Lexus RX 400h – Drivetrain Analysis and Test Results

Authors

Univ.-Prof. Dr.-Ing. G. Hohenberg, Prof. Dr. F. Indra, Technical University Darmstadt

Year

2006

Print Info

Fortschritt-Berichte VDI, Reihe 12, Nr. 622

Summary

A comprehensive scientific study of the Lexus RX 400h full hybrid concept was carried out at the University of Technology Darmstadt. Three essential issues of this study are discussed in this paper: The electromechanic transmission E-CVT is the core of the TOYOTA hybrid system. The fast and infinitely variable adjustment of the transmission ratio allows the optimum operation of the combustion engine regarding fuel consumption and exhaust emissions. Depending on the operating point of the E-CVT part of the engine power is directed through the electrical path with relatively poor efficiency (multiple energy transformation). The operation strategy of the complete system "combustion engine – transmission – electric motors – battery" is crucial for the effectiveness of the hybrid system. In the range between 0 and approx. 60 kph the combustion engine is operated cyclically (about 50% combustion engine and 50% electric drive). Due to this it is possible to avoid using the low part load of the combustion engine with the extremely low efficiencies. Special attention was paid to the start/stop-routine. By use of a technically very demanding combination of valve timing adjustment, lowering the intake pressure and controlling the ignition timing this complex routine is successfully mastered with regard to driving comfort and exhaust emissions. For real-world driving three different phases can be distinguished: In the low range up to 60 kph (phase I) there is a significant fuel consumption advantage mainly due to the cyclic operation of the combustion engine. There are still advantages in extra-urban and highway driving between 60 and 150 kph (constant driving) due to the engine optimization as well as the special operating strategy of the complete system. However, above 150 kph constant driving or 125 kph average speed the efficiency of the hybrid system and thus the fuel consumption is distinctly worse than with comparable vehicles with conventional drivetrains. In theory, by further increasing effort and complexity (such as adding a second gearing) the disadvantages of the Lexus RX 400h in the upper speed range can be avoided. As is generally known, correspondent developments are worked on at the moment. The rate of recuperation to lower the fuel consumption is around 4% and therefore significantly below the theoretically achievable value. Due to the battery lifetime the battery state of charge may only be varied inside narrow limits and that the charging current has to be limited as well.

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