods and a means to compare the results. The secondconsists of carrying out field trials and collecting data. The third dealswith data processing and comparison of the findings.

3.1. Specification and Planning of Field Trials

Among the operational advantages of a hybrid-drive over conventionaldiesel buses, one is of prime concern for this work, namely improvedfuel economy. It is also important to verify the vehicle’s ability toperform smoother and quicker acceleration, more efficient braking andto reach rated maximum speed, since these may represent urban transitlevel of service attributes. Therefore, it was established that twoclasses of field trials should be carried out:

• Dynamic performance trials: in order to determine acceleration/braking rates and capacity to reach maximum speed established bythe manufacturer. In this case, confidence intervals for accelerationand braking times and distances should be determined. An initialsample (n � 5) was established.

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• Fuel economy trials: in order to determine fuel economy (km/l) inoperation. In this case, as fuel consumption depends on averagespeed [5,11], confidence intervals for fuel economy as a function ofaverage speed (km/h) should be determined. This way of expressingfuel economy has ensured greater flexibility for using the results,because once average line speed is available, with similar physicalcharacteristics, it is possible to identify the expected fuel economymore accurately. A large sample (n � 30) was established.

The method used to determine the sampling mean and its confidenceinterval follows the traditional approach when the standard deviationof the population is not known, using the standard deviation of thesample and the student (t) distribution as parameters to estimate theerror (e) [17]. In order to process the findings of the trials presented inthis paper, a significance level of 90% was rated as satisfactory, witha maximum error level of 5% of the sampling mean. These statisticsare considered satisfactory in most transportation engineering studies[18].

It should be stressed that the method presented here is based on thehypothesis that the distribution of sampling means follows a normaldistribution. For large samples (n � 30), as it is the case with thefindings for the fuel economy trial, this can be guaranteed through theapplication of the central limit theorem. For sample sizes under 30, asis the case with the dynamic performance trial findings, it is importantto know if the population subjected to the sampling has a normal orapproximately normal distribution. According to the US Society ofAutomotive Engineering (SAE), this does occur [11].

To guarantee a representative sample some other precautions mustalso be taken: (1) the sample must be selected without bias; (2) thecomponents of the sample must be completely independent of oneanother; (3) there should be no underlying differences between areasfrom which the data are selected; and (4) conditions must be the samefor all items constituting the sample. These precautions were allconsidered in planning and carrying out the field trials.

In Brazil there is no established regulation for assessing bus fueleconomy or dynamic performance. Therefore, as a first approach, adiesel-drive Padron bus, the Padron C, was used as reference for thedata survey. The procedure was adopted of measuring the results foreach of the bus types, applying statistical methods to the data andcomparing the results. Padron C was selected because it is similar tothe Padron H vehicle in terms of size, weight, passenger capacity and

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age. This vehicle is equipped with hydraulic steering, pneumaticbrakes and automatic transmission with four forward drives. Table IIIgives the main characteristics of the Padron C bus tested, which alsorepresents the conventional drive technology operating on the SaoMateus – Jabaquara Metropolitan Corridor, where the potential ofdiesel savings was to be assessed.

As a complementary reference to the fuel economy survey, datafrom national and international literature were used, once the dataacquisition conditions were clear. The comparison is easier and moreaccurate if average speed and payload are known, as will be shown.

Dynamic performance and fuel economy trials were carried out onthe basis of two situations: a vehicle carrying 5500 kg representing anoccupancy rate of 73% and assuming an average weight of 75 kg perpassenger [19] and the empty vehicle carrying only the driver and thecrew, consisting of four people. The occupancy rate selected for thetrials was justified on the basis of the average estimated loads noted inthe Metropolitan Transportation Corridor.

The vehicle load was simulated using drums filled with water. Thevehicle was loaded with empty drums, which were then subsequentlyfilled in order to simulate the loading. This procedure also allowedpartial loading to be simulated.

Before starting the tests, the vehicles were weighed, the real weightof the Padron C vehicle (10 980 kgf) was 13.61% less than that of thePadron H vehicle (12 710 kgf). In order to eliminate this difference, itwas necessary to place 1730 kg of ballast in the Padron C vehicle.

