ABSTRACT Using a clutch to disconnect and shut-off the engine when engine power is not required, the single-motor strong hybrid has the potential for significant fuel economy improvement with reduced costs and less system complexity. However, it is a challenge for the single-motor strong hybrid to maintain acceptable drivability at engine start since it requires diverting motor torque through a slipping clutch to start the engine. In this study, dynamic simulations of the hybrid transmission driveline with hydraulic and motor controls have been employed to assess the feasibility of the single-motor strong hybrid, to address drivability issues specific to this hybrid architecture at engine start, and to develop control methods to manage driveline disturbances to an acceptable level. INTRODUCTION Hybrid electric vehicles (HEVs) are emerging as important personal transportation options for petroleum displacement and energy diversification, particularly critical in this era of elevated demand for energy security and heightened concern over global warming. Hybrid propulsion systems offer considerable fuel economy improvement, reduced vehicle emissions, and performance enhancement [ 1]. A hybrid propulsion system consists of two power sources, the internal combustion engine and the electric machine. Based on the power flow, hybrid powertrains can be categorized into parallel, series, and power-split configurations. The parallel HEV transmits power from the engine directly to the wheels through the transmission (themechanical path), while the series HEV transmits power from the engine through generating and motoring electric machines to the wheels (the electrical path). The power-split HEV uses power-splitting devices (such as a planetary gearset) to split engine power into mechanical and electrical paths. Tamai et al [ 2] presented the development of an engine stop- start system, and showed that it achieved 12-14% fuel economy improvement in the EPA city cycle (an EPA Composite fuel economy of 7%) with comparable drivability (based on axle torque data) over a production small passenger car. Evans et al investigated the electric machine integration [ 3], powertrain architecture and controls integration [ 4] for the hybrid full-size pickup truck to maximize fuel savings at minimum cost and without sacrificing performance or drivability. Tamai et al [ 5] also demonstrated that a hybrid SUV improved fuel economy by 23% and 19% on the City and Highway cycles and reduced 0-60 mph acceleration time by 1 second, and its hybrid functionalities include engine stop- start, regenerative braking, power assist, intelligent battery charge control, etc. Conlon [ 6] applied a generalized model for electrically variable transmissions (EVT) powerflows to evaluate EVT schemes (input, output, and compound power-splits) and their combinations with regard to fuel economy and performance. It is found that the combination of input split and compound split (2-mode) provides significant advantages over a single mode design including reduced motor power for a given vehicle performance. Modeling and Drivability Assessment of a Single- Motor Strong Hybrid at Engine Start2010-01-1440 Published 05/05/2010 Yongsheng He, Norman K. Bucknor and Anthony L. Smith GM R&D Center Hong Yang GM Powertrain Copyright © 2010 SAE InternationalDownloaded from SAE International by Univ of California Berkeley, Wednesday, August 01, 2018Grewe et al [ 7] illustrated the 2-mode hybrid transmission for full-size, fully utility SUVs that integrated two electro- mechanical power-split operating models with four fixed gear ratios. The addition of fixed gear ratios allows the system to use a lower axle ratio and to select either variable models or fixed gears for the highest fuel economy under widely varying conditions, and to further improve power transmission capacity and efficiency for especially demanding maneuvers such as full acceleration, hill climbing, and towing. Hendrickson