A 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 the engine-starter 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 it suffers from poor engine-start performance. However, a parallel HEV with a large starter motor/generator (i.e. P0-P2) provides better engine-start performance at the expense of higher cost. Recently, a P2 HEV with a 12V starter motor (12V+P2) has been considered a balanced solution to this design trade-off. Therefore, this paper systematically evaluates the optimal engine-start performance of three parallel HEVs with various starter motors and discusses 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 the three systems are formulated to assess the optimal engine-start performance. To systematically evaluate and compare the engine-start performance, we define two drivability measures: power deficit and engine-start time. Based on these drivability measures, the evaluation results enable a fair and quantitative comparison of the three parallel HEV architectures under various engine-start conditions, e.g., driver power demand. This novel systematic optimization approach can reduce the complexity of large control problems without sacrificing optimality. The results revealed that 12V+P2 achieves better engine-start performance than P2 only, which is comparable to P0-P2 due to the dedicated engine-starter motor; the engine-start time of 12V+P2 is less than 0.5s. Moreover, 12V+P2 and P0-P2 provide consistently good engine-start performance (low engine-start time and power deficit) across the range of driver power demand. Engine-start performance of P2, however, suffers from a steep trade-off between engine-start time and power deficit. It means that short engine-start time leads to a significant power demand deficit and vice versa. When it comes to the time performance limit ( approximate to 0.2 s) in P2 only system, they must necessarily give up satisfying the power demand. In summary, 12V+P2 can be a good alternative that solves the engine-start issues of P2 type with a reduced cost compared to P0-P2.