3.2. Carrying out Field Trials

3.2.1. Dynamic Performance Trial

The dynamic performance trial consisted of an acceleration test (0–60km/h) and a braking test (60–0 km/h), which provided acceleration andbraking times and distances for an initial sample size of five experi-ments, carried out along approximately 2000 m of straight, flat, dryhighway, properly sign-posted with good asphalt surfacing and novehicles in transit.

Before carrying out the five experiments in the initial sample, thedriver was trained. For the acceleration test, the driver was asked toaccelerate the vehicle steadily and continuously up to 60 km/h. For thePadron H bus, the speed limit was controlled electronically by theCCS, and does not depend on the actions of the driver.

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ASSESSMENT OF HYBRID-DRIVE BUS FUEL SAVINGS 493

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The time values as a function of speed were obtained indirectlythrough reading the voltage curve of the EM, registered by an ana-logue oscillograph. In order to calibrate the oscillograph and ratify thefindings obtained indirectly, an electro-mechanical direct reader (Cor-revit) was used that could provide the times, distances and accelerationrates as a function of the speed. To ratify the use of the oscillographfor the remaining tests, five acceleration experiments were carried outwith the empty vehicle, using the oscillograph and the Correvit systemsimultaneously.

For the braking test, once the vehicle was at 60 km/h, the driver wasasked to brake the vehicle steadily and continuously to an eventualstop. The Padron H bus was fitted with a regenerative braking deviceto generate electricity. Consequently, it was possible to obtain twotypes of braking: regenerative when only the regenerative brake wasused, or dual, when the regenerative brake was used together with theconventional pneumatic brake.

For the trials, the use of the regenerative brake does not depend onthe driver, but is handled through a switch on the vehicle commandpanel. In operating terms, by pressing the brake pedal gently, thedriver brings the regenerative brake into action, until reaching a thirdof the way, which allows the batteries to be recharged and thus savefuel. By continuing to press the pedal more than one third down, thedriver brings the dual braking system into action.

In order to slow down with the regenerative brake, the driver wasadvised that once the speed of 60 km/h was steady, he should removehis foot from the accelerator and simultaneously turn the switch on thevehicle command panel, until the vehicle stops. For dual braking, oncethe vehicle achieved a stable 60 km/h, the driver was taught to pressthe brake pedal steadily and continuously until the vehicle stopped.The five experiments in the initial sample required the driver to gothrough a training session.

3.2.2. Fuel Economy Trial

The fuel economy trial was carried out on the Sao Mateus – JabaquaraMetropolitan Corridor. The length of road used for this trial wasmapped with the help of a GPS in order to obtain the length of thesegments of highway. On this same occasion, the road was inspectedin terms of the physical conditions of the surface and its alignment.

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TABLE IV Distances between bus stops

Number ofName of bus stopTypebus stops Segment Length (km)

1 Garage Metra2 3.00Terminal 1–2Sao Bernardo LE3 2–3Stop 0.65Djalma Dutra4 Stop Matriz 3–4 0.555 0.45Stop 4–5Lauro Gomes6 5–6Stop 0.30Cooperativas7 Stop Brasil 6–7 0.308 0.80Terminal 7–8Ferrazopolis9 Stop 8–9Brasil 0.2010 Stop Cooperativas 9–10 0.3511 0.30Stop 10–11Lauro Gomes12 11–12Stop 0.45Matriz13 Stop Djalma Dutra 12–13 0.5014 0.70Terminal 13–14Sao Bernardo LD2 Terminal 13–2Sao Bernardo LE 1.701 4.00Garage 14–1Metra

Source: [22].

Table IV provides the distances between the five stopping points thatconstituted the test track.

The road selected for the trials had a flat surface paved withconcrete blocks in good condition throughout its entire length, withtraffic lights at crossroads. The road configuration consisted of lanestravelling in the same direction and opposite orientation, separatedlongitudinally from outside traffic and constituting a closed circuit thatbegan and ended at the Sao Bernardo do Campo LE (terminal). Therewere stopping points to allow passengers to board and alight bothways, and a manoeuvre for returning to the Ferrazopolis Terminal.

