By Larry Boulden
Mobile hydraulic systems are used to transfer and control power on a wide variety of nonstationary equipment. You’ll find mobile hydraulics on farm equipment, construction machinery, logging and forestry machines, and in many other places.
Years ago, designing a mobile system was simplicity itself. You’d probably specify a gear pump, manual valves, the necessary actuators, and finish off with an appropriate reservoir, filter, and cooler. But today the vendors serving this key market have brought forth new equipment with capabilities unsuspected a decade ago.
Today, system components offer enhanced capability, such as variable piston pumps with integrated digital control systems. Hydraulic control is used for engine-cooling fans.
Sophisticated simulation procedures are available to optimize the system design, and other simulation programs now can help train the operators for safe, economical operation.
Tomorrow, mobile hydraulic systems will incorporate even more sophisticated capabilities to achieve integrated control solutions, including prognostics, speed control, and anti-cavitation and dry-motor prevention. The trend toward integrating valves into pumps and motors will continue, as will the trend toward the use of dense, compact manifolds and new hose and fitting technology, all of which reduce system part count, minimize the possibility for leaks, and further improve on the already high reliability of the systems. Full system integration to bring functions like power steering, alternator, and air conditioning compressor functions under a single central control is an emerging trend that is sure to accelerate.
Secondary elements like fluids, tanks, filters, radiators, and fans are constantly being improved. Those are the subjects of a future article. This article will discuss some of the trends in pumps, controls, and hydraulic-actuated fans that are changing the face of mobile hydraulic systems today.
Computer-designed, controlled pumps expand capabilities of mobile hydraulics
If you think a pump is just a pump, some of the new offerings may be a big surprise. For example, new variable-displacement piston pumps from Parker-Hannifin with Integrated Digital Electronic Control (IDEC) capability that gives users an unprecedented ability to control pump functions in
Using Windows(tm)-based software, users of the new IDEC pumps can control proportional pressure, proportional displacement, electronic torque limiting, anti-stall, and constant flow settings to optimize pump performance under a full range of operating conditions.
IDEC is fully-integrated into the pump; there are no external wires or plumbing, other than the single wire that commands the flow and pressure output. Sensors for pressure, speed, temperature, and displacement are all built into the pump and calibrated at the factory.
These sensors, working with the software, let the user achieve optimized pressure and flow output, even under conditions of varying speed, temperature, viscosity, and load pressure which are typical of actual working pump applications. Sensor outputs also give users a sophisticated hydraulic system monitoring capability for onboard diagnostics, which can mean extended life and reduced maintenance
downtime for the pump and other system components.
In addition to greater operating efficiency, IDEC pumps also reduce the number of proportional valves required in a system by allowing proportional control of the pump itself. And it offers on-the-fly
electronic changes in the type of control, allowing the pump to function as a load sense, pressure comp, and horsepower control during the operation of different functions in the same system. This feature allows for extremely efficient control by acting as three different pumps in the same system.
Similarly, new developments in gear pumps have greatly expanded their capabilities. While gear-type pumps and motors have periodically been identified as a technology nearing obsolescence, continuing design improvements, supported by sophisticated design and manufacturing technologies, have kept them in the forefront of mobile hydraulic applications. Advanced CAD technology has had a profound impact on gear pump design. They’re smaller, more efficient, more reliable and quieter.
Mobile applications are also the reason these new gear pumps use a cast iron housing rather than the more common aluminum. Cast iron’s greater strength and better thermal stability permits high continuous pressures and higher shaft speeds as compared to aluminum units of similar capacity.
Typical uses for the new pumps and motors include a wide variety of construction machinery, including heavy-duty lift trucks, backhoe-loaders, on and off-road trucks, agricultural equipment, and many similar mobile applications.
Programmable hydraulic fan drives
A marriage of electronic control and hydraulic power is helping OEMs maintain traditional diesel efficiency while meeting ever more stringent emission requirements.
Global emission regulations are driving new technology in the diesel industry as OEMs scramble to comply with a series of mandates scheduled to take effect through 2013. Whether it’s EPA Tier 3 and 4 in the U.S., or the European Stage III and IV, the new requirements are forcing engine builders to re-think every aspect of their product from the basic combustion process itself to all of the sub-systems that support the engine.
One place this re-thinking has shown up is in the cooling system. What used to be a radiator and belt-driven, or shaft-mounted fan rapidly is being transformed into a programmable “thermal management” system on today’s advanced diesels, and the trend promises to accelerate as the advantages of this appoach become more widely known.
Fans are huge power hogs, consuming 10 more of the engine’s horsepower in a typical shaft-mounted configuration. Worse yet, because fan power consumption increases as the cube of fan speed, they tend to use the most power when they are least needed because the vehicle is often in motion when the engine is operating at high RPM providing “ram air” through the radiator.
But the problem is even more complex. For example, when high ambient temperatures require additional cooling at idle, engine speed has to be increased to provide it via a direct-coupled fan. But, increasing engine speed adds more heat to the system, while burning more fuel and generating more exhaust gas to be managed by the emission control system.
Diesels also tend to produce maximum torque at relatively low RPM, one of the characteristics that make them the engine of choice for many applications. But maximum torque also means maximum heat generation and the need for maximum cooling while a direct-drive fan is operating at low speed.
One of the biggest factors for moving to a ‘cool-on-demand’ system has to be the ever rising cost of fuel. When a fan is turned too fast or unnecessarily, the energy expended can be directly correlated to a dollar value, which over time can add up to a considerable amount of unseen cost.
