Hybrid-Hydraulic Engine Technologies


Many engine manufacturers have implemented hybrid-hydraulic technology into multiple vehicle product lines. These engines are found in a variety of applications including light and medium duty utility, telecommunications and municipality vehicles. Hybrid-hydraulic technologies are also found in refuse haulers, sports utility vehicles, family automobiles, delivery trucks, and military vehicles. Finally, this technology is becoming increasing popular in buses, including city transit, shuttle and school buses, due to their frequent braking and low speed travel.

Hybrid hydraulic vehicles, HHV, use pressurized fluid as an alternative power source along with an internal combustion engine (ICE), instead of electric power as in electric hybrids. The main difference between hybrid hydraulics and electric hybrids is hybrid hydraulics recover the kinetic energy of a vehicle while braking and decelerating much more efficiently than an electric system. This process captures and reuses over 70% of the energy normally wasted during breaking and reduces wear on the friction breaks. The engine of a hybrid hydraulic is activated by a hybrid controller when needed, and as a result, engine usage is practically cut in half in stop-and-go city driving. A hybrid controller can command the engine of a hybrid hydraulic system, allowing for optimal vehicle fuel economy and the ability to operate at its maximum efficiency.[1]

There are two types of hybrid hydraulics: parallel hydraulics and series hydraulics. In the parallel hydraulic hybrid, there is a conventional engine and drive train system with hydraulic hybrid technology attached to the drive shaft. In a series hydraulic hybrid, the engine is not directly connected to the wheels. The ICE is only connected to a pump, and is made to operate its most efficient power range to maintain optimal hydraulic pressure in the accumulator.

In most cases, hybrid hydraulic systems tend to be more cost-effective than electrical hybrid systems because there are no expensive materials used, such as batteries. The hydraulic powertrain presence is intended to achieve better fuel economy than that of a conventional gasoline vehicle.

HHVs can perform not only as well as conventional vehicles, but also can be made to provide superior performance. The low cost to manufacture combined with reduced brake maintenance and dramatically increased fuel efficiency results in thousands of dollars saved over the lifetime of the vehicle.

[1] EPA, Hydraulic Hybrid Research, http://www.epa.gov/otaq/technology/research/research-hhvs.htm, (May 22, 2013).