Vehicle Drivetrains Customisation for Transportation Fleets

Transcription

1 Vehicle Drivetrains Customisation for Transportation Fleets S. Faias, J. Esteves, P. Ferrão Abstract Vehicle Drivetrain Customisation goes through the idea that configuration and dimensioning of a vehicle drivetrain can be defined as a function of the drive cycle and the type of service to be provided by the vehicle. In several professional domains, vehicles are used along their life cycle in predefined programmable routine activities. This work is intended to show that a Vehicle Drivetrain Customisation is a solution to lower environmental impacts, reducing energy consumptions and pollutant gaseous emissions. The Vehicle Drivetrain Customisation and the validation of the simulation tool used is based in experimental data collected from the urban drive cycle described by a battery electrical minibus in the city of Oeiras, Portugal. The definition of the activity that will be supported by the vehicle is based in the proposed velocities profile, road grades, weight, autonomy and maximum acceleration required. A simulation tool is used in order to evaluate the different drivetrains performances, optimising the better one to the specific application intended. The results obtained show that the implementation of drivetrain customisation to vehicles describing pre-defined routine activities along their life cycle can be a solution with a significant potential to minimize energy consumption and environmental burdens. Keywords: Bus, energy consumption, HEV, pollution, simulation 1 Vehicle Drivetrain Customisation the concept Vehicle Drivetrain Customisation goes through the idea that configuration and dimensioning of a vehicle drivetrain can be optimised as a function of the drive cycle and the type of service proposed to the vehicle. Vehicles for several professional domains are used along their life cycle in programmable routine activities that can be predefined, like for example post service companies and urban distribution fleets, where this philosophy can be easily implemented [1]. Having the objective of overpassing some of the technological barriers attained by the introduction of alternative drivetrain technologies, Vehicle Drivetrain Customisation is the solution for implementing some objectives to be reached in short-term by road vehicles, like consumptions of less than 2,5 l / 1 km. Obviously, Vehicle Drivetrain Customisation is in apparent contradiction with the actual car manufacturers strategy, that is oriented to large series production is the key sector to the reduction on the vehicle production cost. Several types of alternative fuel and drivetrains already available, but not all of them are the best suited for a specific application. This work intends to show that the search for an optimised drivetrain for given applications allows for lower environmental impacts, reducing energy consumptions and pollutant gaseous emissions. As this optimisation process involves several aspects at same time, it is proposed to use a simulation tool in order to evaluate the different drivetrains performances and allowing optimising it to the specific application intended. The Vehicle Drivetrain Customisation and the validation of the proposed simulation tool are the main objective of this paper, where experimental data collected from the urban drive cycle described by a battery electrical minibus in the city of Oeiras, Portugal is used. 1

2 2 Definition of the Simulation Tool Drivetrains dimensioning, optimisation, evaluation and comparison will request the use of a simulation tool to test simultaneously the several different aspects involved. Advisor is a simulation tool prepared to analyse the power flow throughout the drivetrain, the energy/fuel consumption and pollutant gaseous emissions of vehicles. That simulation tool has been developed by the National Renewable Energy Laboratory of the US Department of Energy [2]. 2.1 Simulation tool operation The simulation tool enables for the distribution of a drivetrain which is submitted to a given drive cycle and provides performance results that can be analysed and compared. First steps go through components dimensioning and the vehicle model definition. The drive cycle, describing the velocities profile and the road grades is programmed and running the simulation allows for evaluating the expected performance. Any variable can be changed and a new simulation can be done in order to optimise the performance as represented in, figure 1. Figure 1 Advisor working Fluxograma 2.2 Simulation tool validation Any tool using a model that intends to represent a real scenario requires validation with experimental results. The validation of the proposed simulation tool was done using experimental data collected by a battery electrical minibus operation on a normal urban bus service run in the city of Oeiras. The experimental data were collected during the electric mini-bus demonstration actions that have taken place from June 22 in 24 Portuguese cities with the goal of showing the benefits of alternative technological solutions in urban public transport fleets. This action is the result of the cooperation between two entities; DGTT, Directorate-General for Inland Transport, and the APVE, Portuguese Electric Vehicle Association [3]. The battery electric minibuses used were Tecnobus - Gulliver U52ESP, as characterized at table 1. Table 1 Battery electric minibus characterization Tecnobus - Gulliver U52ESP Motor Maxim power (@ 139 rpm) Maxim torque (@ 95 rpm) Nominal voltage Maxim current Traction Battery PME TA 18, CC Series Excitation 24,8 kw 235 Nm 72 V 6 A Pb - 2 modules in series 2