Independent studies show that fuel usage reductions of between 10% and 20% can be achieved by using well-designed hydraulic fan systems, where the fan is driven not by the vehicle engine itself, but by a hydraulic system, often called “de-coupled systems.” According to Steve Hansen, Market Specialist for Fan Drives at Parker Hannifin,these de-coupled systems often produce substantial savings in the maintenance and downtime of the vehicle.
Patented circuit solutions for these de-coupled systems allow the fan speed to be increased above what is normally achievable in either direct driven or conventional hydraulically driven fan systems. This unique feature combined with high efficiency products allows the fluids in the radiator pack to be cooled at lower engine RPM without the introduction of more heat.
And, the engine isn’t the only thermal load that has to be managed. Many of today’s systems use a single, multi-path radiator for engine coolant, charge air and the transmission fluid, each of which has a different thermal profile which may, or may not, be related to engine speed. The bottom line is that engine speed and thermal load are not very well coordinated in most diesel applications, and a lot of power and fuel that could be used productively elsewhere is wasted turning a direct-coupled fan.
A number of approaches have been tried to solve this basic problem with direct-drive fans. The most common are clutching systems or temperature-sensitive viscous couplings, or variable pitch fan blades but the basic problem still remains. In all of these systems, fan speed ultimately depends on engine speed regardless of actual thermal requirements — and that is wasteful and ineffective at low engine RPM.
Ideally, the fan should only operate when it’s needed to move air through the radiator, and then only at the minimum speed required to create the necessary airflow. To accomplish this, fan speed has to be both infinitely variable and controlled by the thermal load present in the system. In practical terms, that means the fan cannot continue to be directly coupled to the engine.
The only realistic candidates are electrical and hydraulic fan drives. Both have been proposed, and both have been tried, but for a number of reasons, mainly high power to weight ratio, only the hydraulic solution has found widespread acceptance today.
Actually, hydraulic cooling was first embraced by designers because it allows the “cooling pack” to be located virtually anywhere on the vehicle or equipment since the fan no longer has to be attached to the engine. This flexibility alone was reason enough to incorporate the technology in many applications, but it didn’t take long for engineers to realize that there were other, more compelling, advantages of the approach.
Hydraulic cooling solutions offer very high power density and system efficiency which means they take up relatively little space in the engine compartment and “cooling pack.” They also offer the ability to deliver a broad range of power output from the same package by simply managing fluid pressure and flow. And, while “hydraulic cooling” systems are not new, like the diesel engines to which they are applied, they have undergone significant technological development in the last few years.
Probably the most obvious of these advances is the availability of a new generation of digital electronic controllers to monitor system conditions and adjust fan speed. This controller can communicate with other on-board systems via an industry standard J1939 communications BUS and integrate multiple functions such as Power Steering, Charge air, engine coolant and transmission temperature control. The controller can also manage fan reversal, fan STOP, RPM dictated events and also drive twin fans, which can further save tare HP.
Simpler controls are also available that may be used to monitor inputs from discrete over-the-counter temperature sensors in the various cooling loops and intelligently adjust fan speed as necessary to keep all systems within programmable limits.
Digitally controlled fan systems offer a number of advantages. For example, through the free and easy to use graphical user interface (GUI) various options can be turned on or off or programmed into the system to optimize performance during cold start and warm-up, deactivate the fan to remove its load during a hot start, and even provide automatic, or on-demand airflow reversals to “purge” the radiator of dust and debris accumulated during operations in dirty environments or de-ice during cold weather operation.
Traditionally aluminum or cast iron gear pumps have been used for de-coupled systems. But, the new emission-compliant diesels all tend to operate at higher temperatures than the older engines and that means cooling system performance needs to be stepped up a notch or two to keep up. So, today’s hydraulic cooling systems need to operate at higher pressures than their predecessors, and often at higher ambient temperatures as well. This is particularly true of the pumps, which are nearly always mounted in the engine compartment, and often bolted directly to the engine, air compressor or transmission.
Under those conditions, aluminum is a less than optimum choice for several reasons. Its relatively high coefficient of thermal expansion makes aluminum pumps and motors prone to internal leakage at elevated operating temperatures. The higher operating pressures of the newer systems only make the problem worse.
Cast iron, on the other hand, is much more thermally stable, holds both volumetric and mechanical efficiency values as well as being much stronger. Most of the pumps and motors used in advanced hydraulic cooling systems today take advantage of these properties with the weight penalty more than offset by the improved system performance made possible by higher operating pressures, and the reduced maintenance requirements of the cast iron components.
The next generation of hydraulic cooling will almost certainly use digital controls that not only monitor all of the thermal inputs into the cooling system, but actually control the state
of those thermal sources in real time as well. Using the vehicle’s J1939 bus, the thermal control module will be able to selectively de-rate or deactivate individual thermal sources to maintain the cooling system load within specifications to ensure legal, safe, and efficient operation under all foreseeable conditions.
More stringent emission regulations may have been the catalyst that moved the development of “hydraulic cooling” into the fast lane, but the inherent advantages of this technology are more than sufficient to ensure its role in the long run. As the more advanced components and controls now being developed come online, the contribution of hydraulic cooling to diesel operating efficiency is certain to continue to grow.
Filed Under: Off highway • construction, Hydraulic equipment + components