Electrohydraulics will revolutionize the way off-highway engineers design

  • 16-Sep-2010 09:21 EDT
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Tony Welter, Construction and Mining Segment Director, Eaton Corp.

Here we go again. Tier 4 emissions standards and their international analogues are kicking in and it’s setting off a predictable and familiar frenzy of concern and activity. OEMs are concerned about the cost of accommodating the new engines and aftertreatment systems, users are concerned about the cost of running them, and component suppliers are concerned about the pressure they will inevitably get from both to “do something” to offset the impact of the new regulations.

As the largest single consumer of energy on most construction and off-highway equipment, hydraulics are a natural focal point for both OEM and end-user desires for improved efficiency to offset the cost of regulatory compliance. The good news is that suppliers such as Eaton have been through this before and are ready with both new and improved products to meet the challenges presented by Tier 4 compliance.

Generally speaking, the best way to improve hydraulic efficiency is to increase the operating pressure. Most of the losses in a hydraulic system are directly related to flow volume, and higher pressures require less fluid to do the same amount of work. It’s a simple equation: reduced flow produces less heat and that reduction in heat makes the system more efficient.

Higher pressures can also mean lighter systems because pumps, cylinders, and motors can be physically smaller when operated at higher pressures and still deliver the same level of performance. Or, an OEM could choose to increase the performance of an existing machine without adding weight by increasing the operating pressure.

There is no free lunch, of course. Pumps, cylinders, motor, valves, accumulators, hoses, tubing, and everything else in the system have to be upgraded to handle higher pressures. That process may not be as onerous as it may sound, however, because hydraulic component suppliers have been busy upgrading their products even during economic hard times.

For example, Eaton has recently introduced a new series of braided and spiral wound hose products with pressure ratings that average 24% higher than currently available hoses of comparable size. In specific applications these new hoses have been shown to outperform current competitive products by as much as 70%. In almost all cases, these new hoses can be directly substituted for existing products to accommodate increased system operating pressures with virtually no hose re-engineering.

Even when OEMs choose not to take advantage of the benefits of higher operating pressures, existing products can be re-engineered to deliver increased efficiency. Optimizing the internal flow paths and adding innovative geometry in a gerotor-type motor commonly used in skid-steer loaders, for example, has improved efficiency by up to 15%.

Many such component and system-level improvements are available today, and many more will be coming online in the near future as hydraulic manufacturers respond to changing customer requirements. Industry has seen this before, and if the pattern repeats, improved hydraulic efficiencies will play a major part in offsetting the cost of Tier 4 compliance.

How about if this time we don’t stop at simply offsetting the cost of Tier 4 compliance but instead treat the necessity of change as an opportunity to make the equipment itself more efficient rather than just the hydraulics? Frankly, there is a lot more to be gained by taking a “big picture” approach to the solution than simple improvements in mechanical efficiency.

Let’s look at a hydraulic excavator as an example. Today, all of the work circuit energy used to raise the boom is lost when the boom is lowered, dissipated as heat and turbulence as the hydraulic fluid flows back into the reservoir. But it doesn’t have to be.

Hydraulics are uniquely suited to recapture most of that energy by changing it into pressure in an accumulator. All it takes is a systems-engineering approach and a few added components to capture nearly all of the energy expended in overcoming the force of gravity when the load is lifted.

Anywhere a load is raised, lowered, swung, accelerated, decelerated, or otherwise moved, an opportunity exists to reclaim much of the energy expended. And reclaimed energy does not have to be re-created by burning more fuel. Something very similar already is being done in on-highway commercial vehicles such as refuse trucks and shuttle buses, and there is no reason it can’t be done on off-highway equipment.

Since hydraulics tend to be the largest energy consumers on off-highway equipment, why not make them the centerpiece of a comprehensive energy management system for the whole vehicle?

The marriage of power-dense hydraulic muscle with electronic intelligence has created an exciting technology known as electrohydraulics. It is now possible to monitor the entire vehicle and make intelligent choices about where and how power is directed.

For example, if one asks most operators how they know when the excavator’s bucket is full, or if they are digging at maximum efficiency, the response will be “when I hear the engine bogging.” In fact, a bogging engine is much more likely to be a symptom of system overload than of maximum productivity.

What the operator cannot see is that the engine is simultaneously supplying power for propulsion, boom motion, bucket motion, radiator fans, various power assists, operator comfort systems, and a variety of other functions that may, or may not, be receiving an optimal share of the available energy. The result is machine inefficiency.

An operator cannot possibly monitor and regulate all of those loads simultaneously. But an electrohydraulic control system can. More important, it can intelligently distribute the available energy among them in real time to achieve optimum machine productivity and efficiency.

Electrohydraulics can also make operators more efficient by eliminating much of the eye/hand coordination skills now needed to run a piece of equipment. Electronically controlled machines are much smoother and more stable than manually controlled equipment because even the best human operator cannot react to changing conditions as quickly as a computer-controlled hydraulic circuit.

The technology also removes much of the fatigue-producing manual labor required to operate today’s machines by replacing pedals and levers with joy sticks, touch screens, and other electronic input devices. The next generation of operators will come on the job familiar with these technologies, which will greatly simplify the learning curve they have to negotiate on the way to becoming productive.

These examples only scratch the surface of what is available today in terms of hydraulic components and technologies applicable to off-highway equipment. Electrohydraulics in particular is in its infancy and almost certainly will revolutionize the way we design and build all kinds of vehicles and equipment in the coming decades.

By treating Tier 4, and the inevitable Tier 5, 6, 7, and so on, as opportunities for fundamental improvement in how we design, build, and use off-highway and construction equipment, we can move well beyond mere compliance to new levels of efficiency and productivity. Hydraulics are destined to play an increasingly central and important role in that development of the comprehensive power management system we will all be using tomorrow.

Tony Welter, Construction and Mining Segment Director, Eaton Corp., wrote this article for SAE Off-Highway Engineering.

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