The overall efficiency of hybrid electric vehicles largely depends on the design and application of its energy management system (EMS). Despite the load coordination when operating the system in a hybrid mode, the EMS accounts for state changes between the different driving modes. Whether a transition between pure electric driving and internal combustion engine (ICE) powered driving is beneficial depends, among others, on the respective operation point, the route ahead as well as on the energetic expense for the engine start itself. The latter results from a complex interaction of the powertrain components and has a tremendous impact on the efficiency and quality of EMSs. Optimization based methods such as dynamic programming serve as benchmark for the design process of rule based control strategies. In case no energetic expenses are assigned to a state change, the resulting EMS suffers from being sub-optimal regarding the fuel consumption. However, an exact determination of such engine start costs has hardly been researched, yet leads to a more efficient EMS.
This paper focuses on the experimental evaluation of the energetic expense for engine starts in (plug-in) hybrid electric vehicles (P)HEVs under various conditions and the assessment of its impact on optimal EMSs. First, reproducible engine starts are performed on both a chassis dynamometer and a transient engine test bench for a wide speed and load range. The experiments are carried out for two plug-in hybrid test vehicles in a parallel driving configuration equipped with a 4- and 6-cylinder spark ignited (SI) engine. Subsequently, the results are discussed and broken down to the respective influence of individual powertrain components. Second, the start costs are embedded into a dynamic programming approach for EMSs as a function of the evaluated influencing parameters. The impact of the engine start costs is outlined by analyzing the optimal EMSs for a representative driving route.