3 Capacity Nominal voltage Weight Performance Maxim velocity Autonomy Dimension Length Width Height Weight Net (including traction batteries) Gross (24 passenger + 1 driver) 585 Ah (36+36) V (75+75) kg 33 km/h 6,5 hours 53 mm 27 mm 275 mm 4285 kg 635 kg A model of the battery electric bus drivetrain created using Advisor is presented in figure 2. Figure 2 Drivetrain of the battery electrical bus The model validation was conducted making use of results obtained from different experimental tests and several parameters affecting performance were analysed. The velocity and elevation profile from the minibus drive cycle are presented in figure 3. Figure 4 describes the number of passengers evolution during the drive cycle evaluated and figure 5 presents the comparison of experimental and simulation results. Velocity [km/h] Elevation [m ] Distance [m] a) Velocity profile b) Elevation profile Figure 3 Urban bus service characterization 2 Number of Passengers Figure 4 Number of passengers during the drive cycle 3

4 Power [kw] Experimental 32.5 Simulation Figure 5 Comparison of experimental and simulation results The results presented in figure 5, validate the simulation performed, as the evolution obtained fits with experimental data. The drivetrain energy consumption obtained on this circuit, considering the battery charging process efficiency was about 11,3 kwh/1 km, higher than the typical value, 69, kwh/1 km. This fact is result of the elevation profile, with high number of hills and the average number of passengers of 16,5, about 69 % of the total capability. Consumed Energy [kwh] Experimental Simulation Methodology based for drivetrain customisation A methodology following four main steps is developed in this paper with the goal of the vehicle drivetrain customisation. This methodology is presented in the next paragraphs. 3.1 Service and drive cycle characterization Vehicles for professional use are frequently driven along their life cycle in predefined programmable routine activities. A complete characterization of the service and the drive cycle that will be driven is essential for the vehicle drivetrain customisation. The typical parameters used to describe a service circuit are: Velocities profile Road grades profile Maximum weight carried The service and drive used refers to an urban battery electric minibus run in the city of Oeiras, Portugal. This is performed in a closed circuit, driven continuously during the day, with a stop of half hour at lunchtime. The circuit has about 2,7 km and takes something like 2 minutes to be described. It has no predefined bus stop places; the passengers are caught anywhere they need. The average velocity is 8,2 km/h and the maximum 24,7 km/h, the higher hill of the drive cycle has a grade of 8,4 % and the maximum weight carried corresponds to 24 passengers. The typical velocity profile and the elevation profile of the circuit are represented in figure 3, a) and b) respectively. 3.2 Minimal performance requirements definition Customisation of a vehicle drivetrain, requires the service and drive cycle characterization in order to promote the identification and definition of the minimal the performance requirements. Those requirements, essential for the vehicle drivetrian dimensioning, are the following Full load acceleration Top speed Hill starting ability Autonomy 4

5 The full load acceleration is the time dispended from the stopped position to raise the velocity of 4 km/h, carrying the maximum weight. Top speed depends on the law imposition at the local circulation; in Portugal the limit for urban areas is 5 km/h. The bus circuit does not have predefined bus stop places, so the drivetrain must have the ability to start anywhere with the possibility of carrying the maximum weight. To define the hill starting ability, a start from the stopped position to 1 km/h during 1 seconds and the higher road grade of the circuit was considered. The bus describes the circuit continuously during 5 hours, totalising 4.5 km. In order to have a security gap, autonomy of 5 km was considered. This value is easily obtained by each one of the drivetrain presented. The minimal performance requirements are presented in table 2. Table 2 Minimal performance requirements Full load acceleration (-4 km/h) Top speed 14,7 sec. 5 km/h Hill starting ability 8.4 % Autonomy 5 km 3.3 Drivetrains dimensioning The dimensioning consists in the main drivetrain characteristics determination using the velocity and grade profiles and the minimal requirements previously defined. The main characteristics to be determined are: Traction Motor/Engine Power Energy Source Capability Hybridation Rate Energy Storage System Capability Several types of alternative fuel and drivetrains were considered, evaluated and compared, such as: Battery Electric Conventional Diesel Compressed Natural Gas Series Hybrid Electric Parallel Hybrid Electric The main characteristics of the different drivetrains are discussed in the next paragraphs: 5

