Advanced Hydraulics Lets Machines Meet Clean-Air Standards

Sept. 24, 2010
Advanced hydraulics helps mobile equipment meet tougher emissions standards.

Authored by:
Terry Hershberger
Mobile Applications Engineering Manager
Bosch Rexroth Corp.
Hoffman Estates, Ill.

Edited by Kenneth J. Korane
[email protected]

Key points:
• Tier 4 emission standards will require a holistic approach to equipment design.
• Letting hydraulic and engine controls communicate compensates for poor diesel-engine response, and saves fuel.
• Running hydraulics at higher pressures lets machines maintain performance with smaller engines.

Bosch Rexroth,
Robert Bosch,

Since 1999, the E.U. has had a road map in place for improving the environmental performance of diesel-powered mobile equipment. The ultimate goal is to reduce emissions of nitrogen oxide (NOx), carbon monoxide (CO), hydrocarbons (HC), and soot particles, with reductions scheduled in four stages over 15 years. The U.S. has mandated comparable reductions which differ only slightly in specific limits and deadlines.

In 2014, much-stricter emissions limits come into effect with Tier 4 guidelines. And those limits are already placing considerable demands on manufacturers. For instance, today’s Tier 3 levels for NOx emissions, which are already roughly one-third that of Tier 1 specs, must be significantly cut again. At the same time, customers demand higher-performing equipment that costs less to operate.

In the past, more-efficient diesel engines, better energy management, and both engine strategies (EGR, for instance) and after-treatment strategies (such as selective catalytic reduction) have often been enough to clear emissions hurdles. Additional NOx reductions may seem a realistic goal; however, the new limits are so stringent that only implementing an engine solution or an after-treatment strategy may not always be enough.

Further to this point is that the necessary particulate filters, posttreatment catalytic converters, and other emissions-control hardware will take up drastically more space, which will force manufacturers into significant vehicle redesigns, especially for small to medium-sized construction machinery. In other words, it will take a holistic design approach to reach upcoming emissions goals.

A critical aspect of that holistic redesign will involve the hydraulics. Products such as hybrid propulsion systems and hydraulic fan drives are already proven to raise efficiency and cut emissions. Here’s a look at other electrohydraulic and hydraulic solutions that have blossomed from concept to implementation to address these issues.

Networking controls
Final Tier 4 emissions regulations for mobile equipment will likely result in significantly poorer load response from diesel engines. That is, fears are that tomorrow’s engines won’t respond as quickly to demands for more power from the drivetrain or implements. Further, manufacturers of excavators, telehandlers (rough-terrain forklifts), and other equipment are reducing engine speeds to cut fuel use. The control devices used today for the travel drive and working hydraulics cannot compensate for this “sluggishness.” The potential risk is lower productivity.

Part of the problem is that existing diesel engines do not communicate effectively with the hydraulics. The engine does not know beforehand of changing load requirements, so the machine operator runs the engine faster than necessary to compensate for the possibility of a sudden increase in demand.

Rexroth’s diesel-hydraulic control (DHC) changes that. Developed with engine specialists from Robert Bosch GmbH, Stuttgart, Germany, the DHC manages the engine, as well as drive and working hydraulics, to reduce fuel consumption by up to 20%.

The DHC retains responsive drive and implement hydraulics even with lower engine speeds by letting the engine know of impending load requirements. This is accomplished through the use of matched controllers for engine management as well as the drive and implement hydraulics. The controllers all use a common “map” of vehicle-specific relationships between rpm, efficiency, and torque.

DHC continuously determines the power demands of the drive and implements and calculates the best operating points for the engine and hydraulics based on the map. A joystick for a rough-terrain forklift truck boom, for example, transmits pending work requirements directly to the DHC which, in turn, passes the data to the diesel ECU. This gives the engine time to ramp up for the imminent mechanical load.

Thus, the combined controllers compensate for the expected poorer load response of Tier 4 diesel engines. At the same time, equipment operates with no sacrifice in dynamic response of the drive and implement hydraulics despite lower engine speeds.

Because the engine only provides as much power as the machine actually needs at any given moment, DHC cuts fuel consumption compared with current engines, reducing emissions and operating costs. Potential applications include telehandlers, wheel loaders, commercial vehicles, harvesters and other agricultural machines, and forestry machinery.

Another drive strategy that could help meet Tier 4 regulations involves replacing large engines with smaller powerplants. The trick is doing it without any noticeable loss of performance.

Rexroth’s downsizing concept significantly increases the efficiency of the hydraulics while transferring and using available power more effectively. This makes it possible to maintain machine performance with a smaller engine. Ideally, output will be less than 56 kW or about 75 hp, because engines of this size have less-stringent standards. (The U. S., starting in 2014, and the E. U. (2015) will mandate notably stricter emissions regulations above 56 kW, which will require comprehensive exhaust-gas treatment.)