In order to measure fuel consumption, a flow meter was inserted inthe vehicle engine fuel-feed system. This equipment gave fuel supplieswith an accuracy of one hundredth of a litre, and the duration of thetrial accurate to a tenth of a second. Before carrying out the trials, theequipment was calibrated; adapting it to the diesel engine feed-backsystem and collecting the fuel in a scaled burette. The volume col-lected was compared to the metered volume. In a set of five experi-ments the error was less than 1%.

For each vehicle loading condition (loaded and unloaded) the circuitwas run at least five times. Fuel consumption and time were noted ateach point (see Table IV), in order to obtain a mass of fuel consump-

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tion data based on the trial duration, which could be rated to thedistance covered as average speed.

The fuel consumption data survey for the Padron C vehicle wascarried out simultaneously to the Padron H vehicle, accompanyingeach other as leader and follower. This was intended to minimizedifferences in the traffic conditions encountered by each vehicle. Atthe end of each cycle, the vehicles switched positions, taking it in turnto drive as the lead vehicle.

Part of the energy consumed by the Padron H vehicle electric motorcomes from the batteries. Therefore, it was necessary to ensure thatfuel economy measurements represented the same battery chargingconditions at each metering point. The trial was begun with thebatteries fully charged, with fuel consumption observed at each haltonly after the batteries were fully charged.

3.3. Data Processing and Comparison of the Results

The comparison of the results of the dynamic performance trial issummarized in Fig. 3. The graphs are plotted on the basis of meanvalue data.

The Padron H bus performed poorly in the acceleration trial, atspeeds ranging from 0 to 60 km/h (maximum rated speed). Thisdifference was more emphasized when the vehicle was unloaded,featuring � 26.3% in acceleration time. However, the accelerationtime of the Padron H vehicle for speed intervals under 50 km/h wasless than that observed for the Padron C, with the situation being morefavourable for speeds ranging from 0 to 30 km/h, when it achievedadvantages of (41.6% (loaded) and –38.6% (unloaded) in accelerationtime.

An analysis of the Sao Mateus – Jabaquara Metropolitan Corridorspeed records, where the fuel economy trial was performed, shows thatthe vehicle rarely accelerated at speeds of over 50 km/h, due to theoccurrence of various constraints, such as traffic lights, bus stops andinterference from other vehicles ahead of it. These aspects have ratifiedthe suitability of the performance of the Padron H vehicle, consideringthe operating conditions along this bus corridor.

Regarding the braking test, it was observed that the Padron Hvehicle had a slight advantage when loaded (rated as more critical)with favourable percentage differences above the confidence intervalfor the braking time mean. This had not occurred when empty,

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FIGURE 3 Dynamic performance trial results

although the percentage difference is less than the confidence intervalfor the braking time mean, allowing the findings to be rated as similar.Table V shows complementary data from the dynamic performancetrial.

Figure 4 illustrates the results obtained in the fuel economy trials forloaded and unloaded conditions. For each average speed range, themean values of fuel economy are given. It was decided to present theerror bars with a maximum error of 5% at a significance level of 90%.The dotted line in the loaded condition graph indicates the minimumfuel economy value established as a reference for Padron C buses bythe Ministry of Transportation. The value of 2.0 km/h was obtained for

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TABLE V Dynamic performance trial complementary data

MeanSpeedTrial Attribute Unitrange valueStatus e

Acceleration m/s2Loaded � 0.0580–30 km/h Acceleration rate 1.1660–0 km/h � 22.64Acceleration distance m452.81

m/s2� 0.022Acceleration rate 0.43Unloaded 0–30 km/h Acceleration rate 1.43 � 0.072 m/s2

60–0 km/h Acceleration distance 383.84 � 19.19 mm/s2� 0.026Acceleration rate 0.52

Braking Loaded � 0.22460–0 km/h sBraking time 4.47m� 1.861Braking distance 37.22

Braking rate 3.73 � 0.168 m/s2

Unloaded 60–0 km/h Braking time 4.35 � 0.218 sm� 0.813Braking distance 36.25

� 0.192 m/s2Braking rate 3.83

an average speed of 20 km/h, on the basis of 30 repetitions of ahypothetical traffic cycle where the vehicle accelerates from 0 to 40km/h in 18 s, travels at 40 km/h for 17 s, brakes for 8 s and halts for17 s [13].