6 3.3.1 Battery Electric Drivetrain The Battery Electric drivetrain consists in the simplest case and includes a battery, a converter, a motor and a differential gear [4]. The greatest advantages of this drivetrain are the high start torque, the zero-emission in its running location and braking regenerative possibility. The major constraint in some applications can be its limited autonomy, when compared with other drivetrain solutions. In this paper the Battery Electric bus drivetrain was based on the Gulliver minibus characteristics Conventional Diesel Drivetrain The Diesel conventional drivetrain operation is based on an Internal Combustion Engine (ICE), responsible for the vehicle propulsion. Large autonomy and fully developed technology are the mainly advantages of this drivetrain. The lower engine efficiency and the local pollutant gaseous emissions are the major disadvantages. The dimensioning of the customised diesel drivetrain was performed considering the velocity and grade profiles, the maximum weight carried and the minimal performance requirements presented previously. With the aid of the simulation tool, it has been concluded that a 35 kw maximum power Diesel ICE satisfies the pre-defined constraints Compressed Natural Gas Drivetrain The Compressed Natural Gas (CNG) drivetrain operation is based on an adapted Gasoline ICE. One argument used in natural gas promotion as an automotive fuel is that natural gas reduces carbon dioxide emissions when compared to conventional hydrocarbon fuels. The dimensioning process of the Diesel ICE showed that a 4 kw maximum power CNG ICE satisfies the pre-defined constraints Series Hybrid Electric Drivetrain In a Series Hybrid Electric drivetrain, an Internal Combustion Engine drives an electric generator that delivers power to a battery and/or to the electric motor responsible for the vehicle propulsion [5]. A small IC engine designed only for average power, the possibility of working for short moments as a pure electric vehicle and braking regenerative possibility are some advantages of this drivetrain. The Series Hybrid Electric drivetrain dimensioning considered the drive cycle with the maximum weight. As presented in figure 6, the highest power requested to the traction motor during that drive cycle was about 32 kw, and, as electric motors can work in overload for intermittent regimes, a 25 kw motor was choosen. Figure 6 Traction motor power requested Power [kw] During the drive cycle 1,45 kwh were consumed and 119 seconds dispended. Considering those values, a constant power source with 4,4 kw will be enough to produce all the energy consumed during a drive cycle. Throughout the drivetrain operation the requested power was higher than the power generated as a consequence, the additional power required is to be supported by the intermediate energy storage system. As presented in figure 7, during some periods of time the energy stored is negative, and this means that the intermediate energy storage system must be partially charged at the startup. 6

7 16 12 Consumed Energy Generated Energy Stored Energy Energy [Wh] Figure 7 Series hybrid electric drivetrain energy flow The generator selected to be composed with the internal combustion engine presents an efficiency rounding 82 %, so the power available by the ICE must be of 5,4 kw, and its highest efficiency should be reached when supplying 5,4 kw. For this application, an 11kW motor was chosen. For the intermediate Energy Storage System (ESS) a Lithum-ion battery with 2, kwh and maximum capacity of 7,4 Ah, composed by 25 modules of 1,7 V was selected. Those batteries are characterized by high specific energy, high specific power and high energy efficiency when compared with lead and nickel-based technologies [6] Parallel Hybrid Electric Drivetrain In a Parallel Hybrid Electric drivetrain, an Internal Combustion Engine (ICE) and an electric motor are coupled directly or via a gearbox to the final drive, responsible for the vehicle propulsion [7]. The possibility to work as a conventional vehicle in extra-urban areas or as a pure battery electric vehicle in urban areas makes this drivetrain as the most versatile one. Multiple power combinations of the ICE and the electric motor can be used, making the Parallel Hybrid Electric drivetrain dimensioning the most complex. In order to establish a term of comparison and avoid over dimensioning, the full load acceleration of the Conventional Diesel drivetrain used in this dimensioning process. Figure 8 presents the possible power combinations of ICE and the electric motor, in order to provide a full acceleration of the order of 15 s Full load acceleration - 4 km/h [14.5; 15.] seconds ICE power [W] Elect motor power [W] Figure 8 Parallel hybrid electric drivetrain possible power combinations Provided the possible drivetrain combinations, the next step was to select one to be used. The criteria adopted was related to the lower energy consumption achieved. At this point, the Rate of Hybridisation is introduced and defined as the ratio of the electric motor power to the overall power. A Rate of Hybridisation equal to zero represents a conventional ICE drivetrain and a Rate of hybridisation equal to one corresponds to a pure electric drivetrain [8]. 7

