Comparative study on optimal engine-start performance of parallel hybrid electric vehicles with various starter motors

Abstract

Pre-transmission parallel hybrid electric vehicles (HEVs) have various architectures depending on the type and size of an engine-start motor. Such architectural variations pose a design trade-off between cost and engine-start performance. For instance, a parallel HEV (P2) without a starter motor is cost-effective, but suffers from poor engine-start performance, and a parallel HEV with a large starter motor/generator (a.k.a P0-P2) provides much better engine-start performance at the expense of higher cost. Recently, a P2 HEV with a 12V starter motor (12V+P2) is also being considered as a balanced solution to this design trade-off. In this paper, we systematically evaluate the optimal engine-start performance of three parallel HEVs with various starter motors and discuss how each architecture compares to one another in terms of engine-start performance. First, three parallel HEV powertrain models are developed to characterize the transient behavior during engine-starts. Then, optimal control problems of three systems are formulated and solved to find the optimal engine-start performance. In order to systematically evaluate and compare the engine-start performance, two drivability measures are defined

power demand deficit and engine-start time. The evaluation results based on these drivability measures enable a fair and quantitative comparison between three parallel HEV architectures under various engine-start conditions, e.g. power demand. Comparative study shows that engine-start time of a P2 HEV without a starter motor can be similar to that of P0-P2. However, in this case, the power demand cannot be satisfied in P2 only system. In addition, 12V+P2 shows the same traction performance as P0-P2 and shows an acceptable engine-start time performance. Therefore, if the engine which has high non-linearity can be accurately controlled, it can be a balanced architecture that reduces cost and provides good performance as well.