Torque converter interactions in a parallel post transmission hybrid driveline

Based on work at NC State University, an initial trial group of 15 plug-in hybrid school buses were manufactured by Navistar Corporation to determine basic performance characteristics and provide a basis to improve upon the design. This project was a joint effort with the IC Bus division of Navistar and the many school districts participating in the program. The buses have been in operation for up to four years as of the writing of this paper. The performance of the buses is better than traditional buses in terms of fuel economy and emissions, but not as favorable as originally predicted in computer simulations. There are a variety of reasons for this discrepancy in the performance, some of which were addressed in subsequent design iterations of the buses, such as decreasing final drive ratios and adjusting the capture of regenerative energy. Some other discrepancies remain unexplained. The design of the buses uses a parallel post transmission hybrid system. This design has been proposed in a number of retrofit situations and therefore has application across a wide variety of platforms. The concern related to this design is the performance of the torque converter in the overrunning condition and the resulting energy losses associated with this case. In this paper, the emphasis is on using separately developed mathematical models to create an adequate simulation of the performance of a torque converter in the negative torque state by using converter geometry without the application of lookup tables, which require specific torque converter testing.