The analysis of the results indicates that the Padron H vehicle wasat least 23.3% more fuel efficient than the Padron C vehicle for theoperational conditions established in the trials. This occurred foraverage speeds ranging from 20 to 24.9 km/h and for a loaded vehicle.Comparing fuel economy mean values for the loaded vehicle con-dition, the greatest difference was for average speed ranging from 25to 29.9 km/h (50.6%) and the least for 15 to 19.9 km/h (35.4%).

The most impressive findings are noted when the vehicles wereunloaded, with the Padron H vehicle at least 27.5% more fuel efficientthan the Padron C vehicle, taking average speeds ranging from 20 to24.9 km/h. Under these conditions, the relative difference of fueleconomy mean values ranges between 39.6% (20–24.9 km/h) and45.5% (15–19.9 km/h). It was not possible to reach the 25 to 29.9 km/haverage speed range.

From these results, it is possible to see that fuel economy improvesas average speed increases. Considering the loaded vehicle condition,the improvement in fuel economy was 73.5% if the vehicle ran ataverage speed ranging from 25 to 29.9 km/h instead of 10 to 14.9km/h.

The fuel economy sensibility to load condition is also related toaverage speed range, as shown in Fig. 5. For the Padron H, a relativeaverage difference of about 20% in fuel economy could be expected

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FIGURE 4 Fuel economy trial results

between loaded and unloaded conditions if the average speed range is10 to 14.9 km/h. This difference drops to 3.5% if the average speedrange is 25–29.9 km/h. The same occurs for the Padron C, that is evenless sensitive to load condition.

Considering the fuel economy trial, independent of load conditions

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FIGURE 5 Fuel economy sensibility to load conditions

or average speed range, the fuel economy performance of the PadronH was km/l. In the same conditions, the fuel economy of the PadronC was km/l. A relative difference of 44.8% if the mean values areconsidered.

The Padron H vehicle also presented a better fuel economy than thatstipulated in the benchmark issued by the Ministry of Transportation,being at least 17% better and averaging out at 23% better, if the figurestaken for the 20–24.9 km average speed range are considered. If 2.25km/l is used, the difference drops to 12.4%.

It is also interesting to compare the results for the Padron H bus fueleconomy trials with the findings in the international literature. To doso, the project undertaken by the Northeast Advanced Vehicle Consor-tium (NAVC) was selected [5]. In order to compare the valuespresented here, it should be considered that the NAVC carried out thetests using a dynamometer, based on specific traffic cycles and half-loaded vehicles. For some cases, the findings of the dynamometer testswere compared with benchmarks obtained in the field. In general, itwas noted that the field findings (fuel economy) were lower than thoseobtained through using the dynamometer.

From the vehicles tested by the NAVC, two models were selectedfor comparison: NovaBUS RTS Diesel Series 50 (NovaBUS) andOrion-LMCS VI Hybrid Diesel (Orion HV). The first is a conventionalvehicle driven by an internal combustion engine fuelled by diesel oil,with a tare weight of 12 690 kg, which was tested with a load of 1969

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FIGURE 6 Vehicles compared to international benchmark

kg. The second is a hybrid vehicle with a tare weight of 14 091 kg thatwas tested with a load of 1881 kg.

Figure 6 compares the results based on three test cycles found in theNAVC work: the Central Business District Cycle (CBD), normallyused to assess bus performance with an average speed of 20.3 km/h;the NY Composite Cycle (NYCC), which is similar to the cycle in theCity of New York, and consequently represents a mix of urban andinter-city traffic with an average speed of 14.2 km/h, and the Manhat-tan Cycle (MC), established on the basis of the traffic conditions in theNew York Metropolitan Region with an average speed of 11.1 km/h.

The fuel economy values given in Fig. 6 for the Padron H andPadron C vehicles were taken from Fig. 4 for the pertinent averagespeed ranges. The error bars help characterize the interval estimate ofthe population mean. Compared to the NovaBUS vehicle, the PadronH posts fuel economy values that are higher for all cycles. However,this does not occur when compared with the Orion HV vehicle,performing better only for the cycles with average speeds of 20.3 km/hand 11.1 km/h, with a similar performance for the cycle with anaverage speed of 14.2 km/h.

