Edited by Mary C. Gannon,
Senior associate editor
As they face new challenges and legal requirements, makers of mobile equipment must be concerned particularly with four major things: energy, the environment, costs and functionality. On the one hand, future legal requirements such as Tier 4 Final and the new version of the EU Brake Directive will increase pressures to implement changes. On the other hand, users are demanding higher performance with improved ease of use and lower costs. These are needs that can be fulfilled only if these four aspects are seen together. Bosch Rexroth is meeting these demands by offering new components, functional modules and complete system solutions that make working machines more energy-efficient and functional while cutting pollution and costs.
Knowing what the right hand is doing
The core of the Tier 4 challenge can be found in the drive concept. Existing drive systems, for example, cannot communicate with each other effectively, which means that diesel engines will normally deliver more power than is actually needed. A novel control concept eliminates this unnecessary excess. In the past the right hand — the diesel engine — didn’t know what the left hand — the hydraulic system — was doing. The Diesel Hydraulic Control (DHC) now lets these systems talk with each other. The power management concept lets the drive train and operating functions communicate with the internal combustion engine by way of an electronic control unit. If the driver uses the joystick to send a “work” signal to the hydraulic unit, then the engine is informed at the same time about the upcoming power demand. In this way, the overall system supplies the correct amount of power that is really required, reducing the vehicle’s energy consumption by up to 20%. It also reduces noise and exhaust emissions. A mobile machine with DHC operates at lower engine speed, saving fuel, and achieves about the same performance as a machine without DHC, with its engine running at higher speed.
Two other drive strategies answer the demands of the Tier 4 Final regulation. During downsizing efforts, large engines are replaced by smaller power plants without any noticeable loss of performance. Ideally, output will be less than 56 kW or about 75 hp because the standards for these engines are not quite as stringent. Drastic reductions in both diesel consumption and exhaust emissions can also be achieved by employing a hybrid concept. Bosch Rexroth is developing technology in this field, and is currently testing vehicles in different markets.
Foot off the gas!
With the new EU Brake Directive now imminent, the trend towards rising engine output will turn into a boomerang for developers. Anyone who wants to drive faster will also have to be able to stop safely.
When we want to slow down in a passenger car, we first take our foot off the gas. That’s also true for vehicles with a hydrostatic drive train, but it is becoming increasingly difficult because diesel engine speeds climb from one generation to the next. At the same time, displacement and internal friction have been reduced. This means that the engine’s braking torque is decreasing, thus increasing the hazard of engine damage. Engine speed rises abruptly during the braking process since the vehicle’s entire weight is “pushing” on the diesel engine, pressing it toward redline. By contrast, High Level Breaking (HPB) offers an elegant solution with drastically reduced stopping distances and overspeed protection. Axial piston motors in which the swivel angle can go towards zero are needed here. Software uses the engine’s maximum braking torque during the stopping cycle and caps the diesel’s speed. The braking energy over and above that — generated as a result of deceleration — is dissipated by way of pressure-relief valves.
Braking made easy
Above and beyond pure hydrostatic braking, it is often necessary to use hydraulic power brakes — especially for vehicles moving at more than 40 km/h. These brakes provide the required braking power — effortlessly, reliably and with excellent metering. But they cannot keep individual wheels from locking up under unfavorable operating conditions. That is why Rexroth and Bosch, in a joint development project, have integrated the antilock brake system (ABS) — which is typically installed in passenger cars today — into the power brakes for off-road vehicles. In everyday use the loads on the individual wheels vary tremendously. What’s more, these powerful helpers roll along on high-volume tires and their inertia is far greater than for passenger car tires. Finally, these hard-working vehicles may incorporate any of a number of drive train concepts, including optional four-wheel drive, differential gearing, etc.
Using time-tested components, Bosch and Rexroth have been successful in developing an innovative brake system. It requires no maintenance and, for both the manufacturer and users, means lower costs for conversion and operation. Incorporating hydraulic power brakes with ABS makes fast travelling working machines both safer and more comfortable. The unit no longer takes a “nosedive” during sudden stops. What’s more, this system paves the way for additional functions. These include the electronic stability program (ESP), anti-slip regulation (ASR) and intelligent steering brakes.
Energy created during braking does not need to be wasted, as is demonstrated by the Hydrostatic Regenerative Braking system (HRB) by Rexroth. This makes good sense especially for vehicles that start and stop frequently. Field testing using trash pick-up trucks in New York City showed that this system made it possible to cut fuel consumption by as much as 25%.
Gravity is your friend
As is obvious, savings can be reaped not just in travel and braking. Optimized operating functions can also help satisfy new engine emission regulations. Why should it be necessary to use engine power to lower a load that could otherwise simply give way to the force of gravity? There are other ways to comply with the regulation that the boom be equipped with a device for controlled load lowering, such as the new Green Valve concept for load holding and lowering valves. The first step in this development was to reduce machine cycle times and minimize energy losses. Green Valves more than meet these goals. While conventional valves require pressure of at least 100 bar to lower a working boom, Green Valves get along with little more than zero pressure.
Working with the same flow, the new valves can lower the load quicker, keeping movement fully under control. Controlled lowering is possible even without any flow at all. Pump energy is then available for other functions. Also worthy of note: aside from the new valves themselves, no additional components are required to stabilize the movement.
Testing revealed that the new lowering valve requires less than 1 kW of power while a conventional valve would have required 36 kW for the same procedure.
Fewer components, more comfort
Improving individual components is one thing, but developing a completely new system architecture for the hydraulics is even better. Valve Meets Cylinder (VMC) spreads out the valves, putting them out near the cylinders. That is particularly advantageous in backhoe loaders and telehandlers. In contrast to the existing centralized structure, only one pressure and one return line is required. What sounds fairly trivial triggers an impressive series of effects: fewer components, less space used, less weight, less energy, lower costs, and improved control. A backhoe loader, for example, can make do with just two hoses running along the boom instead of the usual ten. There are no severe pressure drops. Functions can easily be expanded at minimal cost, by adding sensors, for instance. This is true above all because the overcrowded situation at the central valve block is eliminated. And in contrast to the conventional concept, the VMC also uses up to 40% less energy.
This article was written for Bosch Rexroth’s Drive & Control magazine in January 2010. For more information, contact Martin Lhr, Bosch Rexroth Mobile Hydraulics, Elchingen, Germany at martin.loehr@boschrexroth.de or visit www.boschrexroth-us.com.