Construction of a Hybrid Electrical Racing Kart as a Student Project

Transcription

1 Construction of a Hybrid Electrical Racing Kart as a Student Project Tobias Knoke, Tobias Schneider, Joachim Böcker Paderborn University Institute of Power Electronics and Electrical Drives Paderborn, Germany Phone: knoke@lea.upb.de URL: Acknowledgements The authors like to thank Lenze AG (Hameln, Germany) and KARTODROME (Paderborn, Germany) for supporting the project. Keywords «Hybrid electric vehicle (HEV)», «Education methodology» Abstract In the student project presented here a hybrid racing kart was built on the basis of a conventional kart. Technical key aspects of the project were the connectional design of the structure, the sizing of the key components and the implementation of a control strategy. Beside technical aspects, basics of project management and teamwork were central topics. After a short introduction the students accomplished all work on their own responsibility. In particular they were responsible for the costs and schedule. 1. Introduction Today, hybrid electric vehicles are accepted as a step toward environmentally compatible vehicles. Due to the high torque of electrical drives, apart from environmental advantages, hybrid propulsion systems offer also potential for enhancement of driving dynamics [1]. These improved driving dynamics can be used e.g. for racing karts [2]. Within the project presented in this contribution such a hybrid racing kart was constructed by students of electrical engineering. Starting point was an existing conventional kart. This was extended to a hybrid electrical kart using standard components, as used e.g. in automation industry. Beside technical aspects, the basics of project management were a central topic. 2. Curriculum of the Student Project The student project was divided into a specification, a design and a project conclusion phase. Contents of the specification phase were the basics of project management, the definition of the project objectives and the requirement specification. In the design phase a functional specification, the schedule and the technical realization were done. To supervise the progress of the project milestones were defined. After finishing the technical realization in the project conclusion phase the achieved project objectives were evaluated based on the requirement and functional specification. At the end of the project the functional hybrid racing kart was presented. The project focus was not the development of new power electronic components or electrical drives, but the integration of existing components to a functional system. After a short introduction to the basics of project management the students accomplished all work on their own responsibility. In particular the students were responsible for the costs and schedule. EPE Aalborg ISBN : P.1

2 3. Specification Phase Since the participating students usually have got few or no experiences in project work, some fundamental terms of project management were introduced at the beginning of the specification phase [3]: - Project organization (pure project team, matrix) - Roles in the project (steering committee, project manager) - Project documents (requirement specification, functional specification) - Planning and scheduling (bar chart, milestones) - Reporting In a next step the objectives of the project were defined by the students. Apart from the technical objectives in particular also objectives for the total costs and the project duration were posed. The most important technical objectives for the hybrid racing kart were: - Maximum speed: 20 km/h - Operating time: 15 min - Range: 10 km - Solely electrical driving possible - Regenerative braking - Automatic starting of the combustion engine - Reverse drive Additionally the following cost and schedule objectives were formulated: - Total costs: 500 EUR (kart, electrical drives, and energy storage were granted or covered otherwise) - Time effort: 450 h - Project duration: 60 d On the basis of the project objectives the students created a requirement specification, which was reviewed and released by a steering committee. With the release of the requirement specification the design phase began. 4. Design Phase The design phase started with the description of the technical realization in a functional specification subject to the requirement specification. For this purpose, first the students designed a suitable structure for the hybrid racing kart. A possible structure, which considers the project objectives, is depicted in Fig. 1 and Fig. 2. EPE Aalborg ISBN : P.2

