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EPA and Univ. of Toledo Partner on $1M Hydraulic Hybrid Project

A series hydraulic hybrid. A pump/motor provides torque to the driveshaft. Source:NextEnergy

The US Environmental Protection Agency and the University of Toledo have announced a three-year cooperative agreement worth about $1 million to improve and to optimize hydraulic hybrid vehicle components to maximize fuel economy benefits and minimize emissions. EPA will contribute up to $450,000 toward the agreement.

A hydraulic hybrid system uses an accumulator (which stores energy as highly compressed gas) and one or more hydraulic pump/motors rather than the battery pack, electric generator/motor and power electronics used in electric hybrids.

During acceleration, fluid in the high-pressure accumulator drives the pump/motor as a motor, thus providing torque to the driveshaft.

Hydraulic drivetrains are particularly attractive for vehicle applications that entail a significant amount of stop-and-go driving, such as urban delivery trucks, refuse trucks or school buses. A major benefit of a hydraulic hybrid vehicle is the ability to capture and use a large percentage of the energy normally lost in vehicle braking.

The collaborative EPA-UT effort is applicable to both light- and heavy-duty vehicles with high-efficiency hydraulic hybrid systems.

The partnership will focus on four main tasks:

  • Improving efficiency of hydraulic hybrid drivetrain components;

  • Refining hydraulic hybrid drivetrains for smooth and quiet operation;

  • Modeling improvements to hydraulic hybrid operation; and

  • Training a new generation of engineers with knowledge and skills in advanced automotive technology;

A number of hydraulic hybrid development projects are underway in the US and China, including a major push by the EPA to develop a full series hydraulic hybrid in which the engine drives a pump rather than the electric generator in an electric series hybrid.

NextEnergy, Michigan’s non-profit alternative energy technology incubator, also formally launched a Hydraulic Hybrid Working Group earlier this year. (Earlier post.)


Rafael Seidl

Hydrostatic transmissions (e.g.wobble-plate types) have been used in heavy machinery like tractors and backhoes for decades. They are very rugged and continuously variable (even into reverse) but efficiency is pretty lousy.

The EPA system essentially uses half of such a transmission, alternating between pump mode when braking and motor mode when accelerating. The pressure differential between the oil inlet and outlet can be very high (~5000psi = ~350bar), permitting high torque even for low mass flows (i.e. at low RPM).

The other attractions are low cost (compared to ultracaps and high-power battery packs) and, avoiding the me-too syndrome of chasing after the Japanese. The intended markets for hydraulic hybrids are full-size SUVs & pick-ups, urban delivery/garbage trucks and city buses.

There are separate research projects for storing energy as compressed air, targeted at smaller vehicles.

All hybrid powertrains, regardless of storage medium, may be configured as serial (to reduce emissions), parallel (to improve fuel economy) or compound (cp. Prius) relative to the prime mover.

Barry R. Guthrie

I like this hydraulic drive system very much.
However, I'm very concerned about the chamber sealing systems used at these pressures. They are prone to leaking and when exposed to harsh driving vibrations and high and low temperature changes the problems become even more apparent.

Medium duty operation of very frequent stopping and launching is a much more active system than agricultural uses. This will again put much more stress into the seals and reduce the mean time before failure.

The hydraulic chambers and liquid add a great amount of weight to the vehicle reducing overall net efficiency benefits. The tanks could be made with carbon fiber wrapping to help make them lighter but this will increase there costs.

Barry R. Guthrie

Cyclical adiabatic heating is also a big problem. Carbon fiber wrapping can't take the compression temperatures. You would then have to use a heat exchange system to help regulate the adiabatic compressoin and expansion which makes the system more complicated and more problematic, plus adds weight and cost, and lowering the overall efficiency benefit.

The operational graphics look cool, but has big problems.

Blair W

One of the advantages of the hydraulic system is that it removes the drivetrain, instead just using hydraulic lines. This weight reduction should offset most, if not all, of the additional weight of the hydraulic fluid and resevoir.

Barry R. Guthrie

The hydraulic system is a much better choice than Ultra Capaciters and larger battery systems. Especially for larger vehicle sizes. You don't have expensive battery systems that goes bad after 7 years and end up with enormous amounts of heavy metals to dispose of creating a bigger environmental and health problem to worry about than GHG and fuel saving benefits.


Not on my driveway, you Dont!
Every since my brother refused to allow me to park my oil leaking diesel car on his driveway, I have learned oil leaks are bad.
Please don't allow more working fuilds into the environment.

tom deplume

I don't see a reason why biodiesel couldn't be used as hydraulic fluid. Being biodegradeable would minimise the environmental impact.

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