Integrated Powertrain Control for a Mild-Hybrid Urban Vehicle with a Downsized Turbo-Charged CNG Engine

Compressed natural gas (CNG) is considered as one of the most promising alternative fuels for transportation due to its ability to reduce greenhouse gas emissions and its abundance. More specifically, CNG has a considerable potential when used as a dedicated fuel on a downsized turbo-charged SI engine for a small urban vehicle. This approach can be profitably extended by adding a small secondary (electrical) power source to the CNG engine, thus hybridizing the powertrain. This is why IFP has developed a mild-hybrid CNG prototype vehicle based on a MCC smart car equipped with a starter-alternator and ultra-capacitors (the StARS system). This solution offers some interesting functions such as “Stop'n'Start”, regenerative braking, torque boosting and engine assistance, and yields significant benefits in terms of fuel economy, reduced exhaust emissions and better driveability. The unfolding of this project, named VEHGAN, has involved interesting and challenging issues from an engine control point of view, such as coping with distinctive features of natural gas engines, mastering the automatic manual transmission (AMT) control system and integrating a new power source, with its new functions, into the torque-oriented engine control structure. A paper presented at the 2007 SAE World Congress had already given an overview of the project and some preliminary results, mostly focusing on the AMT control part. This paper focuses on the StARS system and on-board energy management strategies, and presents the latest results of the VEHGAN project.