Dynamic Simulation of Electric Bus Vehicle

Transcription

1 Dynamic Simulation of Electric Bus Vehicle Thanatchai Kulworawanichpong* & Suchart Punpaisarn** *School of Electrical Engineering, Suranaree University of Technology, THAILAND. thanatchai{at}gmail{dot}com **School of Electrical Engineering, Suranaree University of Technology, THAILAND. suchartlek17{at}gmail{dot}com Abstract In this paper, dynamic analysis and simulation of electric bus vehicle are illustrated. They consist of mathematical models integrating with large-scale vehicle movement and calculation of electric traction performances. The electric bus vehicle movement is studied by applying the Newton s second law of motion. Traction force of the electric bus vehicle is supplied from electric motors instead of a diesel engine. This dynamic modeling and simulation can be used to estimate vehicle performance during vehicle running services. To exhibit these analysis and simulation, an electric mini-bus of 1-ton weight is employed for simulation to provide a single-trip service distance of 9.4 km. The bus route used in this work is obtained by acquiring GPS tracking data on-board the bus. As a result, the electric bus vehicle requires energy from a set of on-board batteries of 10. kwh per trip and draws the peak power of 155 kw from the traction motor. Keywords Dynamic Simulation; Electric Bus Vehicle; Newton's Second Law of Motion; Traction Performance; Vehicle Resistance. Abbreviations Global Position System GPS); Suranaree University of Technology (SUT). I. INTRODUCTION Abus transport vehicle uses a conventional internal combustion engine propulsion system. This type of buses normally engages a diesel power-train and is also known as diesel buses. Because of its safety, reliability and efficiency, diesel is the predominant power source for public transit, school and intercity bus services nationwide. Nearly all buses and heavy-duty large trucks use diesel engines because of the massive torque available. The sufficient engine torque is the key to help the vehicle pull huge amounts of weight. Diesel fuel is one of the most efficient and energy dense fuels available today [Corbo, 1998; United States Environmental Protection Agency, 008]. Because it contains more usable energy than gasoline, it delivers better fuel economy. Although diesel fuel is considered more efficient than gasoline, the diesel engine still needs regular maintenance to keep them running. Within the last decade, electric bus vehicles have been introduced and replacing conventional diesel bus vehicles for test in some city bus services. The electric engine, so-called electric motor, does not result in a bus that is simply more environmentally friendly yet of a lower quality, in fact the overall performance is arguably improved. In addition to their main quality of a reduction in air pollution due to the lack of emissions, electric transport has proven itself adept at ascending steep hills, making the electric bus very popular. The electric engine causes far less vibration throughout the vehicle, making for a more comfortable journey for those on board without the rattling often experienced when a bus is at lights or a stop. A reduction in vibration also increases the life and reduces maintenance requirements of the bus, making it a cost-effective option for operators. Although the initial introduction of an electric transport system and fleet can be costly, as a long-term mode of public electric transport buses are surprisingly cost-effective in terms of lifespan and upkeep. One of the most common causes of approval for the electric bus is its lack of noise. The electric buses are noticeably quiet, lowering noise pollution and increasing comfort for those onboard. In addition, regenerative braking demonstrated with the electric bus means that the motor acts as a generator, channelling excess energy back into the battery on the bus. Diesel buses would see this energy expelled as friction during braking, meaning that the electric bus saves around 30% of energy through this difference alone [Chandler et al., 00; Hallmark, 01]. In this paper, the study of traction performance and dynamic simulation of bus vehicle is focused in order to investigate an amount of energy consumption of a single trip of the bus service. This information will help electric bus designers to decide how large of the on-board battery capacity and what the traction motor rating are. So that, this paper organizes a total of five sections. Next section, Section two, illustrates the mathematical model representing the electric bus vehicle movement and traction performance calculation. Section three gives the brief of MATLAB platform which is a software tool to develop simulation codes. Section four presents simulation results and discussion. Conclusion is in Section five. ISSN: Published by The Standard International Journals (The SIJ) 99

