14 Shuttles at SFO to Run on Hythane
House Passes Energy Bill with New CAFE Regulations

Parker Hannifin and FedEx Ground to Test Class 6 Hydraulic Series Hybrid

Chassis layout for an EPA/Parker Hannifin Class 6 hydraulic series hybrid yard hostler. Click to enlarge.

Parker Hannifin Corporation and FedEx Ground, a unit of FedEx Corp. will test a Class 6 vehicle employing hybrid hydraulic technology that seeks to improve fuel economy by more than 50% and significantly reduce engine emissions.

The new vehicles will rely on a Parker series hybrid hydraulic drive that eliminates the conventional transmission, along with its weight and maintenance, while improving drive train power density. Primary pump/motor components developed by Parker leverage additional technologies developed by the US Environmental Protection Agency (EPA) to create a series hybrid drive.

In September, the EPA, in partnership with Parker Hannifin and others, launched a project to develop hydraulic series hybrid systems for Class 6 port yard hostlers—the heavy-duty diesels that move goods and products from ships to trucks at ports. (Earlier post.) Class 6 yard hostlers differ from on-road Class 6 vehicles in that they have lower top speeds in their unique drive cycles (around 25 mph).

The hydraulic hybrid drivetrain uses hydraulic accumulators to store energy. As hydraulic fluid enters either accumulator, the nitrogen (N2) in that accumulator compresses and its pressure rises. Like other hybrid systems, energy saved when applying the brakes is reused to help accelerate the vehicle.

Compared to batteries as an energy storage device for hybrids, the hydraulic accumulators offer a higher power density, but a lower energy density. The accumulators support a higher energy flow, not limited by overheating, and are more efficient at charging/discharging.

The system uses bent-axis pump/motors that can deliver 330 hp at 5,000 psi at 45 degrees and 510 hp at 7,000 psi at 45 degrees. The system adjusts to power demands using the variable position yoke assembly. At 0 degrees, no power is produced or absorbed; at 45 degrees, maximum power is produced or absorbed.

A full suite of Parker products, including specifically-designed pump/motors, hoses, fittings, valves, electronic controls, and Parker high-efficiency accumulators are incorporated into the design. A working demo vehicle is projected to be available for FedEx Ground within the next 12 months.

The United States Environmental Protection Agency (EPA), as part of a Cooperative Research and Development Agreement (CRADA), will continue providing Parker additional technical assistance in the development of the vehicle as it has for four years. These include in-kind engineering services, test equipment and facilities. Initial EPA simulation modeling of city driving confirms greater than 50% potential improvement in fuel mileage efficiency.

Parker expects the technology to have applications with other classes of vehicles.

FedEx is participating in the WestStart CalStart Hybrid Truck User Forum’s (HTUF) hybrid parcel delivery truck working group (WG), which is currently focused on hydraulic hybrids only—the hybrid-electric parcel delivery truck industry already being relatively mature.

Key performance parameters set out by the HTUF WG include:

  • >50% fuel economy improvements compared to baseline Class 4/6 vehicles<
  • Walk-in Van Chassis
  • Reliability ≥ baseline truck
  • Lifecycle costs ≤ baseline truck
  • No reduction in cargo capacity
  • Serviceability/maintainability ≥ baseline truck
  • Durability ≥ baseline truck
  • Cargo bed height ≤ baseline truck
  • Range ≥ 400 miles/day
  • Exterior and interior noise levels ≤ baseline truck
  • Acceleration ≥ baseline truck
  • Gradeability ≥ 5% at 37.5 mph for a Class 6 truck

This relationship with FedEx Ground marks the latest chapter in Parker’s 10-year history of energy saving hydraulic drive systems for commercial vehicles. We are extremely pleased FedEx Ground is working with us as a collaborator for this program.

—Dr. Joseph A. Kovach, Vice President of Technology and Innovation for Hydraulics

The day prior to the FedEx announced, Dr. Kovach had presented Parker’s hydraulic hybrid technology to financial analysts and shareholders during the company’s annual Investor Day at the company’s Cleveland headquarters.


  • Hydraulic Hybrid (FedEx Ground WestStart CalStart presentation)


Harvey D

Would this be an acceptable-affordable solution for large 5000+ lbs pick-ups & SUVs, at least until lower cost efficient batteries-ultracaps become available?


it also allows the vehicle to be maintained by the normal maintenance staff, since high pressure hydraulics are nothing new to any regular old mechanic familiar with brakes.

unlike 600V electrics that have to be sent to specialists.


This should be a good application of hydraulic hybrids.

Outstanding questions are:

Can they put in large enough high pressure tanks to store useful amounts of energy? (Usually they're lucky to get 2,000 horsepower seconds stored).

Will the tanks displace too much space under the chassis?

How do they do thermal management (heat that radiates through the tank is lost energy, but tanks that overheat fail)?

How do they manage noise? Hydraulics are pretty darn noisy.

How long before the hydraulics wear out. Have hydraulics ever been used continuously for 400 miles/day, for 10 years?


I'm no engineer, but I assume series hydraulic would not be very efficient for highway cruising, but is well-suited for low speed, high torque start-stop operation.

Gary Shank

The EPA was supposed to have sponsered a Hybrid Hydraulic drive in a UPS truck in 2006. Where can I find the performance data on this? All I see on the EPA website is press releases.


A UPS hydraulic hybrid would be nice. Last time I saw a fleet of UPS trucks take off in the morning, I saw a trail of thick black smoke coming out of their tailpipes.


Why should a hydraulic engine with a closed loop, hydraulic oil drive, be noisy?

I agree, hydraulic engines running off compressed air are noisy, but just because of the very same reason that combustion engines are noisy - the low pressure reservoir is the ambient air, and the air released to it makes that noisy, primarily.

With a closed loop hydraulic, you'd expect the same noise level as old-style automatic transmission (ie. next to none), as no medium should be released and carrying sound away...


If you have ever been around hydraulic equipment lifting heavy loads, you know what the noise is like. I would imagine that there is resonance in every system and with the power of hydraulics under load, the sound can be formidable.


Couldnt see anything that may give a clue as to how this transmission works,as the previos comment states heavy on the press release, light on information.
wasting my time.

The comments to this entry are closed.