Simulation Study of a Series Hydraulic Hybrid Propulsion System for a Light Truck

The global energy situation, the dependence of the transportation sector on fossil fuels, and a need for rapid response to the global warming challenge, provide a strong impetus for development of fuel efficient vehicle propulsion. The task is particularly challenging in the case of trucks due to severe weight/size constraints. Hybridization is the only approach offering significant breakthroughs in near and mid-term. In particular, the series configuration decouples the engine from the wheels and allows full flexibility in controlling the engine operation, while the hydraulic energy conversion and storage provides exceptional power density and efficiency. The challenge stems from a relatively low energy density of the hydraulic accumulator, and this provides part of the motivation for a simulation-based approach to development of the system power management. The vehicle is based on the HMMWV platform, a 4×4 off-road truck weighing 5112 kg. The development of the component models and system integration in SUMULINK are discussed before addressing the configuration (single propulsion motor or two) and component sizing. The power management is based on a thermostatic state-of-charge (SOC) approach, but the optimum threshold power and SOC for accumulator charging are determined based on detailed system analysis, rather than the conventional wisdom of operating the engine at the “sweet spot”. The results indicate significant advantages of reduced threshold power. Relatively low target SOC led to improved ability to capture the braking energy. Engine shut-downs are considered too. The fuel economy predictions for the optimized hybrid system indicate improvements of 68% (with engine shut-downs) under urban driving conditions, and tangible benefits in highway driving.