INTRODUCTION The eAssist is a key part of GM's overall strategy for powertrain electrification, which includes a wide range of propulsion architectures from light hybrid systems like eAssist all the way to fully electric and fuel cell propulsion system. The role of eAssist in this strategy is to be a low cost, high-value hybrid solution that can be applied to mainstream vehicles around the globe. This type of “light electrification” addresses the need for achieving significant increases in efficiency in the otherwise conventional powertrains that are likely to remain the dominant form of automotive propulsion for many years. The eAssist shares with the original GM 36V Belt Alternator Starter (BAS) mild hybrid system the basic arrangement of a single MG connected to the engine crankshaft through a belt [ 1]. However, eAssist was designed to operate at a higher voltage and deliver much higher electric boost and regenerative braking power than 36V BAS. In what follows, the key requirements, architecture selection, component design, system integration, and development lessons of GM's eAssist system will be described and explained. KEY REQUIREMENTS & ARCHITECTURE The key requirements for the eAssist system were greater fuel efficiency, increased electric boost, lower hybrid system costs, and compatibility with a wide range of existing powertrains and vehicle platforms. The increased fuel efficiency was defined as an increase relative to the conventional vehicle's fuel economy. This was particularly challenging when considering how conventional vehicle fuel economy had evolved as more advanced fuel economy mechanisms were utilized, including some of those used previously only for hybrid vehicles. The increased electric boost was required to not only improve the vehicle's acceleration performance, but also to provide the opportunity for additional fuel savings. Lower hybrid system costs and compatibility with a wide range of powertrains and vehicle platforms also increased the potential for broad market coverage. In addition to the key program requirements for the eAssist system, the technical solution was further constrained by the strategy to use a single MG attached directly to the engine. This helped in optimizing the hybrid system for use with a conventional engine/transmission powertrain with minimal changes. These additional constraints supported the goals of lower hybrid system cost and compatibility with a broad range of platform applications. 2012-01-1039 Published 04/16/2012 Copyright © 2012 SAE International doi:10.4271/2012-01-1039 saealtpow.saejournals.org Development of General Motors' eAssist Powertrain Shawn Hawkins, Fred Billotto, Daniel Cottrell, Alan Houtman, Stephen Poulos, Roger Rademacher , Keith Van Maanen and Daryl Wilson General Motors Company ABSTRACT General Motors' (GM) eAssist powertrain builds upon the knowledge and experience gained from GM's first generation 36Volt Belt-Alternator-Starter (BAS) system introduced on the Saturn VUE Green Line in 2006. Extensive architectural trade studies were conducted to define the eAssist system. The resulting architecture delivers approximately three times the peak electric boost and regenerative braking capability of 36V BAS. Key elements include a water-cooled induction motor/ generator (MG), an accessory drive with a coupled dual tensioner system, air cooled power electronics integrated with a 115V lithium-ion battery pack, a direct-injection 2.4 liter 4-cylinder gasoline engine, and a modified 6-speed automatic transmission. The torque-based control system of the eAssist powertrain was designed to be fully integrated with GM's corporate common electrical and controls architectures, enabling the potential for broad application across GM's global product portfolio. ____________________________________ 308CITATION: Hawkins, S., Billotto, F., Cottrell, D., Houtman, A. et al., "Development of General Motors' eAssist Powertrain," SAE Int. J. Alt. Power. 1(1):20