981122 Development of the Hybrid/Battery ECU for the Toyota Hybrid System Akira Nagasaka, Mitsuhiro Nada, Hidetsugu Hamada, Shu Hiramatsu and Yoshiaki Kikuchi Toyota Motor Corporation Hidetoshi Kato Denso Corporation Copyright © 1998 Society of Automotive Engineers, Inc. ABSTRACT For energy saving and global warming prevention, Toyota has developed Toyota Hybrid System (THS) for mass- produced passenger cars, which achieves drasticimprovement in fuel efficiency and reduction in exhaustemissions compared to conventional gasoline engine cars. The THS has two motive power sources which engage depending on driving conditions. It’s power is suppliedeither from an engine (controlled by the engine ECU) oran electric motor (controlled by the motor ECU) which ispowered by a high-voltage battery (monitored by the bat-tery ECU). These ECUs are controlled by a hybrid ECU.Each ECU has been developed with a fail-safe system inmind, to ensure driver safety in case of vehicle break-downs. Among these ECUs, this paper reports particularly on the newly introduced ECUs: hybrid ECU and battery ECU. In the development of these ECUs, special attention was focused on fail-safe performance. INTRODUCTION Reducing carbon dioxide (CO2) emissions to alleviate global warming has become an international issue in recent years. From an automaker’s point of view, the most important factors in reducing CO2 emissions are improving fuel economy and achieving cleaner exhaustemissions. Toyota has developed a highly advanced hybrid system for powering mass-produced passenger cars. The THS uses a gasoline engine, electric motor, and electric gen-erator to achieve nearly twice the fuel efficiency of con-ventional gasoline engine cars and significantly reduceexhaust emissions. The THS has two motive powersources which engage depending on driving conditions.An improved efficiency gasoline engine provides themain power to drive the wheels, as in conventional auto- mobiles. Drive power can also be supplied by an electricmotor, which derives its electricity from the nickel-metalhydride battery and an electric generator. This paper reports on the development of the hybrid elec- tronic control unit (ECU) , which gives commands to theECUs that directly govern the hybrid drive system, and ofthe battery ECU, which monitors the charging/discharg-ing state of the battery, particularly the development pro-cesses that focus on the fail-safe mechanism. SYSTEM OUTLINE Before we introduce the development processes of thehybrid ECU and the battery ECU, we will explain the con-struction of this hybrid control system and summarize ofits operation. As shown in Fig.1, the THS is driven by both an engine and a motor, with the engine providing the primary power. The system divides the engine’s power with a power splitdevice that uses a planetary gear to send power to boththe drive shaft and the generator. Some of the electricity from the generator drives the motor; the remainder, afterbeing converted to DC by an inverter, is stored in the bat- tery. Figure 1. Toyota Hybrid System Downloaded from SAE International by Stony Brook Univ, Monday, August 13, 2018With an electronically controlled continuously variable transmission and an electric motor that assists the engine, the THS vehicle achieves smooth accelerationand deceleration, as well as excellent response. In partic-ular, initial acceleration takes advantage of the motor’s characteristics — high torque at low speeds. For maxi- mum acceleration when the throttle is fully opened, power from both engine and motor drives the vehicle, with energy from the battery boosting the motor’s output. (Fig. 2) Figure 2. Operating Conditions When starting out, driving at extremely low speeds, going down a moderate slope, or operating in other conditionsin which the engine is not at peak efficiency, the engineshuts down and the motor drives the vehicle. (A) For normal driving conditions the power split device sep- arates the engine power into two path