Hydraulic Hybrid Powertrains

Hydraulic Hybrid Powertrains: Once competitive with electric hybrids, now relegated to niche applications

Well into the early 2000’s hydraulic and electric hybrid powertrains were in direct competition in the heavy on-highway vehicle sector. Hydraulic hybrids had a distinct advantage over electric hybrids for urban stop and go driving with the power density of hydraulic accumulators far exceeding the power density of battery technology at the time. This enabled hydraulic hybrids to more efficiently recover kinetic energy from heavy vehicle braking, and then use that stored energy to assist in accelerating the vehicle during takeoff.
In the subsequent decades, battery technology has made great strides in cell chemistry enabling even heavy vehicles such as city busses and refuse trucks to recover and reuse virtually all the kinetic energy available during aggressive stop and go operation. Given the rapid technological advancement in electric hybrids Czero pivoted from early work in the hydraulic hybrid sector to electric hybrids while transferring and leveraging much of the experience gained in including powertrain modeling, control, and power management.

Hydraulic hybrids for on-highway vehicles

Czero engineers started developing hydraulic hybrid powertrains in the early 1990’s with multiple Czero personnel having industrial and academic experience in on-road and off-highway hybridization. One of Czero’s early products was a drop-in hydraulic hybrid conversion kit developed and demonstrated over a 6-month timeframe. This work came just as electric hybrid technology was rapidly improving and this product was soon discontinued to focus on supporting vehicle electrification.

Czero U.S. patent 8622859
Czero's parallel hydraulic hybrid powertrain

Hydraulic hybrids for off-highway and niche applications

While electric hybrids have completely supplanted hydraulic hybrids in almost all on-road applications, there are still a subset of applications where hydraulic hybrids offer significant benefits. These niche applications are typically found in machines which already have a significant amount of hydraulic equipment in place. Hydraulic hybrids enable recovery of kinetic and potential energy which would otherwise be wasted by storing the energy as pressure in a hydraulic accumulator. This stored energy can then be utilized by the working hydraulics to reduce overall fuel consumption. Examples of applications where Czero personnel have worked on hydraulic hybrid machines include side loader refuse trucks, reach stackers, subsea equipment, agricultural equipment, and forestry equipment among others.

      Hydraulic hybrid reach stacker circuit
      Reach stacker

      Hydraulic to electric hybrids

      Although hydraulic hybrids for on-road applications are no longer under development, much of Czero’s expertise in this area has been transferred to assisting clients with powertrain electrification. Example projects include trade studies for the next generation of heavy tracked combat vehicles, trade studies on electric hybrid aircraft powertrains, control and power management of forestry and mining equipment, and fast charging among many others. 

        Tracked vehicle powertrain trade study
        Electric aircraft powertrain trade study

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