As already stressed, this comparison should be considered withcaution, due to the differences between the traffic cycles presented bythe NAVC and the conditions for the trials carried out in Brazil, withsimilar average speed ranges, although obtained differently. Moreover,as the dynamometer trials were carried out with light loads, it is

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FIGURE 7 Fuel efficiency of Padron H -- estimate for 96 passengers

expected that the figures would drop, which to a certain extent favorsthe Padron H vehicle.

As stated in Section 3.1, the Padron H is 1730 kg heavier than thePadron C, which means that the same total gross weight for eachvehicle represents different passenger capacity. In this case, the totalgross weight tested of 18 210 kg for the Padron H represents some 76passengers, while for the Padron C it represents about 96 passengers.Relationships between fuel economy, load condition and averagespeed range have also been shown (Fig. 5). Therefore, it is possible toinvestigate the fuel efficiency of the Padron H in terms of litre perpassenger per kilometre and compare them to those for the Padron C.Figure 7 presents these estimates considering both vehicles carrying 96passengers. In this case, the Padron H is at least 20.7% more efficientthan the Padron C at an average speed range of 10–15.9 km/h. Thebest performance occurs for an average speed range of 25–29.9 km/h(35.5%).

4. ASSESSMENT OF HYBRID-DRIVE BUS FUEL SAVINGS

In order to estimate the fuel savings that could be obtained through theuse of the Padron H bus, it was decided to take two alternatives intoconsideration: replacing the Padron C bus fleet in the Sao Mateus –

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Jabaquara Metropolitan Corridor, and replacing the Padron C bus fleetin the Sao Paulo Metropolitan Region.

4.1. Use in the Sao Mateus – Jabaquara Metropolitan Corridor

Using the data available on the Sao Mateus – Jabaquara MetropolitanCorridor [15,20–22] it was possible to obtain the passengers per tripratio, allowing an estimate of the average occupancy of the vehicles inthe course of a year, and the average annual mileage per vehicle inthousands of kilometres. The average mileage per vehicle tends tohover around 74.8 ( 103 km/year for a confidence interval of 2% witha significance level of 90%. This directly reflects a specific character-istic of this operation, where the vehicles travel only within theboundaries of the Metropolitan Corridor.

Examining the average occupancy per vehicle, the best fit wasobtained for a simple linear regression estimate, shown in Eq. (1). Thisleads to an average occupancy rate of 73.3 passenger/vehicle during2002, which justifies the loading selection adopted for the trials, basedon the 100 passenger/vehicle capacity for the Padron H.

y[pax/vehicle] � 1.8392.x[year] � 3608.8 (1)

where y � average occupancy per vehicle in passenger/vehicle; andx � year of observation.

For 1992 through 1998 the Padron C fleet average speeds and meanfuel economy were 22 km/h and 1.94 km/l respectively. These figuresare in keeping with the findings for the trials, which should becompared with the figures for average speeds ranging from 20 to 24.9km/h. The average occupancy rates for the vehicles over the periodvaried between 55% and 66% [15,20,21].

The current Padron C fleet consists of 94 vehicles with an averageage of eight years. The corridor also handled articulated buses andtrolley buses (conventional and articulated), all with the Padronconfiguration. In 1998, the company operating this corridor acquired10 articulated trolley buses as part of a fleet modernization process,which continued during subsequent years with the acquisition of sevenarticulated buses running on diesel oil, and more recently, the purchaseof three hybrid-drive Padron buses in 2002, somewhat similar to thevehicle tested [22].

The findings outlined above were applied to the possibility ofreplacing the entire conventional Padron C bus fleet over the next 10

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FIGURE 8 Fuel savings through use of Padron H buses in Sao Mateus -- JabaquaraMetropolitan Corridor

years, from 2003 to 2012, through the acquisition of 10 vehicles ayear, assuming the continuity of the fleet renewal policy adopted bythe Corridor’s operator and an occupancy rate of around 10% of theproduction capacity of the hybrid vehicle plant (10 to 15 vehicles amonth).

