Controls Development for Clutch-Assisted Engine Starts in a Parallel Hybrid Electric Vehicle

In a parallel hybrid electric vehicle, higher fuel economy gains are typically achieved if significant electric drive (or engine-off) operation is possible, shifting the engine operating schedule so that it only runs at medium to high load for best efficiency. To enable efficient engine-off driving, a typical configuration will have a disconnect clutch between the engine and the rest of the driveline. In some configurations, when engine-on operation is requested the disconnect clutch is applied in conjunction with the traction motor/generator to crank the engine (i.e., a flying engine start). In this paper we describe the development of a control system for a flying engine start using an engine disconnect clutch. The clutch is located between the engine and electric motor, which is connected to the input of a multispeed transmission. We first describe an initial control algorithm evaluation using a driveline model. After this initial evaluation, the control algorithm was incorporated into an existing hybrid vehicle control system architecture and tested on a HIL bench to validate code implementation. Finally, we describe experiments in which we tested the control algorithm in a test cell using a mule transmission. We discuss in qualitative terms the refinements in the control algorithm resulting from the experimental results.