2 II. ELECTRIC BUS DYNAMIC SIMULATION The key dynamic variables of bus vehicle movement are position, velocity and acceleration rate. During single vehicle motion, the relationships among these variables are only subject to the straightforward kinematic equation according to the Newton s second law of motion [Henclewood, 007; Mashadi & Crolla, 01]. Let consider figure 1, a bus vehicle is moving up on an incline road surface. This motion can be expressed mathematically by using the free body diagram describing all the forces acting on the bus vehicle as shown in (1). Power electronic inverters are necessary tools for controlling the torque-speed characteristics of the traction motors. Figure -b) illustrates the controlled characteristics of the tractive effort of the electric traction motor [Vu, 01]. a) Diesel Engine Figure 1: Free Body Diagram of the Bus Vehicle Movement FT FR Meff a (1) FR FRR Fgrad Fdrag () F T denotes the tractive effort of the bus vehicle F R denotes the resistance force of the bus vehicle F RR denotes the rolling resistance force of the bus vehicle F drag denotes the aerodynamic drag force of the bus vehicle F grad denotes the gravitational force (gradient force) of the bus vehicle M eff denotes the total mass of the bus vehicle a denotes the bus vehicle acceleration.1. Tractive Effort Tractive effort or tractive force [Repčić, 011] is produced at the tyre-road interface. It is caused by the applied torque from traction motors to the wheel axis slip in the contact area. It notes that F T F max, otherwise the wheel slipping rather than spinning. The bus vehicle accelerates through the application of tractive forces. Diesel engine and electric motor are the propulsion system to generate the traction forces for diesel and electric vehicles, respectively. Diesel engine typically works at larger torques and low speeds. In a certain gear, the tractive force is considerably degraded with a wide range of the operating speed. It is clear that the tractive force at each gear varies with vehicle speed. At low gear, the tractive force is greater and produces greater acceleration. The non-smooth tractive effort of a diesel engine resulting from all gears is shown in figure -a). AC motors, induction or permanent magnet synchronous, are generally used as traction motors for medium-duty and heavy-duty electric vehicles. One of the inherent properties of electric motors is the production of torque at zero speed. b) Electric Motor Figure : Tractive Effort of the Bus Vehicle [Soylu, 011] (3) is used to simplify the tractive force for use in association with the vehicle movement. T m m F T (3) v F T is the tractive effort T is the overall power transmission system efficiency m is the torque produced by the traction motor m is the rotational speed of the traction motor s shaft v is the longitudinal speed of the bus vehicle.. Resistance Force There are resistive forces, F R, opposing of the vehicle motion [Duysinx, 01]. The resistive forces can be categorized into three types: frictional forces, commonly called rolling resistance, air resistance or dynamic drag forces and gravitational or gradient forces. ISSN: Published by The Standard International Journals (The SIJ) 100

3 The rolling resistance is the resistance to motion of rotating parts. It can be categorized into two main resistances: i) frictional torques (bearing torques, gear teeth friction, brake pads) and ii) tyre deformation. (4) is the mathematical representation of the rolling resistance. FRR frw f0 f1v W (4) W is the wheel load f R is the rolling resistance coefficient f 0 and f 1 are two constants (for simplification, f 1 is neglected) f 0 = (truck vehicle running on asphalt or concrete road).3. Aerodynamic Drag Forces The motion of a vehicle is taking place in the air and the force exerted by air on the vehicle will influence the motion. The aerodynamic resistance force results from three basic effects: i) the pressure different in front and behind the vehicle due to the separation of the air flow and the vortex creation behind the vehicle, ii) skin friction representing the surface roughness of the vehicle body and iii) internal flow of air entering the internal parts of the vehicle [Hucho, 1998; Kalm, 007]. It is common to express the aerodynamic resistance force in the basic form as (5). 1 Fdrag aircd AF vair (5) air is air density (kg/m 3 ) C d is an aerodynamic drag coefficient A F is the projected frontal area of the vehicle v air is the speed of air relative to the vehicle body.4. Gravitational or Gradient Forces The gravitational force [Tautkus, 011] on a slope will act in opposite direction for uphill and downhill motion of the vehicle. The positive and negative signs are for the downhill and uphill motions respectively. The gravitational force is a constant force as long as the slope is constant. (6) is the mathematical representation of the gradient force. F M g sin (6) grad M eff is the vehicle mass (kg) g is the gravitational constant (9.81 m/s ) is the slope angle.5. Equation of Motion eff The relation between velocity and time is a simple one during constantly accelerated, straight-line motion. Constant acceleration implies a uniform rate of change in the velocity. The longer the acceleration, the greater the change in velocity. Change in velocity is directly proportional to time when acceleration is constant. If an object already started with a certain velocity, then its new velocity would be the old velocity plus this change. This is the easiest of the three equations to derive formally. Start from the definition of acceleration, expand the Δv term, and solve for v as a function of t. The first equation, (7), is the relation between velocity and time. The displacement of a moving object is directly proportional to both velocity and time. Acceleration compounds this simple situation. Displacement is directly proportional to time and directly proportional to velocity, which is directly proportional to time. Time is a factor twice, making displacement proportional to the square of time. The second equation of motion, (8), is the relation of displacement and time. Combining the above two equations gives rise to a third, (9). Therefore, displacement is proportional to velocity squared when acceleration is constant. x t t x t v t t 1 a t (7) v t t v t a x t t x t (8) t is the time step x(t) is the displacement at time t v(t) is the velocity or speed at time t a is the acceleration of motion START Load system input parameters Initialize all variables e.g. t = 0 Calculate kinetic variables: speed, displacement, acceleration Calculate resistance forces: rolling resistance, drag, gradient forces Calculate tractive force, input power, energy consumption Store data t > Tstop END t = t + t Figure 3: Structure of Simulation Program III. SIMULATION MODEL AND PROGRAMMING STRUCTURE The bus vehicle movement simulator presented in this paper is simplistic. The power propulsion system is assumed to be a set of inverters, battery storage systems and traction motors. However, it neglects any complicated models of these equipment. The battery is represented by an ideal energy source. The efficiency of the on-board inverter and the ISSN: Published by The Standard International Journals (The SIJ) 101

