Target Setting Process for Hybrid Electric Drives Using TPA, Jury Study, and Torque Management

The idea of improved efficiency without compromising the “fun to drive” aspect has renewed the auto industry’s interest toward electrification and hybridization. Electric drives gain from having multiple gear ratios which can use advantageous operating set points thus increasing range. Furthermore, they benefit significantly from frequent decelerations and stopping as is experienced in city driving conditions. To recuperate as much energy as possible, deceleration is done at high torque. This presents an interesting but serious sound quality issue in the form of highly tonal whine harmonics of rapidly changing gears that do not track with vehicle speed thus being objectionable to the vehicle occupants. This paper presents an NVH target setting process for a hybrid electric transmission being integrated into two existing vehicles, one belonging to the premium segment and another aimed at enthusiasts with off-road applications. The demand for power has shifted from mechanical domain into electrical domain, and as such, the solution to electric drive NVH issues also lay partly, in the way these drive systems are calibrated. A time-domain Transfer Path Analysis (TPA) model was developed for both vehicles, by virtually installing the hybrid transmission into the vehicle, thus predicting interior noise in several gears and simulating the brake regen coast downs at varying torque levels. Road and wind noise masking was added to these predictions taking care that the summations were correctly done at same vehicle speeds for which a program was written using a commercially available numerical computing tool. Extensive jury studies were then conducted to determine NVH no-fly zones and the torque management strategy for the two vehicles during brake regen events. Requirements and strategy for the two vehicles were different since they presented different levels of road and wind noise masking. To validate the NVH targets, another jury compared the finalized strategy with a premium target vehicle fitted with a similar hybrid system. This target-setting approach was useful in getting an upfront idea of the NVH risks without any system modifications. It then circumvented the need of re-developing expensive acoustic package and/or gear optimization that would be otherwise needed to mitigate the risks, with co-operation from calibration teams, while still being able to meet their regenerative braking targets in every gear for the two vehicles.