8 The energy consumption evolution as a function of the Rate of Hybridisation is presented in figure 9. 2 Consumption [L/1 km] Rate of Hybridisation [%] Figure 9 Energy consumption evolution for different Rate of Hybridization It would be expected that higher Rate of Hybridization may correspond to lower energy consumptions, but this is not always true, because a smaller ICE doing the same job can impose an operation point far from the optimal efficiency one. In the present situation, a Rate of Hybridization of 1 % allows 15 litres/1 km, the lowest energy consumption, obtained with a maximum power Diesel ICE of 27 kw and an electric motor of 3 kw. For the intermediate ESS a Lithum-ion battery with 2, kwh and maxim capacity of 7,4 Ah, composed by 25 modules of 1,7 V was selected. The sum of the ICE and the electric motor power is transmitted to gearbox during the acceleration periods, when the power required is low and the ESS state of charge is also to low, ICE supplies the gearbox and the motor (that will work as generator) to charge the ESS. At low velocities if the ESS state of charge allows, the ICE can be turned off and the vehicle work as pure electric vehicle, regenerative braking can also contribute to ESS charging. Figure 1 presents the energy flow evolution obtained during the parallel hybrid electric drivetrain operation. 18 Energy [Wh] ICE-output Elect. Motor-output Gearbox-input ESS-input Figure 1 Parallel hybrid electric drivetrain operation Customised Vehicle Drivetrain Characterization The general characteristics of the customised drivetrains as obtained from modelling the drive cycle selected are presented in table 3. Table 3 Customised drivetrains characterization ICE Components Diesel Customised Drivetrains CNG Series Hybrid Parallel Hybrid P MAX [kw] P OPT [kw] 5,4 Generator P MAX [kw] 4,4 Motor P MAX [kw] 25 3 ESS Energy [kwh] 2 2 Capacity [Ah] 7,4 7,4 Vehicle Total mass [kg]

9 3.4 Analysis of the Results Evaluation and comparison of the different drivetrains performances was used to understand the key points to be considered in customisation. General-purposed and customised drivetrains are compared in order to demonstrate drivetrain customisation benefits. The results from energy consumption and pollutant gaseous emissions are considered to decide the best suited vehicle drivetrain for the service Performance Comparison A 115 kw maximum power Diesel minibus available at the Portuguese market will be used as the general-purpose drivetrain reference, together with the Gulliver Battery Electric minibus. Performance comparison of the several drivetrains is presented in table 4. Table 4 Drivetrains performance Full load acceleration ( - 4 km/h) Minimal Performance Requirements General-Purpose Diesel Battery Electric 14,7 sec. 4,1 sec. 17,8 sec. ( - 3 km/h) Diesel Customised Drivetrains CNG Series Hybrid Parallel Hybrid 14,7 sec. 13,4 sec. 12,1 sec. 15, sec. Top speed 5, km/h 111,7 km/h 33 km/h 5,5 km/h 57,8 km/h 52,8 km/h 5,5 km/h Hill starting ability 8.4 % 18 % 12 % 13 % 16 % 15 % 15 % Autonomy 5 km 3 km 6 km 15 km 15 km 15 km 15 km Fuel/Energy Consumption Comparison Usually, the bus operation does not occur in the worst conditions, so the simulations to obtain comparative consumption results will take place considering a constant number of twelve passengers. The several drivetrains Fuel/Energy consumptions are presented in table 5. Table 5 Drivetrains Fuel/Energy consumption General-Purpose Fuel / Energy Consumption [ /1 km] Diesel Battery Electric Customised Drivetrains Diesel CNG Series Hybrid Parallel Hybrid 37,8 litres 11,3 kwh 21,2 litres 25,9 m 3 (n) 17,9 litres 16,6 litres The characterization of the energy consumption is very important for a vehicle evaluation, not just from the economical point of view, but also from its environmental impacts. The vehicles evaluated use different technologies and energy sources, so a correct comparison for the energy point of view must be made considering a well to wheel analysis. This imposes transforming the tank consumption in primary energy consumption. The primary energy consumptions calculation considers extraction, transportation and refining efficiency for the diesel fuel and for the compressed natural gas [9]. The determination of the primary energy consumed by the Battery Electric drivetrain considers the typical mix of power plants used in Portugal namely the based on combustion processes like Coal (34,3%), Natural Gas (14,1%) and Fuel Oil (11,7%), the renewable one like Hydraulic (29,%) and the obtained by Cogeneration (11%) [1]. Figure 11 shows the results obtained for primary energy consumption for the different drivetrains considered. 9