4 Road Gradient Profile (m: SSL) The SIJ Transactions on Computer Science Engineering & its Applications (CSEA), Vol., No. 3, May 014 traction motors for this application is normally high. Thus, all these can be represented by a single parameter of the overall power-conversion efficiency. The simulation program structure can be summarized as described in figure 3 [Punpaisarn & Kulworawanichpong, 014]. In this study, existing buses are considered as test vehicles. It assumes that a GPS tracking device is equipped on-board bus vehicles [Fuse & Shimizu, 000]. The speedtime and road altitude curves are vital components to conduct this simulation. An example of the vehicle speed trajectory and road altitude is shown in figure 4. Figure 4: Example of GPS Tracking Data IV. RESULTS AND DISCUSSION Figure 6: GPS Tracking Module and its Tracking Software In this paper, study of a single trip bus vehicle service is carried out. The campus bus service in Suranaree University of Technology, Nakhon Ratchasima, Thailand, is selected for this test as shown in figure 5. The GPS tracking module used in this work is a skylab GPS module SKM55 with high sensitivity of -165 dbm. The GPS tracking device and its tarcking interface via google map can be shown in figure 6. The bus route is 9.4 km long as described in figure 7-a). The speed trajectory and road altitude of the bus vehicle service acquired from the GPS tracking device are given in figure 8 and figure 7-b), respectively. The test was performed on an Intel, Core Duo,.4 GHz, 3.0 GB RAM with MATLAB software. a) Bus Route Figure 5: SUT s Campus Bus Service Distance (km) b) Road Altitude Figure 7: Test Route in SUT Campus ISSN: Published by The Standard International Journals (The SIJ) 10

