Hybrid Electric Vehicles, 2nd Edition

6Special Hybrid Vehicles

6.1 Hydraulic Hybrid Vehicles

Although this book is primarily dedicated to issues related to hybrid electric vehicles, it should be appreciated that non‐electric hybrid vehicles are also viable and can sometimes be more beneficial than electric hybrid vehicles. Basically, there are a few reasons for going hybrid in the first place:

• A normal internal combustion engine (ICE) vehicle uses the engine over a wide speed range and hence the efficiency over this range is not the highest achievable efficiency for a particular engine.
• The maximum efficiency of ICE propulsion is very low, on the order of 30%.
• The electric energy storage system, including the battery and electric propulsion motor, has a high efficiency on the order of 80–90%.

Had it not been for the large size and cost, attributed to low energy storage per unit weight or volume capability of the battery, the pure electric vehicle probably would have replaced current vehicular technology. The next option, therefore, is the hybrid electric vehicle, where the ICE can be used to optimally charge the battery, and propulsion can be shared with the electrical method.

So, naturally, the question arises: is there anything else, other than ICE propulsion, available? The answer is that the hydraulic system is one such option. In a hydraulic system, the energy is stored in the form of a compressed fluid in a cylinder or by similar means. To pressurize the fluid needs energy, which comes from the ICE, to activate ...