Control of Engine Starts for Optimal Drivability of Parallel Hybrid Electric Vehicles

Abstract

The pretransmission parallel hybrid electric vehicle (HEV) with a single electric motor requires relatively little changes from existing powertrain configurations. This configuration, however, has a challenging drivability issue during enginestarts because the electric motor must simultaneously provide the demanded propulsion torque and start the engine. Depending on the propulsion power level, such enginestart process may require a tradeoff between drivability and quick start. The goal of this study is to find theoretical performance limits and corresponding optimal control strategies that achieve the balance between these two conflicting goals. We first develop a simplified parallel HEV powertrain model to predict the engine, clutch, and vehicle dynamics. Assuming that the clutch torque can be accurately estimated and perfectly cancelled, an optimal enginestart control problem is formulated to minimize enginestart time while supplying the driver demanded torque. This nonlinear constrained optimal control problem is solved both analytically and numerically. For some special cases, the optimization problem can be solved analytically to obtain a closedform solution. For the numerical method, dynamic programming (DP) is used, and both analytical and numerical solutions show that selecting a proper level of constant clutch pressure is the key to achieve nearoptimal drivability performance. Furthermore, the DP control policy is found to be timeinvariant, and thus can be implemented in the form of a full state feedback controller.