5 Electric traction power (kw) Tractive force (kn) Bus speed (km/h) The SIJ Transactions on Computer Science Engineering & its Applications (CSEA), Vol., No. 3, May time (s) Figure 8: Speed Trajectory of the Bus Vehicle under Study The simulation shows that the total time of the test bus service is 7 minutes approximately. The maximum tractive effort is about 14.3 kn as shown in figure 9 for the simulated traction force curve. The maximum power drawn from the battery is about 155 kw as described in figure 10. This pins out the limitation of the peak of the traction motor used for this electric bus vehicle design. The total energy consumption of the battery for one trip service is about 10. kwh. This value can be used to estimate the energy capacity of the onboard battery time (s) Figure 9: Simulated Traction Force of the Bus Vehicle time (s) Figure 10: Simulated Power Supplied by the On-Board Battery of the Bus Vehicle V. CONCLUSION This paper describes analysis and simulation of electric bus vehicle movement and traction performance calculation. The purpose of this simulation is to obtain necessary information for evaluating capacity of on-board battery and traction motors. This study is based on the Newton s second law of motion in which three main resistance forces (rolling resistance, aerodynamic drag resistance and gravitational resistance) are taken into account. From the calculation, tractive force created by the traction motors, power and energy supplied by the on-board battery are obtained. This information can be used accordingly to design motor and battery sizing of the electric bus vehicles. ACKNOWLEDGEMENTS This work was financially supported by Provincial Electric Authority of Thailand (Grant ID: B ) to Thanatchai Kulworawanichpong. REFERENCES [1] P. Corbo (1998), Lean Burn Natural Gas Engines as a Possible Power Unit in Urban Fleets of Heavy Duty Vehicles with Low Environmental Impact, International Journal of Vehicle Design, Vol. 0, Pp [] W.H. Hucho (1998), Aerodynamics of Road Vehicles, SAE International. [3] T. Fuse & E. Shimizu (000), A New Technique for Vehicle Tracking on the Assumption of Stratospheric Platforms, International Archives of Photogrammetry and Remote Sensing, Vol. XXXIII, Part B5, Amsterdam. [4] K. Chandler, K. Walkowicz & L. Eudy (00), New York City Transit Diesel Hybrid-Electric Buses: Final Results, DOE/NREL Transit Bus Evaluation Project. [5] D.A. Henclewood (007), The Development of a Dynamic- Interactive-Vehicle Model for Modeling Traffic Beyond the Microscopic Level, University of Massachusetts-Amherst, USA. [6] E. Kalm (007), Design of an Aerodynamic Green Car, Lulea University of Technology. [7] United States Environmental Protection Agency (008), Vehicle and Engine Compliance Activities, Progress Report. [8] N. Repčić (011), Tractive Effort Curve in Gearbox Analyze, 15th International Research/Expert Conference Trends in the Development of Machinery and Associated Technology (TMT 011), Prague, Czech Republic. [9] S. Soylu (011), Electric Vehicles - Modelling and Simulations, InTech. [10] A. Tautkus (011), Longitudinal and Lateral Dynamics, Powering the Future with Zero Emission and Human Powered Vehicles Terrassa, Erasmus LLP Intensive Programme. [11] S.L. Hallmark (01), Assessing the Costs for Hybrid versus Regular Transit Buses, Tech Brief, Center for Transportation Research and Education, Iowa State University. [1] P. Duysinx (01), Performance of Vehicles, Research Center in Sustainable Automotive Technologies, University of Liege. [13] B. Mashadi & D. Crolla (01), Vehicle Power Train Systems: Integration and Optimization, Wiley. ISSN: Published by The Standard International Journals (The SIJ) 103

6 [14] T.M. Vu (01), Vehicle Steering Dynamic Calculation and Simulation, Annals of DAAAM for 01 & Proceedings of the 3rd International DAAAM Symposium, Vol. 3, No. 1. [15] S. Punpaisarn & T. Kulworawanichpong (014), Traction Performance and Electric Bus Vehicle Dynamic Simulation, International Symposium on Fundamental and Applied Sciences, 8 30 March 014, Tokyo, Japan. Thanatchai Kulworawanichpong. He is an associate professor of the School of Electrical Engineering, Institute of Engineering, Suranaree University of Technology, Nakhon Ratchasima, THAILAND. He received B.Eng. with first-class honour in Electrical Engineering from Suranaree University of Technology, Thailand (1997), M.Eng. in Electrical Engineering from Chulalongkorn University, Thailand (1999), and Ph.D. in Electronic and Electrical Engineering from the University of Birmingham, United Kingdom (003). His fields of research interest include a broad range of electrical power systems, railway electrification, traction system and electric vehicle, power electronic, electrical drives and control, optimization and artificial intelligent techniques. He has joined the school since June 1998 and is currently a leader in Electric Transportation Research and Electrical Power System, Suranaree University of Technology, to supervise and co-supervise over 15 postgraduate students. Suchart Punpaisarn. He received B.Eng. in Electronics Engineering from Vongchavalitkul University, Nakhon Ratchasima, Thailand (1998) and M.Eng. in Electrical Engineering from Kasetsart University, Bangkok, Thailand (005). He worked as an Assistant Chief Engineer, JVC (Components) Thailand Company, Nakhon Ratchasima, Thailand ( ) - Level IV Engineer and a Senior Engineer, Seagate Technology (Thailand) Ltd., Nakhon Rachasima, Thailand ( ) Level V Engineer. Currently he is a Ph.D. student conducting his research in electric bus vehicle analysis, simulation and design. His research interests include control system, process and industrial control and instrument, electronic and digital circuit design, robotics, signal and image processing, artificial intelligence and optimization. ISSN: Published by The Standard International Journals (The SIJ) 104