10 Primary Energy Consumption [MJ / km] Gen. Purp. Diesel Gen. Purp. Battery Electric Cust. Diesel Cust. CNG Cust. Series Hyb. Cust. Parallel Hyb Figure 11 Primary energy consumption The results presented in figure 11, clearly show that drivetrain optimisation provide significant increases in energy efficiency. The lowest consumption corresponds to the battery electric minibus and an impressive value of 42% of Diesel general-purpose consumption is obtained. At pure energy consumption point-of-view, electric drivetrains are the most efficient, however they have the low autonomy constraint in several applications. In this case must be considered also, like presented in table 4, that battery electric drivetrain doesn t satisfy all the Minimal Performance Requirements, namely the Top Speed and the Full Load Acceleration. Drive cycles with many stops and starts and with some climbing hills are usually indicated for battery electric and hybrid vehicles, making no surprise the lower consumption of the customised drivetrains presented by the parallel hybrid, 44% of the Diesel general-purpose, although the diesel engine may show some possibility to be further optimised. The benefits of customization are clearly perceptible when comparing the general-purpose and the customized Diesel drivetrains consumptions. Customization, in the studied case, allows a reduction of 38% of the primary energy consumption. Considering a hypothetical scenario where the total energy produced in Portugal was obtained by Natural Gas combustion power plants, the battery electric minibus consumption would be 52% of the Diesel general-purpose, like presented in figure 12. Primary Energy Consumption [MJ / km] Gen. Purp. Diesel Gen. Purp. Battery Electric Cust. Diesel Cust. CNG Cust. Series Hyb. Cust. Parallel Hyb Figure 12 Primary energy consumption considering only Natural Gas power plants Pollutant Gaseous Emissions Comparison One of the major drivetrain customisation objectives consists in the reduction of pollutant emissions to atmosphere. The global warming effect and the depreciation of urban quality of life have direct relation with the automobile exhaustive use. Here, each drivetrain pollutant gaseous emissions are presented and compared. The electric minibus is a zero-emission vehicle in its running location; however, from a life cycle point of view, emissions that result from energy production must be considered [11]. The specific emissions obtained are presented in figure 13. 1

11 The argument of carbon dioxide emissions reduction used in compressed natural gas promotion as an automotive fuel is here confirmed, and again the drivetrain customisation benefits are demonstrated. Within the customised drivetrains, the parallel hybrid presents the lower carbon dioxide, 44 % of the Diesel general-purpose emissions. CO [g / km] Gen. Purp. Diesel Gen. Purp. Battery Electric Cust. Diesel Cust. CNG Cust. Series Hyb. Cust. Parallel Hyb Figure 13 Specific emissions of carbon dioxide In figure 14 are presented the results when considering the scenario with the total energy obtained by Natural Gas combustion power plants, and it is significant the specific carbon dioxide emissions aggravation related to the battery electric minibus. CO [g / km] Gen. Purp. Diesel Gen. Purp. Battery Electric Cust. Diesel Cust. CNG Cust. Series Hyb. Cust. Parallel Hyb Figure 14 Specific emissions of carbon dioxide considering only Natural Gas power plants 4 Conclusions and Perspectives The implementation of drivetrain customisation to vehicles describing pre-defined routine activities along their life cycle can be a solution with great energy and environmental benefits. Fuel consumptions and pollutant gaseous emissions of a customised drivetrain vehicle can be reduced in 56% if the drivetrain and the energy source are carefully selected, when compared with Diesel general-purpose vehicles. Although general-purpose vehicles present lower acquisition cost, the reductions referred below, in a life-cycle perspective allow for decreasing running costs due to, reduced fuel consumption [1]. Other economic benefit may be obtained at a near future, with the perspective set by Kyoto Protocol, where companies will pay for their fleets pollutant gaseous emissions, although considering possible higher acquisition costs The results obtained show that Parallel hybrid electric vehicle is the best suited for the proposed application. However, it is expected that, depending from the required drive cycle, other drivetrain configurations will present better performances. 11

