The Environmental Protection Agency recently unveiled a hybrid diesel UPS delivery van that combines a high-efficiency diesel engine with a hydraulic propulsion system, which replaces the vehicle's conventional drivetrain and transmission. Such a vehicle could reduce the fuel costs of package delivery services like UPS by as much as 70 percent and cut harmful emissions.
In laboratory testing, the hybrid achieved a 60 percent to 70 percent improvement in fuel economy and a reduction of more than 40 percent in emissions, versus current UPS trucks. The EPA and UPS plan to conduct real-world testing this year.
A hydraulic hybrid has several advantages. One is that it can accept and deliver huge amounts of energy quickly, which batteries cannot. And its storage ability does not degrade over time, which is a fact of life with batteries available today. Generally speaking, though, hydraulic systems do not store as much total energy as an electrical battery does, because the storage tanks are bulky.
A hydraulic hybrid is more specialized than the gasoline-electric hybrids sold today. It works better on heavier vehicles, and in stop-and-go traffic; backers say the ideal vehicle for this system is a garbage truck, but that it could work well in vehicles as small as S.U.V.'s.
In the series hydraulic hybrid diesel, a high-efficiency diesel engine is combined with a unique hydraulic propulsion system, replacing the conventional drivetrain and transmission. The vehicle uses hydraulic pumps and hydraulic storage tanks to store energy, similar to what is done with electric motors and batteries in hybrid electric vehicles. In this design, energy is stored in a series of pressurized tanks, rather than in nickel-metal hydride or lithium-ion batteries; the energy moves not as high voltage current in copper wires but as hydraulic fluid pressurized to thousands of pounds per square inch. Fuel economy is increased in three ways: vehicle braking energy is recovered that normally is wasted; the engine is operated more efficiently; and the engine can be shut off when stopped or decelerating.
The U.P.S. van has four "accumulator tanks" of 22 gallons each which can be pressurized as high as 5,000 pounds. When fully charged, the system holds 2,000 horsepower-seconds of energy, according to Benjamin M. Hoxie, engineering manager for hydraulic hybrids at Eaton, an automotive supplier that built the prototype, using technology developed by the E.P.A..
When the truck is in operation, its diesel engine, running at constant speed, runs a pump to fill the storage tanks with fluid. The tanks contain nitrogen gas to When the driver presses on the accelerator, pressurized fluid is released from the high pressure tank and routed to the pump. The pressurized fluid pushes a piston down in its cylinder, recycling some of the energy to turn the vehicle's wheels.
The company says that because the diesel engine runs at constant speed, it will have a head start in meeting the stricter pollution standards that take effect in 2010.
At the introduction of the U.P.S. truck, Eaton announced that by next year it would commercialize a related technology, a "hybrid launch assist," which could be retrofitted on existing vehicles. It would capture braking energy and deliver it to the wheels again when it was time to accelerate.
EPA, UPS unveil hybrid delivery truck
EPA Unveils Unique Hydraulic Hybrid Diesel Delivery Truck with UPS, International Truck and Engine, Eaton and U.S. Army, press release, June 21, 2006
A New Wrinkle in Hybrids Does Away With Batteries, Matthew L. Wald, NY Times< June 26, 2006