For this time period, and if the trends continue, the application ofEq. (1) implies that the average occupancy rate will remain in therange of 75.1–91.7% per vehicle. The average traffic speed in thecorridor was considered in the range from 20 to 24.9 km/h. Takingthese together, it is possible to calculate estimates giving the greatest,average and smallest savings in fuel use through using Padron Hbuses. These are shown in Fig. 8.

During the 10 years in operation, average savings of 5.9 million l ofdiesel oil could be expected, representing an average reduction of11.8% in the consumption of this fuel for the operation of the entirefleet in this corridor (articulated and Padron C buses). From the 10thyear onwards, annual diesel oil savings would average out at 1 millionl, accounting for some 20% of total annual diesel oil consumption.

Taking the price of diesel oil in effect at the time when the analysiswas carried out (at US$0.325 [23]), the reduction in diesel oil con-sumption would represent savings of US$1.93 million in 10 years, oran annual reduction of US$193,480 in operating costs. Assuming thatthe Padron H vehicle has an additional acquisition cost of approxi-

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mately 30%, the proposed fleet renewal would require additionalannual investments of US$130,000 that could be more than offsetthrough fuel savings.

In addition to lower operating costs, diesel oil savings would alsoresult in lower CO2 emissions, the main greenhouse gas, and that couldbe rated as a direct environmental benefit through using hybrid buses.The estimated reductions in these emissions was drawn up on the basisof the calculation taking the CO2 emission factor caused by burningdiesel oil at 3188 g/kg [24] and a specific mass of 0.83 kg/l [23]. In10 years of operation, a drop in emissions of around 15 718 tons ofCO2 is expected, with an average cut of 2720 tons a year from the 10thyear onwards. Supposing the value of US$5 for each avoided ton ofCO2, this reduction in emissions represents a small annual gain ofUS$7859, or 4% of the gains obtained in fuel savings.

4.2. Application to the Sao Paulo Metropolitan Region Fleet

The Sao Paulo Metropolitan Region consists of the state capital and 39Municipal Districts. It has a fleet of approximately 14 300 buses, with5% of this fleet being Padron C buses [1]. It is these that could bereplaced by Padron H vehicles without adversely affecting systemoperations. As shown in the Sao Mateus – Jabaquara MetropolitanCorridor, lower operating costs resulting from fuel savings could morethan offset the increased investments needed to acquire the hybridvehicles. Consequently, it was decided to assess the reduction in dieseloil consumption for this new alternative.

The operating data survey indicates that the fleet in question has auseful life of seven years, an average mileage of 55 000 km/year, withan average occupancy rate of 50% and average speed of 14 km/h [1].These calculations were carried out on the basis of replacing the entirePadron C bus fleet over a seven-year period, through annual acquisi-tions of 98 Padron H type vehicles each year. The consumption figureswere taken based on average speeds ranging from 10 to 14.9 km/h.Fig. 9 illustrates the findings for fuel savings.

In the course of 10 years in operation, average savings of 54.2million l of diesel oil are expected, which is some 9.2 times higherthan the figure obtained for the Sao Mateus – Jabaquara MetropolitanCorridor. It is noted that in this case, the fleet renewal takes place morequickly, as the average age of the vehicles is lower. Consequently,from the seventh year onwards, annual diesel oil savings remain

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FIGURE 9 Fuel savings through the use of Padron H buses -- Sao Paulo MetropolitanRegion

constant at 7.7 million l of diesel oil. It is estimated that the annual cutin operating costs would reach some US$2.2 million for an increasedinvestment of around US$1.3 million.

For the estimated reduction in CO2 emissions for this alternative, thefactors used for this calculation are the same as those for the previousalternative. During 10 years in operation, a reduction in emissions isexpected of around 143 000 tons of CO2, with an average annual dropof 20 400 tons from the seventh year onwards. Taking the figure ofUS$5 per avoided ton of CO2, the cut in emissions would represent anannual gain US$71 500, approximately 3% of the gains through lowerexpense in fuel consumption.