3 electrical drive (E1) accelerator / brake pedal double layer capacitor electrical drive (E2) chain wheels combustion engine PLC clutch drum brake Fig. 1: Structure of the hybrid racing kart accelerator / brake pedal double layer capacitors inverter electrical drive (E1) PLC drum brake electrical drive (E2) clutch combustion engine (ICE) Fig. 2: Design drawing of the hybrid racing kart The combustion engine can be separated by the clutch from the driving axle, so that solely electrical driving is possible. In addition, reverse driving is possible with opened clutch by the electrical drive E1. The electrical drives E1 and E2 in combination with the double layer capacitor allow recuperation of braking power (regenerative braking). Automatic starting of the combustion engine is possible via the electrical machine E2. For practical realization, the project was divided into three subprojects: 1. Subproject energy storage: Selection, sizing and mounting of the energy storage 2. Subproject propulsion system: Selection, sizing and assembly of the main components of the drive train (electrical drives, combustion engine, clutch, chain wheels) 3. Subproject control strategy: Design and implementation of a control strategy EPE Aalborg ISBN : P.3

4 Energy Storage As energy storage for the hybrid racing kart batteries can be considered as well as double layer capacitors (DLC). Latter were selected because of their high power density and sufficient number of charge/discharge cycles. The layout of the energy storage had to fulfill two requirements. On the one hand the energy capacity had to be considered and on the other hand the DLCs had to provide the required power. The storage power rating arises approximately from the rated power of the electrical drives (s. Table II) as 900 W. The energy capacity of the DLC was sized considering that is possible to accelerate solely electrically from 0-20 km/h. The technical data of the kart listed in Table II, in particular the voltage limits, result in a necessary capacity for the DLC of 12 F. DLC modules which were available for the project have a maximum voltage of 15 V, a capacity of 58 F and a maximum power of 750 W. To comply with the voltage conditions, a series connection of 3 modules is necessary. This results in an energy storage with a maximum voltage of 45 V, a capacity of 19 F and a maximum power of 2250 W. The DLCs are connected to the DC link of the electrical drive inverter without any additional power electronics (e.g. DC/DC converter). To limit the resulting voltage range of the DC link the DLCs are operated in a range of V so that 35% of the available energy capacity is used. To operate the kart in different conditions (s. Table I) and to protect the DLCs from overcharge, the students developed a circuit of relays and resistors shown in Fig. 3. As space on the kart is limited, the resistors can be connected in different combinations, resulting in the required values for resistance and power. With this combination of resistors, it is possible to charge the completely discharged DLC by the electrical drives with limited charging current, provide resistive braking power of about 1 kw and offer the possibility to discharge the DLC after use. An overvoltage protection circuit (OVP) monitors the voltage of the DLC, cutting off the DLC from the DC link in case of failure of the control of the kart. In case of an emergency stop, the supply current of the relays is cut off. The types of the relays (neutral closed/neutral open) were chosen that way that they fall into a safe state, disconnecting the DLC from the drives and connecting the resistors, thus providing electrical braking power. Table I: Discrete states of the relays S1-S5 (O = open, C = closed) Operating condition S1 S2 S3 S4 S5 Emergency stop/overvoltage O C O O C Start of combustion engine O O O O * Charging of discharged DLC O O C O O Normal operation C O O O * Discharging of DLC O O C C O Resistive braking C C O O C EPE Aalborg ISBN : P.4

5 Inverter Fig. 3: Protection circuit Propulsion System Due to their high power density, permanent magnet synchronous motors were used as electrical machines. For safety reasons within this student s project, the system voltage used on the kart must not exceed 60 V. Therefore inverters and drives for low voltage were used. Their rated values are listed in Table II. To adjust the rotational speed between the wheels and the drives chain wheels were used. The necessary ratios arise as a result of the maximum speed of the kart and the speed range of the drives/engine. The used clutch (s. Fig. 1 and Fig. 2) cannot slip, so the driveaway has to be done solely electrically with drive E1. Control Strategy As a result of the structure of the hybrid racing kart depicted in Fig. 1, degrees of freedom arise, which require a control strategy. For hybrid vehicles many control strategies are known ([4], [5], [6], [7]). The control strategy used here (s. Fig. 4) is based on the ''electrical assist'' strategy [4]: - Below 8 km/h the kart drives solely electrically (via E1), the combustion engine is switched off and the clutch is open. - Within the range of 8 14 km/h the kart drives in a combined mode with combustion engine and both drives, the clutch is closed. - Above 14 km/h the kart is solely driven by the combustion engine, the clutch is closed. The maximum speed is limited to 22 km/h - If the state of charge of the DLC falls below a lower threshold, it is reloaded by combustion engine and drive E2. Depending on the speed the clutch is opened or closed. - If the drive power is negative (e.g. while braking), the clutch is open and the brake power is recuperated via the drive E1. EPE Aalborg ISBN : P.5

