Study on the feasibility of a hydraulic-electric hybrid powertrain for a passenger vehicle

Abstract

Fluctuations in fuel prices, concerns over global warming and the environmentally conscious movement in general have led to the development and implementation of several alternative vehicular powertrains. Amongst them, electric vehicles (EVs) and electrical + internal combustion engine (ICE) hybrid vehicles (HEVs) have become particularly prominent. Electric vehicles have several advantages, like producing zero emissions at the local level, having higher well-to-wheel efficiencies, producing less absolute pollution under most possible electrical generation fuel combinations. However, despite having all these benefits, the high cost and low power and energy density of batteries has held the full scale deployment of EVs back. Additionally, since batteries can only be charged using voltages above their discharge potential and at relatively low charging rates, combined with the limitations of real life DC-DC converters, electric vehicles can only regenerate a relatively small portion of the energy available from braking, particularly under low speed urban driving. Batteries also have short lifetimes and require special care in order to prolong them. To solve these problems, hybrid rechargeable energy storage systems (H-RESS) have been proposed, most prominently a combination of a battery and a super capacitor bank. However, super capacitors are expensive and do not maintain a constant voltage during charge or discharge, so voltage converters are also necessary. Another alternative to overcome these limitations could be the construction of an electric-hydraulic hybrid powertrain. Hydraulic components are naturally power dense, efficient, very reliable and comparatively low cost. They also require no exotic materials or particularly advanced manufacturing technologies. ICE-Hydraulic powertrains have existed in heavy construction vehicles for many years and have been successfully applied to freight and passenger vehicles successfully in the past. However, electro...