5. CONCLUSION

The results presented here confirm the operational advantages ofhybrid-drive over conventional diesel buses in urban operations, astheir dynamic performance may be rated as satisfactory for urbantraffic conditions, particularly for the conditions in the transportationcorridor taken for trials, with the fuel economy results being quitepromising. Compared with the Padron C bus selected as a reference,fuel savings of over 20% may be assumed with confidence, for thetraffic conditions in which the tests took place.

It is hypothesized that the impressive performance of the Padron H

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vehicle in the fuel economy trial is due mainly to two factors: propersizing of the motor generator group, which reconciles a dynamicperformance when travelling through the corridor with an operatingscheme for the internal combustion engine that minimizes specific fuelconsumption, in parallel with the use of the regenerative brake as asource of energy for charging the batteries.

It was possible to reach a ratio between mean fuel economy (km/l)and average speed ranges (km/h) that allows a closer approximationfor calculating the fuel savings expected through the use of thesevehicles, provided that the average traffic flow speed is known. Forexample, fuel savings ranging 35–40% were noted for average speedsranging 10–14.9 km/h. These average speeds are precisely those foundfor urban traffic in major Brazilian cities such as Sao Paulo, Rio deJaneiro and Belo Horizonte [1,25,26], further recommending the con-sideration of this technology.

For the Sao Mateus – Jabaquara Metropolitan Corridor and the fleetin the Sao Paulo Metropolitan Region, the use of hybrid busesindicates a significant reduction in diesel oil consumption and aproportional reduction in operating costs and CO2 emissions, althoughin this case the expected gains through carbon credits are far lower(around 6–8%) than the gains through lower operating costs.

The deployment of this technology in other transit corridors iscertainly a realistic possibility for Brazilian cities. For example, thepotential for using Padron H buses in order to reduce diesel oilconsumption could be illustrated by their use in Curitiba (five corri-dors, 60 km), Belo Horizonte (15 corridors, 73 km), Goiania (twocorridors, 40 km), Campinas (one corridor, 5 km), Porto Alegre (eightcorridors, 28 km), Uberlandia (six corridors, 31 km), Sao Paulo (49corridors, 165 km) and Recife (six corridors, 15 km) [1,15,20,21].

An indirect benefit of reducing diesel oil consumption is to decreaseatmospheric pollutant emissions (CO, NOx, HC and PM) at the locallevel, as well as at the regional (mainly sulphur oxides – SOx) andglobal levels (CO2). It is possible to obtain an estimate of the reduc-tions in the emissions of CO2. For the local pollutants, it would benecessary to conduct a survey of the reductions in emissions of theseatmospheric pollutants.

It is worthwhile recalling that hybrid traction technology can bequite versatile, allowing other types of fuels and engines to be used.Consequently, a hybrid bus could be developed and tested, fitted withan internal combustion engine running on, for example, ethanol. Thisalternative is most appropriate in the Brazilian context.

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As hybrid vehicles are some 30% more expensive than conventionalvehicles, proper estimates of expected fuel savings are the main inputdata for calculating the feasibility of these additional investments innew fleet. Obtaining the fuel savings figures as presented providesinput for an economic feasibility study of this technology. Accordingto the manufacturer, other items that help lower the operating costs ofthe vehicle include: engine maintenance, consumption of lube-oils, tireand brake linings. The highest additional cost is the batteries [12].

To identify further the significant benefits of hybrid-drive technol-ogy it is also necessary to make additional tests such as noise andatmospheric pollutant measurements. The procedure proposed toevaluate operational performance can be easily adapted to include suchtests.

A reliability analysis is another important consideration for choos-ing a new technology. It was not possible to do this as part of theresearch reported here, since it also depends on a larger operationalfleet running over a longer period of time.

It is also timely to suggest that additional trials could be carried outunder other road conditions, including gradients and urban trafficconditions on shared thoroughfares, which is a typical situation inBrazilian state capitals.

References

[1] NTU (2003) Dados Estatısticos da Confederacao Nacional dos TransportesUrbanos (Confederacao Nacional dos Transportes Urbanos [NTU], Brasılia).

[2] FABUS (2002) Informacoes Estatısticas. Associacao Brasileira dos Fabricantesde Onibus (FABUS, Sao Paulo).

