Last year, partners from the federal government and private industry announced they were working together to develop new hydraulic hybrid technology for an urban delivery vehicle — technology that promises dramatic improvements in fuel economy and emission reductions. The goal of this partnership was to establish the best possible business case — highest efficiency and lowest cost — for heavy hybrid vehicles.

Today, that technology is on the road in the form of a UPS delivery truck. In June, engineers unveiled the first-ever series hydraulic hybrid diesel urban delivery vehicle, which was developed through years of research by UPS, the U.S. Environmental Protection Agency, Eaton Corp., International Truck and Engine, and the U.S. Army.

UPS-branded P-100 truck prototypes represent the first-ever hybrid delivery vehicles to use hydraulic technology. The first vehicle will operate a daily delivery route in the Detroit area for a few months. Then, the prototype will be returned to EPA's lab in Ann Arbor, Mich. for analysis. In 2007, similar testing of a second prototype vehicle will occur.

"The hydraulic hybrid technology is quite promising, and we're eager to see how the vehicle performs in a real-world setting," said UPS chief operating officer John Beystehner. "We have led our industry in testing alternative fuel vehicles because fuel conservation is critical to our business. We believe the impact of this initiative will go far beyond our industry."

EPA laboratory tests show that the technology has the potential to dramatically improve the fuel economy of urban vehicles used in applications such as package delivery, shuttle and transit buses, and refuse pickup. By capturing and reusing virtually all the energy created during operations, engineers estimate the EPA-patented technology will achieve 60 to 70% better fuel efficiency in urban driving, with a 40% reduction in carbon dioxide emissions.

A partnership approach
UPS, Eaton Corp., and International Truck and Engine have been working with the EPA and the U.S. Army's National Automotive Center to develop the hydraulic technology for several years. Eaton has actually been working with the EPA since October 2001 under a Cooperative Research and Development Agreement involving hydraulic hybrid systems. As part of the company's role in designing and developing hybrid technologies, Eaton engineers have been co-located at the EPA's Ann Arbor facility.

This partnership approach has benefits beyond advancing alternative fuel technology for commercial fleets.

How it works
Similar to a hybrid electric vehicle, which includes a gas or diesel engine and battery, the hydraulic hybrid vehicle (HHV) includes a diesel engine and a hydraulic power system that, in laboratory testing, has achieved significant fuel economy over traditional UPS vehicles.

Hydraulic hybrid technology includes two power sources that propel the vehicle — a fuel-efficient diesel combustion engine and hydraulic components. This technology replaces a conventional drive train with a hydraulic one, which eliminates the need for a mechanical transmission and driveline.

These vehicles can store energy from the hydraulic system, even after the vehicle is turned off. This storage allows the vehicle to start with this energy, instead of relying on the engine to propel the vehicle.

The innovative technology behind the first prototype vehicle is simple. The main components of the system consist of:

  • A high-pressure accumulator stores energy, as a battery would in a hybrid electric vehicle, by using hydraulic fluid to compress nitrogen gas stored inside each accumulator.
  • A low-pressure reservoir stores hydraulic fluid after it has been used by the pump/motor.
  • A rear drive pump/motor converts high-pressure hydraulic fluid into rotating power for the wheels and transmits braking energy back to the high-pressure accumulator.
  • An engine pump transmits pressurized hydraulic fluid to the rear drive pump/motor, the high-pressure accumulator, or both.
  • A hybrid controller monitors the driver's acceleration and braking actions and commands the hybrid system components.

Three key design features in the full series hydraulic hybrid enable it to provide maximum fuel efficiency:

Regenerative braking — When stopping the vehicle, the hybrid controller uses the energy from the wheels by pumping fluid from the low pressure reservoir into the high pressure accumulator. When the vehicle starts accelerating, this stored energy is used to accelerate the vehicle. This process allows hydraulic hybrids to recover and reuse over 70% of the energy normally wasted during breaking. Figure 2 illustrates the efficiencies of hydraulic hybrids.

Optimum engine control — The engine pump pressurizes and transfers fluid from the low pressure reservoir to the rear drive pump/motor, and under certain operating conditions, to the high pressure accumulator. In the full series hybrid design, there is no conventional transmission and drive shaft connecting the engine to the wheels, which frees the engine to be operated at its "best efficiency" speed, to achieve optimum vehicle fuel.

Shutting off the engine when not needed — The unique full series hybrid design not only allows the engine to be operated most efficiently, but also enables the engine to be completely shut off during certain stages of operation — such as when decelerating and when not moving at a stop. As a result, in certain stop-and-go urban city driving, engine use is cut almost in half.