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By Dan Hebert, PE, Senior Technical Editor
Especially in tough economic times, Machine Builder Nation actively seeks out new and often non-traditional markets. An area where public and private money is being spent, and where machine builders and their suppliers have related experience, is in hydraulic hybrid vehicles.
Who knows more about applying and controlling hydraulic power than the machine builders that are immersed in the technology?
Hydraulic hybrids have some compelling advantages over electric hybrids that just might allow them to win the race for the best technology.
As with electric hybrids, hydraulic hybrids use a gas engine, but they use the power more efficiently because of three main factors. First, hydraulics decouple vehicle power demand from power supply, allowing the gas engine to run at its sweet spot. Second, hydraulic hybrids can run with the gas engine completely shut off. Third, hydraulic hybrids are extremely efficient at capturing braking energy.
Hydraulics decouple vehicle power demand from supply by storing energy in high-pressure hydraulic accumulators at around 3,000 psi. Unlike gas/electric hybrids such as the Toyota Prius, hydraulic hybrids can operate without a mechanical transmission because of load decoupling.
When engine load is decoupled from road load, the engine can be run nearer to its maximum efficiency point. Decoupling the engine from road load also allows hydraulic hybrids to run with the gas engine completely off.
Gas/electric hybrids capture regenerative braking energy, just like hydraulic hybrids, but with much poorer efficiency. The high power density of hydraulic accumulators permits capture of about 70% of the braking energy. Because batteries have comparatively poor power density as compared to hydraulic systems, electric hybrids capture only about 25% of braking energy.
At sustained speeds, hydraulic hybrids don’t confer significant benefits over other vehicle propulsion systems. Hydraulic hybrids instead excel in stop-and-go driving where the three advantages detailed above come to the fore.
That’s why the earliest applications of hydraulic hybrids are being seen in applications like garbage trucks, taxis and delivery vehicles. David Spinweber, manager of product development & system engineering for mobile hydraulics at Bosch Rexroth, says garbage trucks are a particular area of interest because they already have hydraulics onboard for lifting garbage cans and bins.
FedEx, UPS and Waste Management have been evaluating hydraulic hybrids developed by Parker Hannifin and Eaton. The U.S. EPA recently showed a hydraulically driven UPS truck at the International Fluid Power Exposition in Las Vegas.
Bosch Rexroth is working with the state of New York and the U.S. Department of Energy to evaluate the suitability of hydraulic hybrid taxis for use in New York City. Vehicles are nearing completion and will be street-tested this year.
Given the inherent advantages of hydraulics over batteries, what’s holding hydraulic hybrids back? According to Spinweber, one of the main issues is precise control of the hydraulic system.
Bosch Rexroth obviously has some expertise when it comes to automation of hydraulics systems. That’s why they are one of the leading players in this nascent industry, along with other familiar names like Parker Hannifin and Eaton.
But when it comes to applying, controlling and automating hydraulic systems in machines, the main experts are not suppliers, but are instead machine builders. And after all, a motor vehicle is nothing more than a machine, albeit a very sophisticated one, so it seems that machine builders should be at the forefront of this new industry.
Machine builders with expertise in hydraulics, especially in the area of automation and control, might want to tune in and see if there are opportunities in what promises to be an industry of the future.
Some of the current hybrid hydraulic work is being sponsored and funded by various U.S. government agencies including the EPA and the Department of Energy. States are getting into the act too with funding provided by the New York State Energy Research and Development Authority for the aforementioned hydraulic hybrid taxis. So the best way for a machine builder to get in on the action could be through local, state and federal government agencies.
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