Originally Posted by Jospeh_Ogando_Sr_Editor

Not all hybrid vehicles rely on electric motors, batteries and wires. Some propel the vehicle with a combination of hydraulic pump-motors, fluid lines and accumulators

Joseph Ogando, Senior Editor -- Design News, July 16, 2007
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The trucks that deliver our packages and haul our garbage have a couple of strikes against them from a fuel-economy standpoint. Weight is the big one. Fully loaded heavy-duty vehicles cover a weight range from 14,000 to more than 70,000 lb And on top of the weight, many of these vehicles have a fuel-burning duty cycle that requires continuous starts and stops.
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To consumers and even some engineers, hydraulics may seem like outmoded technology in an increasingly electronic world. Yet hydraulic pump-motors and accumulators can provide a low-cost, reliable way to apply torque and store energy — which is exactly what hybrid vehicles require. And hydraulics offer a significant power-density advantage over electrical systems, at least for now. “It looks like hydraulics will make a lot of sense, at least for the heavy end of the heavy-duty truck spectrum,” says John DeCicco, a Ph.D. mechanical engineer who is a senior automotive strategies fellow for Environmental Defense.
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Aside from the cost and weight of the system itself, though, hydraulics have something else going for them — power density. “Nothing out there beats hydraulics when it comes to power density,” says Kovach, who explains high-pressure accumulators and pump motors have at least 10 times the power density of batteries and electric motors.
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