| Based on the work of both NC State University and independent non-profit Advanced Energy, a group of 15 plug-in hybrid school buses were constructed to determine basic performance characteristics and 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 ADVISOR modeling.;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 basic 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 current work looks at the energy lost to drag in the torque converter by extending existing models to a more general case and adding models of the overrunning regime. In this study, a majority of the emphasis is on creating an adequate model of the performance of a torque converter in the negative torque state without the application of lookup tables, which require specific torque converter testing.;An automatic transmission has a hydraulic torque converter with a complex flow that aids the vehicle by multiplying torque at low speed from the combustion engine and facilitates idling of the engine while the vehicle is not moving. From 1960 to 1990, the torque converter was studied and modeled in great detail, but in each case, no modeling was conducted in the overrunning regime, where the speed of the turbine exceeds the speed of the impeller. Throughout the late 1990s and 2000's, several drivelines have been built which use a parallel post transmission hybrid system. There is a high percentage of time where the electric motor is the prime mover of the vehicle and in this case, the speed of the motor will cause the torque converter to overrun. Vehicle computer models such as DOE's ADVISOR and PSAT utilize lookup tables for this purpose. The modeling approach developed here allows the user to generate an accurate model for the torque, speed, flow and losses for the torque converter from two independent performance variables and the geometric parameters of the specific torque converter in question.;The torque converter model is compared in DOE ADVISOR 2002 with a similar torque converter model which uses a lookup table. The performance results for the positive torque state show strong correlation with the results of the lookup table version in both a macro (drive cycle) scale as well as in specific performance results against real world data and short duration events; however, the model shows that the overall energy losses of the converter can be 19 times greater than the traditional lookup table predicts. This is an important new finding that helps explain the losses in the torque converter when the vehicle is being driven by an electric motor downstream of the torque converter. |
