When most people think of “hybrid,” they no doubt think of one of the small gasoline-electric hybrids, such as the Toyota Prius. Hybrid systems are also available for trucks, like the Ram PHEV pickups that have been delivered to various cities and utility companies.

There’s another hybrid system that isn’t as well known, but has the potential to be even more fuel efficient in certain types of commercial operations: the hydraulic hybrid. UPS, FedEx and Purolator recently took delivery of the first commercially available hydraulic hybrid Class 6 vans to evaluate their use in package delivery applications.

A hydraulic hybrid doesn’t use an electric motor or batteries: it uses a clean fluid pumped back and forth between a low-pressure reservoir and a high-pressure, nitrogen-charged accumulator to drive a hydraulic pump that also acts as a motor, directly driving the wheels. It gets its energy from vehicle braking – up to 70% of the brake energy that would normally be wasted can be captured by the system.

During braking, fluid is pumped from the reservoir to the accumulator. When it’s time to move again, the pressurized fluid in the accumulator is forced back into the reservoir, driving the pump, which drives the wheels.

Hydraulic hybrids are best-suited for operations with lots of stop-and-go driving, such as city buses, garbage trucks and delivery trucks because they capture more energy from regenerative braking than electric hybrids. The systems also weigh less and don’t have the disposal challenges of heavy-metal batteries. While stop-and-go driving normally means engine wear and terrible mileage, the opposite is true for the hydraulic hybrid. Because the frequency of stops raises the amount of energy available to drive the vehicle, the main engine can be shut off and the vehicle can operate on the hydraulic system alone. Not only does the system cut down on fuel usage and diesel emissions, the use of the braking energy reduces brake wear and may even extend tire life as less brake heat is transmitted to the wheels.

The hydraulic system is also powerful: it’s been in use for a few years on big Class 8 municipal refuse trucks weighing up to 70,000 pounds.

The new trucks use Parker-Hannifin Runwise serial drive systems integrated into Freightliner Custom Chassis MT-55 platforms and have walk-in van bodies installed by Morgan Olson. The new Parker hybrid drive system is designed specifically for applications such as parcel delivery. It incorporates an infinitely variable transmission that Parker says will seamlessly blend hydraulic power with engine power depending on demand. The onboard controller uses an advanced engine-off strategy, minimizing run time and reducing fuel use by 40% or more.

So when will we see a consumer vehicle with hydraulic hybrid drive? Not any time soon; the electric hybrid is better-suited for everyday driving that involves fewer stops and generally higher speeds.

To learn more about hydraulic hybrids, check out this article at HowStuffWorks.com.