Advanced, highly efficient hydraulic components are one of the keys to downsizing the engine while retaining performance. For instance, new axial-piston pumps can transfer significantly more power than previous versions of the same nominal size. That lets engineers increase pressure to compensate for lower engine output not only at start-up, but over the entire working range.

Hydrostatic drives built with these high-pressure pumps and motors significantly improve the efficiency of the travel drive and, in turn, boost the machine’s overall efficiency by up to 15% in various applications. The same applies to the implement hydraulics operating at higher pressures.

This means that downsizing the engine and upgrading the hydraulics are inextricably linked. A smaller engine lowers emissions and fuel consumption and requires less space — all without sacrificing machine performance.

In recent tests on a compact loader, hydraulic pressures to 400 bar let engineers reduce the diesel engine size from 68 to 55 kW. Yet overall efficiency increased roughly 20% and tractive force by 19%, and speed, acceleration, and handling capacity remained essentially the same. The change also let engineers reduce engine rpm while retaining full functionality, for added fuel savings.

Many machines in the 57 to 74-kW (75 to 99-hp) power class are likely candidates for this type of solution.

Hydraulic flywheel
Reducing engine size is just one approach to the Tier 4 challenge. For instance, diesel engines and their exhaust-treatment systems work best at more-constant speeds. Operating within a small speed band, with barely detectable load changes, reduces fuel consumption and exhaust emissions.

The challenge is that typical mobile machines have greatly varying power demands. Fortunately, today’s hydraulics make it possible to temporarily store excess energy and make it available later for assisting the engine or performing other tasks.

A setup Rexroth calls the hydraulic flywheel (HFW) does just that. It is a separate open-circuit hydraulic system consisting of proven components such as an axial-piston pump, control block, hydraulic accumulator, and controller, as well as a tank, filters, cooler, and connections to the hydrostatic travel drive or working hydraulics.

Using open-circuit pumps which can swivel over zero (center), it is a simple matter to change its operation from pump to motor while maintaining the same direction of rotation. First, the axial-piston unit picks up torque at the engine shaft and hydraulic oil flows to an accumulator. This stored energy is then unloaded by reversing the oil flow and using the axial-piston unit as a motor, to convert energy into drive torque and release it to the engine shaft.

An electronic controller regulates the flow rate of the axial piston unit such that a preset torque is applied to the drive shaft during both charging and recovery.

The HFW comes in two versions. In mobile machines that move infrequently, HFW can temporarily store unused power from the engine and return it to the drivetrain for smoothing power peaks and valleys. This improves machine dynamics while increasing productivity; it also prevents parallel power peaks, thereby helping to keep the engine operating at constant speed.

In mobile machines with frequent travel cycles and cyclically repeating drive movements, recovering braking energy is often the more important job. Here, HFW generates additional drag torque during overrun modes. This effectively protects the engine from overrevving while at the same time storing the machine’s kinetic energy. Second, it returns the stored energy to the drivetrain, and this “power boost” can improve machine performance.

HFWs recover braking energy and assist in optimizing the engine’s operating point. Thus, it reduces the burden on the engine, lowers fuel consumption, and cuts exhaust gas emissions. These advantages increase machine efficiency and lower operating costs. Potential applications include compact wheel loaders, backhoe loaders, telehandlers, rollers, and excavators.

Embracing gravity
One question that has long vexed machine builders: Why not let the force of gravity lower a boom and its load instead of using engine power? The idea sounds simple. However, it contradicts traditional concepts of load-holding and lowering valves.

To meet safety requirements and ensure smooth motion, today’s machines literally push down the boom against a counterbalance valve. This requires engine power to build up pressure and open the valve. It’s not uncommon to use 100 bar and even up to 240 bar in equipment such as telehandlers, backhoe loaders, and cranes. The process consumes 55 kW or more of power, depending on cylinder position, speed, and pilot ratio of the counterbalance valve.

Rexroth’s new Green Valves all but eliminate energy-consuming high oil pressure and flow. Engineered to replace traditional counterbalance valves, the design lets a machine harness gravity to lower a boom, while at the same time improving stability and control. This results in quicker boom movement with smoother starts and stops. In addition, the valves require little oil flow for lowering, so more pump capacity is available for other simultaneous movements. This can shorten machine cycle times.

Tests show that the new lowering valves consume less than 1 kW of power. By drastically reducing the energy required from the diesel engine to lower a boom, the valves save fuel and help meet emission standards. As well, the valve’s stability and controllability eliminate the need for damping devices such as orifices, thereby reducing costs and installation space. Suitable for any type of hydraulic circuit, the valve is compact, simple to install, and interchangeable with current counterbalance valves. These are just a few examples of advanced hydraulic systems that assist the mobile-machine builder in addressing the effects of emissions regulations.

© 2010 Penton Media, Inc.

About the Author

Kenneth Korane

Ken Korane holds a B.S. Mechanical Engineering from The Ohio State University. In addition to serving as an editor at Machine Design until August 2015, his prior work experience includes product engineer at Parker Hannifin Corp. and mechanical design engineer at Euclid Inc. 

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