Dynamic modeling and simulation of transmotor based series parallel

Editor's Notes

• #2: Abstract— Pollution and limited fossil fuels are the critical issues that lead HEVs to emerge. Nowadays new designs and topologies of HEVs are suggesting and developing. In this paper we have dynamically modeled and simulated a new structure of series-parallel HEV based on a special machine named Transmotor and applied it to Toyota Prius 2004. In This structure, generator and planetary gear in common series-parallel HEV structure are replaced with a Transmotor resulting in a simpler HEV structure. Transmotor performs as a speed coupler between ICE and wheels. Performance of this structure has been compared with those given in U.S. DOE reports for Toyota Prius 2004. Keywords; Transmotor; Hybrid Electric Vehicle (HEV); Series-Parallel; Simulation;
• #4: In recent decades dozens of papers about HEVs have been published. The main goal in HEV design is to increase the efficiency of ICE and use the wasted part of energy as much as possible. One of the first industrialized HEVs was Toyota Prius and after that various HEVs have been presented and even industrialized from Toyota Corporation and other vehicle manufactures. Toyota Prius has series-parallel topology. In normal series-parallel HEV, there is a planetary gear which provides speed coupling between Internal Combustion Engine (ICE) and vehicle wheels. The planetary gear is a mechanical device and has some drawbacks like decreasing vehicle efficiency and also increasing the car’s overall cost. In this paper we simulate a new structure of series-parallel HEV, which has no planetary gear and a Transmotor will be replaced with planetary gear and generator. The idea is based on [1] and here in this paper we simulate it in MATLAB Simulink and compare the results with US Department of Energy (DOE) report [2].
• #5: A Transmotor Static Model: Transmotor is an electric motor with a floating stator which forms a double-rotor machine with stator as outer rotor and traditional rotor as inner rotor. Through the air gap, electric power converts into mechanical power as shown in Figure 1.The stator electrical frequency in Transmotor is the relative speed of rotor (as inner rotor) to stator(as outer rotor). According to the action and reaction effects, in steady state rotor and stator torques are the same. The speed relationship can be expressed as    ωr and ωs are outer and inner rotor speeds respectively and ωe is the inner rotor (rotor) relative speed to the outer rotor (stator) and it is indeed the electrical frequency of stator windings. In steady state the torque relationship can be expressed as   Transmotor used as a speed coupler [1]
• #6: Dynamic Model: For simulation purposes, dynamic model of Transmotor have been used. The dynamic model of Transmotor is same as common synchronous PM machine but it has one extra mechanical equation for stator rotation. Equations (3)-(9) show this model.
• #7: B ICE Model The ICE model is based on Toyota Prius 2004[3]. The model uses a look-up table for its speed-torque characteristic and according to its throttle and speed the output torque will be defined. The look-up table is presented in Figure 2[3].
• #8: C Energy Management This is the heart of the vehicle, subsystem which most of the decisions are made in it. Energy management decides which part should provide torque and how much torque should be provided. In braking mode it decides whether the motor could provide the whole negative torque and act as a generator or mechanical brake should be used. Most of the decisions are based on SOC and speed of the vehicle.
• #9: Topology of Transmotor Based HEV The topology we have used in this paper is shown in Figure 3 [1]. The Engine power through Transmotor transfers to wheels and combined with traction motor, provides the moving force. The Transmotor acts as a speed coupler and the gears which connect it to differential, act as a torque coupler. The whole system acts as a torque-speed coupler which decouples simultaneous speed and torque of engine from required speed and torque on wheels. Therefore this topology results in a series-parallel vehicle [1].   New topology of series-parallel HEVs with Transmotor
• #10: Operating Modes This vehicle operates in several modes. We describe them through a start - stop drive cycle. In starting and low speeds, only traction motor provides propelling torque. In this mode the engine is off and Transmotor provides no electric energy. When vehicle reaches a specific speed (20 km/h), or when the battery’s state of charge (SOC) comes down to a specific amount (50 percent), the engine starts and Transmotor acts as a speed coupler. It transfers engine power to differential. In this mode whenever the SOC of battery gets lowered the Transmotor recharges it. In braking mode, traction motor acts as a generator and recharges the battery. If braking needs more negative torque and the traction motor could not provide it, a mechanical brake will provide the rest and act as an emergency brake.
• #11: Simulink model The Simulink model has been shown in Figure 4. This model is based on MATLAB HEV model [4]. It consists of six main blocks; Energy management, Battery and DC-DC converter, ICE, Transmotor, Motor and Vehicle Dynamics. Specifications for this model are based on DOE report of Toyota Prius 2004 and a brief list of them has been presented in TABLE I~III [2]. Simulink model for series parallel HEV with Transmotor
• #12: Specifications for this model are based on DOE report of Toyota Prius 2004 and a brief list of them has been presented in TABLEs [2].
• #13: Simulation results To verify this model and simulation results, we have used US DOE report for Toyota Prius 2004 as a reference. In the Figure the Drive power (a), ICE generated power (b), generator and battery output power (c) and hybrid indicator (d) has been shown. During time intervals that drive power gets negative values, the ICE will produce no power. In braking mode, the battery gets the maximum acceptable power through traction motor (which acts as a generator) and the rest will be wasted through mechanical brake.
• #14: Simulation results Battery SOC and the charging process in braking mode have been showed here.
• #15: Simulation results Here in the Figure, the model output of vehicle speed is compared to related figure of DOE report. The digitized one is DOE result and the smooth one is our model result. The result shows the accuracy of the Simulink model due to DOE data.
• #16: Conclusion In this paper we have dynamically modeled and simulated a new series-parallel HEV topology, based on a special machine named Transmotor in MATLAB Simulink. Applying the new structure to the parameters of Toyota Prius 2004, its performance indices are then compared with those given in DOE report for Toyota Prius 2004. Compared to the real Prius 2004 results published in DOE report, the results are similar and topology could be treated as a replacement for generator and planetary gear to enhance the vehicle performance and simplicity. The main drawback of the proposed topology, when compared with the usual generator based topology, is a few percents larger variation in battery SOC.
• #17: References M. Ehsani, Y. Gao and A. Emadi “Modern Electric, Hybrid Electric, and Fuel Cell Vehicles: Fundamentals, Theory, and Design” SE, CRC Press, 2010, pp. 141-150. U. S. Dept. Energy, “Evaluation of 2004 Toyota Prius Hybrid Electric Drive System”Tech. Rep. ORNL/TM- 2006/423, May 2006. [Online] Available at: http://inspire.ornl.gov/OriginalDocument/f38948a5-d6a2- 4f80-8cc9-34b77e3862a3 K. Muta, M. Yamazaki and J.Tokieda, “Development of New-Generation Hybrid System THS II – Drastic improvement of Power Performance and Fuel Economy” Tech. paper series, 2004-01-0064, Detroit, MI, March 8-11, 2004 MATLAB Simulink Software,” Hybrid Electric Vehicle model” Version 2008 and 2009