Energy Efficient Hydraulic Hybrid Drives

Energy efficiency of propulsion systems for cars, trucks and construction machineries has become one of the most important topics in today’s mobile system design, mainly because of increased fuel costs and new regulations about engine emissions, which is needed to save the environment. To meet the increased requirements on higher efficiency and better functionality, components and systems have been developed over the years. For the last ten years the development of hybrid systems can be divided into two major technologies, electric hybrids and hydraulic hybrids. Today, there is a hard competition between these two branches. Therefore, it is interesting to compare these two technologies to figure out their individual capabilities. This paper will discover the advantages of hydraulic hybrid systems over electric hybrids and how to succeed with the hybrid system design. One obvious advantage with hydraulic hybrid systems is that they create a unique opportunity to optimize the engine loading at all speeds. About the power density, hydraulic machines have around 5 times higher density than electric machines. This gives the opportunity to use direct hydraulic drive and remove mechanical gearings needed in an electric hybrid system. This fact has a large impact on the system overall efficiency. By using hydraulic accumulators, a part of the potential load energy can be reused, which make it possible to design a system with improved endurance compared with a similar one without such devices. This kind of regenerative capabilities increase the power supply capability. The additional power can be used to improve the performance of the system, without increasing the primary engine power. Another advantage of using a hydraulic accumulator is that its round-trip efficiency is higher than for an electric battery, especially at frequent acceleration and braking