6 Torque ICE E2 ICE E1 5 E Speed / km/h Fig. 4: Control strategy For the realization of the control strategy a programmable logic controller (PLC) was used. Communication between the drives and the PLC is done via Controller Area Network (CAN) [8]. Table II: Technical data of the hybrid racing kart Maximum speed Tare weight Payload (driver) Air drag coefficient 22 km/h 120 kg 75 kg not considered Rolling resistance coefficient 0.02 Wheel diameter (rear) Combustion engine Electrical drives Inverter System voltage m 2.2 kw, rpm 450 W, 1.6 Nm, 2700 rpm, 30 V, PMSM 620 W, 48 V, 13 A V 5. Results The functional hybrid racing kart is depicted in Fig. 5. The technical project objectives defined during the specification phase were achieved, however costs and schedule were not kept. The total costs of the project were approx. twice as high as originally planned and the duration of the project was extended by 30 %. EPE Aalborg ISBN : P.6

7 Fig 5: Hybrid racing kart The students obtained the following learning outcomes by the project: - Basics of project management and teamwork - System understanding of hybrid electric vehicles - Selection and sizing of electrical energy storages - Integration of an energy storage in the complete system (including safety measures) - Control strategies for hybrid electric vehicles - Basics of field busses, in particular CAN - Programming of an PLC according to IEC To evaluate the project the students were asked for a feedback. The project was evaluated positively, as becomes clear at the following quotations: ''Project was very interesting'', ''great to do some practical work'', ''very informative'', ''learned a lot, in particular about CAN''. Suggestions for improvement were given regarding the used components: ''Electrical drives should have more torque'', ''Develop own inverters instead of using industrial types''. 6. Outlook The inverters used in this project are not able to utilize full potential of the electrical drives. In particular overload operation as well as flux weakening operation is not possible. Therefore the students will develop own inverters in a further project. The focus of this new project will be on the design and implementation of a frequency inverter for permanent magnet synchronous motors. EPE Aalborg ISBN : P.7

8 References [1] T. Franke, H. Glonner, D. Nowak, and F. Osterreicher, Electrified power train - challenges and opportunities for the electrical industry, in Power Electronics and Applications, 2005 European Conference on, Sept. 2005, pp. P.1 P.12. [2] W. D. Jones, Students fast-track hybrid race cars, IEEE - The Institute, vol. 30, No. 3, pp , September [Online]. Available: [3] D. Lock, Project Management Handbook. Gower Technical, [4] L. Guzzella and A. Sciarretta, Vehicle Propulsion Systems: Introduction to Modeling and Optimization. Springer, [5] N. Schouten, M. Salman, and N. Kheir, Fuzzy logic control for parallel hybrid vehicles, Control Systems Technology, IEEE Transactions on, vol. 10, no. 3, pp , May [6] B. K. L. Rahman Z. and E. M., A comparison study between two parallel hybrid control concepts, in SAE 2000 World Congress, March [7] B. Baumann, G. Washington, B. Glenn, and G. Rizzoni, Mechatronic design and control of hybrid electric vehicles, Mechatronics, IEEE/ASME Transactions on, vol. 5, no. 1, pp , March [8] K. Etschberger, Controller-Area-Network. Grundlagen, Protokolle, Bausteine, Anwendungen. Fachbuchverlag Leipzig, EPE Aalborg ISBN : P.8