Abstract
At present, numerous researches is fulfilled by vehicle manufacturers and supported by different programs at governmental level to improve the efficiency and environmental performances of current vehicles maintaining the actual properties and cost characteristics. Hydraulic hybrid as demonstrated by actual research results is a high power density low cost solution given by the possibility of efficient brake energy recovery. The paper presents the possibility to apply the pneumatic energy storage as prime mover for the solution of hydraulic propulsion. In order to identify the performances and running parameters for a small scale vehicle, a multi-domain model was developed having as main goal the correlation of the pneumatic storage characteristics and the hydraulic structure. A second important goal at the simulation level was to identify the best condition, considering the efficiency point of view, for the pneumatic – hydraulic energy conversion. It was found that pneumatic storage considering energy density is more efficient if it is converted as early as possible in hydraulic energy, because hydraulic loses are less then pneumatic ones. In order to identify the optimum conversion solution, both the rotational and translational options were considered. At the translational solution inertial effects were considered, simulated and adapted in order to improve the efficiency of the pneumatic – hydraulic conversion. This option had as result the hydraulic pump as a hydraulic constant frequency power source. The adaptability of the hydraulic hybrid powertrain to vehicle propulsion was made by an accumulator solution. The accumulator is considered also the solution for brake energy recovery. The recovered hydraulic energy is used both to propel directly the vehicle and to assist the constant frequency hydraulic power source. The simulation results were considered for an experimental model that was tested on an ATV structure. The pneumatic source was used for a translational actuator that propels the hydraulic translational pump. It could be observed that the conclusion observed during simulation was confirmed by experiment, the pneumatic energy is a more efficient storage solution if it runs at constant frequency and if it is early converted into hydraulic one.
Keywords: Pneumatic Storage, Hydraulic, Hybrid-hydraulic Vehicle