By definition, a hybrid vehicle draws on more than one source of energy. The most common and best-known type is the Hybrid Electric Vehicle (HEV), which uses batteries as secondary energy storage, charged by the kinetic energy normally lost during braking, in a process called regenerative braking.
Lesser-known Hydraulic Hybrid Vehicles (HHV) use a hydraulic motor/pump combination connected to the driveline as the secondary power source. HHVs capture kinetic energy by pumping fluid from a low-pressure tank into high-pressure accumulators.
While HEVs have taken off in the passenger-car market, HHVs are increasingly recognised as the fuel-saving alternative of choice in large, slow, stop-start vehicles, from garbage trucks to buses and even excavators.
Hydraulic technology is nothing new. Hydraulic excavators, for example, use a fossil-fuelled engine to drive hydraulic pumps that power accessories such as the swing motor. By comparison, hydraulic hybrid excavators improve efficiency by capturing the energy lost during swing braking, ‘storing’ the energy in high-pressure accumulators, then releasing it on the return dig cycle.
Other HHVs work on the same principle of storing hydraulic fluid under pressure as ‘potential’ energy. In ‘series’ HHVs, the engine kicks in when needed to pressurise the accumulator that drives a separate motor. In ‘parallel’ HHVs, the engine powers the wheels through a standard transmission, while the hydraulic components are attached to the driveshaft to assist in stopping and accelerating the vehicle.
WHO’S DOING IT?
Caterpillar recently launched the 336E H hydraulic hybrid excavator, claiming fuel savings of up to 25 per cent. Hyundai has also launched its own hydraulic hybrid excavator, Hi-POSS, slated for production next year.
Formed in 2000, a small company called Permo-Drive, based in Ballina in far-north NSW, developed its own hybrid hydraulic technology for heavy diesel vehicles, with the potential to improve fuel efficiency by 25 per cent. Permo-Drive was forced into liquidation in 2012 when it couldn’t secure enough investment or funding.
Another Ballina-based company, Hydraulic Hybrid Technologies – the director of which was a former shareholder of Permo-Drive – says it is close to delivering Australia’s first hydraulic hybrid garbage truck.
Source: Boretti, A. and Stecki, J., “Hydraulic Hybrid Heavy Duty Vehicles – Challenges and Opportunities,” SAE Technical Paper, 2012 (plus additional research)
In the picture: Hydraulic hybrid components in a demonstration garbage compactor. Source: Hybrid Hydraulic Technologies Ltd