The diagram on the left shows how stored energy in the hydraulic high-pressure accumulator is used by the hydraulic motor to propel the vehicle at low speeds. The diesel engine remains off until needed. Power is also developed by the diesel engine and transferred to the wheels mechanically, shown on the right, giving high transmission efficiency for high-speed driving.


Series HHVs rely entirely on the hydrau-


lic system pressure to drive the wheels; the engine does not directly drive the vehicle. A hydraulic engine pump provides addi- tional fluid pressure to the pump/motor when required.


ADVANTAGES OF HHV SYSTEMS HHV technology is very versatile and


can be applied to military vehicles, bus- es, delivery trucks, refuse haulers, and domestic/sport utility vehicles. Cost effectiveness is very high since manufac- turing skills are already available in the U.S. It’s projected that fuel efficiency will increase from 60 percent to an estimated 100 percent while reducing emissions up to 40 percent. HHV manufacturing does


not require expensive, high-tech materials and can have a potential payback period of less than three years when produced in high volumes. While braking, the HHV system cap-


tures and stores energy coming from the wheels. During acceleration, the stored energy is reused to accelerate the vehicle by using more than 70 percent of the en- ergy usually wasted during braking. Tis regenerative braking process reduces wear on the friction brakes. With no transmission and drive shaft,


the engine can be shut down when not needed. A hybrid controller will activate the engine only when required, triggered by the driver removing his or her foot from the brake and depressing the throt-


In June, STN contributing editor John Whelan sat down with Kris Hus, prod- uct manager at Freightliner Custom Chassis Corporation, to discuss the Parker Advanced Series Hybrid Hydraulic Systems that his company is cur- rently developing.


STN: The engine shut-down and start-up process in a series hydraulic hybrid system promises to save wear on the engine, brakes and increase fuel efficiency. What about average fuel savings? Kris Hus: This will depend on the route and application. However, in a dense, urban stop-and-go environment, the engine-off feature is anticipated to improve fuel economy by approximately 15 to 30 percent over the baseline hydraulic system.


STN: Is there a second source for starting the engine with the Series HHV? KH: Since engine-off is expected to happen repeatedly during operation, it is un- desirable to use the starter repeatedly. Therefore, the system is programmed to use the hydraulic pump to start the engine back through the drive line.


STN: Are HHVs currently optimized for highway plus stop-and-go driving? KH: Yes, the planetary gearbox allows the vehicle to be driven solely from hy- draulic pressure at slow speeds and creates a direct link to the rear axle at high speeds. However, improvements in fuel economy will be realized at their great- est during stop-and-go operations. STN: Are alternate fuels being integrated for even better mpg?


tle. Proponents say this will drastically cut down on engine use, especially with stop- and-go city driving. Te hybrid controller will also operate


the engine only at maximum efficiency and when required for the best fuel economy possible. Te industry term for this mode of operation is called the “sweet spot.” Another company working on this


technology is Freightliner Custom Chassis Corporation, a division of Daimler Trucks North America and a sister company to Tomas Built Buses. “Since engine-off is expected to hap-


pen repeatedly during operation, it is undesirable to use the starter repeatedly. Terefore, the system is programmed to use the hydraulic pump to start the


KH: At this point, design efforts are focused on clean diesel.


STN: Does the increased mpg outweigh the higher GVWR? KH: Yes. Although the system adds approximately 1,500 pounds of weight, the 35 to 70 percent increase in fuel economy easily offsets the weight gain.


STN: What role are electric hybrids playing right now in the industry, including their current role with school buses? KH: Currently, the electric-hybrid is the most mature fuel-saving technology available in production.


RESOURCES YouTube Video: “How HHV” Systems Work www.stnonline.com/go/827


YouTube Video: “UPS HHV” www.stnonline.com/go/828


“Hydraulic Hybrid Videos” — EPA www.stnonline.com/go/829


www.stnonline.com 39


“How Hydraulic Hybrids Work” — EPA www.stnonline.com/go/830


“Hybrid Truck Initiative [PDF]” www.stnonline.com/go/831


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