Anincreasing corporate focus on energy efficiency is a significant design trendaffecting mobile hydraulics based on the belief that fuel costs will continueto rise and there will continue to be upward pressure on fuel prices as globaldemand for energy increases. End users' interest in sustainability and theimpact of greenhouse gas regulations adds this focus on energy efficiency.

For a company such as Caterpillar - a manufacturer ofconstruction and mining equipment, diesel and natural gas engines, andindustrial gas turbines - looking specifically at the fuel burned in theaggregate of all its machines, around 60 to 70 percent of that fuel used isconsumed by the hydraulic system. So as Caterpillar looks to control owning andoperating costs of its machinery, a major area to hone in on is hydraulicsystem efficiency.

TanaUtley, vice president and chief technology officer at Caterpillar Inc., saysthat, "We view hydraulics as an important part of our future because it's hardto find a technology with greater power density that is continuously variable.Hydraulics will be part of our future platforms for a long time because thetechnology allows a very flexible routing of power."

Key Areas of Focus

To understand Caterpillar's energy-conscious approach tohydraulics, let's first look at the three "hows" driving Caterpillar plans.

According to Utley, the first "how" is thinking abouthydraulic system efficiency and it starts with attention to details such as allof the bends, leak sources, radii and transitions in flow that can be targetedto improve efficiency at the component level.

Thesecond "how" entails looking at the system architecture, and how the componentsare put together. Beyond that, Caterpillar looks at how to control thisarchitecture, which includes the integration of electronics into hydraulicsystems.

Thefinal "how" is to enable capabilities for even greater owning and operatingcost benefits. Hydraulics, and especially electro-hydraulics, is the platformfor semi-autonomous and fully autonomous functions on Caterpillar machines.

Thereare six key areas where Caterpillar is focusing to increase hydraulic systemenergy efficiency:

Tominimize metering losses, one area of focus is the integrated electro-hydraulicvalve control that enables intelligent control of valve position to minimizerestrictions. "We are doing that today in our hydraulic systems," says Utley.

Anotherarea of focus is hydraulic excavators that use a cross-sensing system with twoseparate hydraulic implement pumps. This system enables Caterpillar to separatethe circuits and the pressures during certain operations to control meteringlosses.

Thoughfixed and variable displacement piston pumps have been around for a long time,another issue Caterpillar is working on is the next generation of variablepiston pumps, with a focus on volumetric and mechanical efficiency. Utley saysthe objective here is to be able to deliver higher pressure and quickerresponse times. "We are also developing this family of pumps in concert withour engines of the future so that our engines will run at their optimumoperating points," she adds.

"Withwhat we are facing with EPA's Tier 4 regulations, hydraulics and how we tuneour hydraulics are beginning to have an important influence, not just onoverall machine performance, but also in how we address those regulatorychallenges," Utley says. "We use computational fluid dynamics (CFD), amathematical model of how fluid flows through a system, to know - before we cutany iron - the complete design of the system and we can analyze how the fluidflows through the circuits. The designer is able to see expansion and losses asthe hydraulic fluid expands, quantify the losses, and change the orifice sizeand the resulting fluid flow."

Toenable the recycling of lost energy, Caterpillar has begun using regenerationcircuits in its hydraulic excavators including both the boom and the stick thatsave energy during the boom-down and stick-in operations. According to Utley,this increases efficiency, and reduces cycle times and pressure losses thatresult in lower fuel costs.

"Ourwheeled excavator uses a dedicated variable displacement piston pump and afixed displacement hydraulic piston motor to power the swing mechanism," Utleysays. "This provides a recuperative technology with a closed hydraulic circuitthat allows us to maximize swing performance without reducing power to theother hydraulic components. Swing braking energy is transferred directly backto the engine where it can be used in other circuits."

OtherEnergy Improvements

Fansconsume a lot of energy, so Caterpillar has begun using hydraulic demand fanswhich are electronically controlled.
The units can vary fan speed to cool the engine as needed, thus saving fuel.

"We also carefully design lines and tubes to make surethat the fluid flow is very smooth and is as linear as we can possibly make it,especially at connection points and transitions," says Utley. "Computationfluid dynamics also helps us to understand where losses occur, and to improveour design to reduce losses."

Oneof Caterpillar's goals is to reduce the top engine rpm, especially inapplications such as hydraulic excavators.

"Historicallywe have had a high engine operating speed to accommodate the heavy hydraulicdemands that we get in those applications and to ensure we have sufficient pumpresponse," says Utley. "Instead of tuning the engine to run all of the time ata high rate of speed, we are now tuning the engine at a lower rpm, whichenables us to optimize the engine performance. It also produces the very bestfuel consumption at a lower engine rpm where it more naturally runs at a sweetspot."

Anotherenergy question continually facing Caterpillar is how to supply the right kindof power for a heavy transient demand in a hydraulic system. Currently,Caterpillar believes the answer is to speed the engine up temporarily whichallows it to run at the sweet spot most of the time and provide the transientpower only when needed.

Hydraulic Efficiency

Lookingbeyond direct hydraulic efficiency, Caterpillar also considers the efficienciesof what the hydraulic system itself enables. This includes technologies such asCaterpillar's Accugrade system, which uses electro-hydraulic systems as afundamental building block. Accugrade and auto-dig features rely on the abilityof the machine to sense the position of the blade and control it based on thedifference between what the computer thinks is the blade's ideal position andthe actual position.

Utleysays it is not uncommon using Accugrade technology to see a 20 to 30 percentimprovement in the efficiency of an earth-moving job. "In hydraulic efficiency,a five or 10 point improvement is tremendous," she says. "We have achieved 20percent with a basic electro-hydraulic system; adding the positioningtechnology has enabled us to implement innovations such as Accugrade to driveadditional improvement gains."

Hydraulicsare not just used by Caterpillar to power machine work, but also for operatorcomfort.

"Thinkof the geometry of these machines when the operator is carrying a load and mustdrive over bumpy surfaces," explains Utley. "On a tough worksite, the machinebegins to bounce, which can cause the operator to slow down and even startdumping out the contents of the machine's bucket."

Oneof the technologies Caterpillar uses on its 938H wheel loader is a ride controlsystem that offers an option of using four nitrogen-charged accumulators set todifferent pressures that enables the machine's hydraulics to provide optimumcushioning over uneven terrain at all loads and speeds. "Since hydraulics is ameans of distributing power," says Utley, "it takes hydraulic energy to dothat. But in order to optimize energy consumption, we activate the system onlywhen the operating speed is greater than 3 mph and the operator is in carrymode."