Hydraulics Researchers Stress Energy Efficiency

Energy efficiency was the focus of the 7th International Fluid Power Conference, held recently in Aachen, Germany. So it’s not surprising that many of the technical discussions centered on hydraulic-hybrid vehicles (HHVs).

HHVs offer greater power density than electric hybrids and are therefore more efficient at recovering and reusing braking energy, explains Larry Hartter of FEV Inc., Auburn Hills, Mich. While electric hybrids typically return 30% or less of the braking energy back to the wheels, hydraulic hybrids can return as much as 70%.

However, with R&D progressing on several fronts, there is no consensus on the best HHV design. FEV favors serial hybrids. These typically have one or more pump/motors (P/Ms) connected to the driving wheels, a P/M connected to the internal-combustion engine, along with interconnected accumulators. There is no direct mechanical link between the engine and wheels.

When compared with other designs, the series hydraulic hybrid offers better fuel economy than parallel (launch-assist) hybrids that only recycle braking energy, says Hartter. And series hybrids eliminate the additional costs and weight of hydromechanical (power split) hybrids.

Though the series hybrid recovers braking energy, decoupling the engine from the wheels is the primary reason for a series HHV’s high fuel economy, says Hartter. This lets the engine operate where it is most efficient, that is, at higher torque with fewer engine transients.

Initial road tests of a hydraulic-hybrid delivery vehicle working a typical route in Detroit demonstrated a 45 to 50% decrease in fuel consumption. This shows that series hybrids are a viable option for commercial vehicles such as delivery trucks and transit buses, says Hartter.

Although fuel economy is the key driver behind hybrid technology, productivity and drivability demand equal billing, says Andreas Johansson with Parker Hannifin AB, Trollhättan, Sweden. Parker’s RunWise hydraulic-hybrid transmission addresses those issues, he says, and reduces fuel consumption from 30 to 50% in Class 8 refuse vehicles, compared to baseline vehicles.

The RunWise transmission has a two-speed hydrostatic drive for urban driving combined with mechanical direct drive for efficient operation at highway speeds. It also has high-pressure accumulators that capture braking energy and reuse it to help accelerate the vehicle. A digital controller handles pump/motor operation, gear shifting, and accumulator charging and discharging, as well as operations of the diesel engine.

Compared with conventional trucks, tests show the RunWise transmission accelerates refuse trucks faster without affecting fuel savings, says Johansson. There is also less brake wear, thanks to hydraulic-energy recovery, and less noise. The RunWise should pay for itself in fuel and maintenance savings in 4 to 5 years, says Johansson.

Vincent Duray of Eaton Corp., Eden Prairie, Minn., explained the advantages of its parallel-hydraulic hybrid called Hydraulic Launch Assist (HLA). It contains hydraulic components and controls similar to those in a series hybrid. However, it augments, but does not replace, a truck’s existing drivetrain.

Parallel hybrids use regenerative braking to improve fuel economy in refuse trucks by 15 to 30%, based on duty cycle, says Duray. It also reduces brake wear.

Although a series hybrid saves more fuel by managing engine speed, admits Duray, parallel hybrids offer several advantages. For example, it readily retrofits to existing vehicles. HLA’s modular design lets it install in new or existing vehicles without extensive alterations or design changes to other major vehicle subsystems. Parallel hybrids connect to the vehicle’s drivetrain between the transmission and differential.

Parallel hybrids also improve acceleration and significantly reduce heat generation, engine-power output, and loads on the torque converter and transmission. This reduces failures in driveline components or, as an alternative, permits smaller components that lower initial investment and weight. And the vehicle continues to function even if the hybrid system fails.

As noted previously, hydraulic-hybrid vehicles and many other machines rely on hydropneumatic piston or bladder accumulators to recover and reuse energy. But under a wide range of operating conditions, thermal losses in the accumulators themselves can be substantial, explains Alexander Stroganov of Lumex Ltd., St. Petersburg, Russia.

His company has developed a fluid-power storage device that significantly reduces thermal losses. Called a compressible metal regenerator, it provides nearly isothermal gas compression and expansion. The design relies on a series of flexible-metal leaves dividing the gas reservoir within an accumulator shell — a design readily manufactured via conventional methods, says Stroganov.

Segmenting the gas volume into thin layers with metal elements, he explains, improves heat transfer and reduces temperature differences. This increases reversibility of gas compression and expansion and, therefore, efficiency. Test results in work cycles typical of hydraulic-hybrid vehicles show the regenerator’s power efficiency was in the 97% range, versus about 80% for gas/oil accumulators.

Hybrids weren’t the only focus at Aachen. Engineers from Voswinkel GmbH, Meinerzhagen, Germany, introduced a new type of hydraulic-hose fitting that reportedly improves flow efficiency in hydraulic circuits. They note that standard fittings, when crimped, slightly collapse the nipple and restrict the ID. The company’s new uvos-m fittings have a corrugated socket that, when crimped onto hose, tightly grips the reinforcement without nipple ID collapse, according to the company’s Fritz-Georg Weiland.

In size DN 19, the fittings increase internal-flow area about 10%. For flow rates of 200 lpm, pressure losses drop by 24% in the DN 19 and by 34% in DN 25 fittings. On a mobile machine with just two hydraulic lines, the DN 25 uvos-m can save about 130 liters of fuel/1,000 hr of operation, says Weiland.

Even the type of fluid makes a difference. Paul Michael of the Milwaukee School of Engineering explained that although straight-grade hydraulic fluids are used in many fluid-power applications, multigrade hydraulic fluids are a better choice in commercial vehicles and mobile equipment where temperatures vary widely. Even though they cost more than straight-grade fluids, savings with these so called energy-efficient hydraulic fluids can be substantial.

At low temperatures, multigrade fluids reduce hydromechanical losses due to viscous drag. And at high temperatures, volumetric efficiency improves in many kinds of pumps. Field trials in skid steer loaders and excavators show this equates to fuel-efficiency improvements of 14 to 22%, and machine productivity improves as well. Savings are said to exceed 10,000 Euros over the life of the fluid.

Copyright 2010, Penton Media Inc. All rights reserved.