ABSTRACT A cost effective means of achieving fuel economy gains in conventional powertrain is to utilize a 12 volt start/stop (S/S) system to turn the engine off and on during periods of vehicle idle. This paper presents powertrain integration issues specific to a 12 volt S/S system and the powertrain hardware content and calibration strategies required to execute a 12 volt S/S system for start ability, reduced noise and vibration (N&V) and vehicle launch. A correlated lumped parameter modeling methodology is used to determine engine startup profiles, starter hardware and intake cam park position requirements based upon vehicle level response to the startup event. Optimization of the engine startup is reported for a multitude of powertrain configurations, including transverse and longitudinal arrangements with manual, automatic and dual clutch transmissions. Details are provided on the level of hardware and calibration necessary to satisfy a set of prescribed performance metrics, particularly seat track vibration and smooth vehicle launch. INTRODUCTION Automotive hybrid systems with only start/stop functionality at idle typically provide a 4 to 10% improvement in fuel economy over a combined EPA cycle, see [ 1 and 2], and approximately comparable percentage reduction in CO2 emissions, [ 3]. Unlike S/S systems with motor-generators running off the engine accessory drive referred to as belt alternator-starters, BAS or BSA, a 12 volt S/S system utilizes a conventional starter on flexplate/flywheel ring gear with no regeneration benefit. The 12 volt S/S system simply shuts off the engine during periods of zero vehicle speed and restarts the engine once the driver performs a lift foot of the brake (automatics - AT's, automated manual transmissions - AMT's and dual clutch transmissions - DCT's) or presses the clutchpedal on manual transmissions - MT's. The level of performance required in certain markets will require the 12 volt auto start system to be as transparent as possible, meaning imperceptible N&V at driver interfaces (steering wheel, seat track and interior noise) with no compromise to vehicle launch performance for automated transmission applications. N&V CONSIDERATIONS The basic source-path-receiver model for a conventional automotive powertrain startup event is shown in Figure 1. With this basic model, the source is engine torque pulsations from compression and/or combustion. The path is vehicle sensitivity, defined as the ratio of driver seat track acceleration to torque at the powertrain mounts. The receiver is the driver, quantified by the level of peak-to-peak acceleration at the driver's seat. Source level vibrations are produced by the slider crank mechanism of the reciprocating internal combustion engine and are a result of block force reactions to instantaneous cylinder pressures. The engine speed spin-up profile will have a significant influence on the severity of the receiver response at the seat track depending on how the frequency content of the torque pulsations align with vehicle sensitivity. For 12 volt S/S, either source level vibrations originating from the engine can be reduced or path sensitivity can be reduced to produce auto starts that are nearly transparent to the driver and vehicle occupants. At a source level, the engine speed spin profile can be managed through cranking speed and combustion to avoid dwelling at engine speeds that align with high vehicle sensitivity. Peak cylinder pressures, whether motoring or firing, can be reduced since this will lead to a reduction in net block torque acting on the powertrain mounts, see [ 5, 6, 7]. However, a cylinder pressure reduction strategy is complicated by other metrics, Optimizing 12 Volt Start - Stop for Conventional Powertrains2011-01-0699 Published 04/12/2011 Darrell Robinette and Michael Powell General Motors Company Copyright © 2011 SAE International doi:10.4271/2011-01-0699 SAE Int. J. Engines | Volume 4 | Issue 1 837Downloaded from