12 References [1] S. Faias, J. Esteves, P. Ferrão, J. Guilherme, L. Santos, Evaluation Procedure for Alternative Propulsion Drivetrains Application to a Postal Service Fleet,.EET 24 European Ele-Drive Transportation Conference Urban Sustainable Mobility is Possible Now!, Estoril, Portugal, March 24 [2] T. Markel, A. Brooker, T. Hendricks, V. Johnson, K. Kelly, B. Kramer, M. O Keefe, S. Sprik, K. Wipke, ADVISOR: a systems analysis tool for advanced vehicle modeling, Journal of Power Sources 11 (22) [3] APVE, Associação Portuguesa do Veículo Eléctrico, January 25 [4] U. Gorlach, The relation between efficiency and range of electric vehicles; Elektrot. Zeitschr. A Vol. 94 (1973) p [5] P. Striefler, Electric experimental bus OE 32, Electric Vehicle Study Day, Brussels 1972, paper [6] G. L. Hunt, The Great Battery Search, Spectrum, IEEE, volume 35, Issue:11, November 1998 [7] Mehrdad Ehsani, Khwaja M. Arman, Hamid A. Toliyat, Propulsion System design of Electric and Hybrid Vehicles, IEEE Transactions on Industrial Electronics, Vol. 44, No. 1, February 1997 [8] G. Maggetto, Electric Vehicle Technology: A Worldwide Perspective, Electric Vehicles A Technology Roadmap for the Future (Digest Nº 1998/262), IEE Coloquium on, 5 May 1998 [9] Organization for Economic Co-operation and Development, Electric Vehicles: Technology, Performance and Potential, OECD/IEA, 1993 [1] Rede Eléctrica Nacional, Dados Técnicos 2, January 21 [11] Direcção Geral do Ambiente, Programa Nacional para as Alterações Climáticas, Portugal, 21 Authors Sérgio Faias, Electromechanical Engineer; ISEL-Instuto Superior de Engenharia de Lisboa; CAUTL- Centro de Automática da Universidade Técnica de Lisboa ISEL - Rua Conselheiro Emídio Navarro, Lisboa, Portugal Phone: (+351) ; Fax: (+351) ; sfaias@deea.isel.ipl.pt Sérgio Faias was born in Sesimbra, Portugal on April 22, He received is graduation in Electromechanical Engineering from Escola Superior de Tecnologia de Setúbal, Portugal, in 21. He is Assistant at ISEL from 21. He is also researcher from the CAUTL. He is preparing is MSc Thesis in Mechnical Engineering Energy from Instituto Superior Técnico, in the field of Alternative Vehicle Drivetrains. Jorge Esteves received his graduation, M.Sc. and Ph.D. degrees in Electrical Engineering from Instituto Superior Técnico, Lisboa, Portugal, in 1983, 1986, and 1992, respectively. Since 24, he is Coordinator Professor at the Department of Electrical and Automation Engineering from Instituto Superior de Engenharia de Lisboa. He is also researcher from Centro de Automática da Universidade Técnica de Lisboa and Director of Dispatch and Networks at the Portuguese Energy Regulator Authorithy (ERSE). His main research interests are electrical machines modelling, variable speed electrical drive and generator systems and applications of electric and electronic systems to the transport domain. He is President of the Board of the Portuguese Electric Vehicle Association. Paulo Ferrão is Associate Professor at the Department of Mechanical Engineering of Instituto Superior Técnico, Technical University of Lisbon. Paulo Ferrão is leading a research group on Industrial Ecology at the IN+, Center for Innovation, Technology and Policy Management, and in this context, special attention has been given to the automobile eco-technologies. 12