[3] MME (2002) Balanco Energetico Nacional 2002 (Ministerio das Minas e Energia[MME], Brasılia).

[4] IEA (2002) Bus System for the Future. Achieving Sustainable Transport World-wide (International Energy Agency [IEA], Paris).

[5] NAVC (2000) Hybrid-Electric Drive Heavy-Duty Vehicle Testing Project, FinalReport (Northeast Advanced Vehicle Consortium [NAVC], West Virginia).

[6] Jefferson, C.M. and Barnard, R.H. (2002) Hybrid Vehicle Propulsion (WIT Press,Ashurst Lodge, Ashurst).

[7] O’Keefe, M.P. and Vertin, K. (2002) An Analysis of Hybrid Electric PropulsionSystems for Transit Buses, Milestone Completion Report (National RenewableEnergy Laboratory, Golden, CO).

[8] Ribeiro, S.K. et al. (2000) Transporte e Mudancas Globais (Mauad, Rio deJaneiro).

[9] Ribeiro, S.K. et al. (2001) Transporte Sustentavel. Alternativas para OnibusUrbanos (COPPE/UFRJ, Centro Clima, Rio de Janeiro).

[10] Ribeiro, S.K. et al. (2001) Estudo das vantagens Ambientais do Gas NaturalVeıcular (O Caso do Rio de Janeiro. COPPE/UFRJ, Centro Clima, Rio deJaneiro).

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[11] SAE (1996) Automotive Handbook, 5th edn (Society of Automotive Engineering[SAE], Warrendale).

[12] ELETRA (2002) Technical Information on Brazilian Hybrid Vehicles (EletraTecnologia de Tracao Eletrica [ELETRA], Sao Paulo).

[13] GEIPOT/EBTU (1983) Standardization Study of City Buses, Final Report (Em-presa Brasileira de Planejamento de Transportes [GEIPOT]/Empresa Brasileirados Transportes Urbanos [EBTU], Brasılia).

[14] FINEP (1983) Aspectos Metodologicos para Implantacao de Trolebus em Corre-dores Urbanos (Financiadora de Estudos e Projetos [FINEP], Comissao SEPLAN/Energia, Consorcio Setepla/Engeconsult, Rio de Janeiro).

[15] ANTP (2000) Anuario dos Transportes Urbanos 2000 (Associacao Nacional dosTransportes Publicos [ANTP], Sao Paulo).

[16] Wright, L. (2002) Latin America Busways: Moving People Rather than Cars.Urban Transport for Growing Cities (G. Tiwari, ed.) (Machillan India Ltd, NewDelhi).

[17] Stevenson, W.J. (1981) Estatıstica Aplicada a Administracao (Editora HarbraLtda, Sao Paulo).

[18] King, L.E. (1994) Manual of Transportation Engineering Studies: AppendixC/Statistical Analyses (Prentice Hall Inc, New York).

[19] IBGE (2003) Caracterısticas Medias da Populacao Brasileira (InstitutoBrasileiro de Geografia e Estatıstica [IBGE], Rio de Janeiro).



[22] METRA (2003) Informacoes Operacionais da Empresa (Sistema Metropolitanode Transporte Ltda [METRA], Sao Paulo).

[23] ANP (2003) Anuario Estatıstico Brasileiro do Petroleo e do Gas Natural 2002(Agencia Nacional de Petroleo [ANP], Rio de Janeiro).

[24] IPCC (1996) IPCC Guideline for National Greenhouse Gas Inventory. ReportInstructions (Intergovernmental Panel on Climate Changes [IPCC], London).

[25] IPEA/ANTP (1996) Reducao das Deseconomias Urbanas com a Melhoria doTransporte Publico (Instituto de Pesquisas Economicas Aplicadas [IPEA]/Asso-ciacao Nacional dos Transportes Publicos [ANTP], Rio de Janeiro).

[26] SecTran (2000) Polıtica de Transporte de Passageiros para a Regiao Metropoli-tana do Estado do Rio de Janeiro (Secretaria de Transporte do Estado do Rio deJaneiro [SecTran], Rio de Janeiro).

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assessment of hybrid‐drive bus fuel savings for brazilian urban transit

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