Optimum Design of Hybrid Powertrain Systems for Non Emission Vehicles 2008-01-0875
Vehicle simulation software was used to analyze different types of vehicle configurations and power component sizing in different settings. This paper will show a method to obtain the optimal ratio, from the point of view of energy efficiency, between primary and auxiliary energy systems output power, depending on the driving cycle. First, the evolution from the conventional vehicle to the fuel cell one was justified, considering intermediate configurations such as conventional engine hybrid and pure electric vehicle. After simulating all these configurations, the fuel cell option was chosen as the optimal one. In the next stage, fuel cell vehicles using different auxiliary energy systems were also analyzed and, finally, the use of Li-ion battery as a secondary energy system was found the most energy efficient configuration. This vehicle configuration was simulated in settings ranging from urban to suburban. The primary and auxiliary energy system output power was tuned in order to achieve maximum energy efficiency at each driving cycle. The “auxiliary energy system output power vs. overall output power” ratio evolution through different cycles was represented. This ratio varies from about 80% in urban cycles to 5% as cycles become less urban. It is concluded that the contribution of the auxiliary energy system vs. primary one is very high in urban cycles due to the energy demand variability although this importance decreases as cycles become less urban since energy demand is almost constant for those cases. The optimal output power selection depending on an specific driving cycle is relevant, as fuel consumption differences up to 2.75 times have been noticed when comparing -in the same driving cycle- a vehicle optimally design for that driving cycle with another vehicle optimized for a different cycle, even though both vehicles use the same components and the only difference is the output power of their energy systems.