Bosch has developed a number of technologies to improve the efficiency of the internal-combustion engine. One of these is start-stop technology, which contributes to significantly reduced fuel consumption and has become a standard feature in a variety of new automobiles. While the starter systems that have become so widespread in automotive technology are not available for mobile work machines, Bosch Rexroth has applied the idea of start-stop technology to hydraulics, and in fact took the time to emphasize at this year's ConExpo the integration of Bosch and Rexroth technologies toward off-highway solutions.

Rexroth development engineers had to consider that off-highway machines not only drive, their main function is to work hard and they must have enough power to effectively dig, lift, or carry. If no energy is required for the machine’s driving and work functions, the internal-combustion engine could be turned off—provided there is a sufficient power reserve for a hydraulic start.

The hydraulic fly wheel system (HFW), which always operates in the open circuit, builds up the required power reserve. HFW includes a Rexroth axial piston pump, control block, hydraulic accumulator, and control unit. It is connected to the travel drive or working hydraulics using only basic machinery components such as a tank, filter, and cooling system. The HFW system briefly gathers energy and makes it available to the machine later on as needed, for instance to provide a boost, support the diesel engine in the event of a power demand peak, or to power a start-stop function.

As a result, energy is applied in a much more efficient manner. First, the axial piston unit picks up torque at the combustion engine shaft. The hydraulic oil flow generated is fed to an accumulator, where pressure and thus the amount of stored energy increase accordingly. If the accumulator is unloaded, the axial piston unit serves as a hydraulic motor—a special characteristic of several mooring-capable components—and converts the oil flow back into energy for the combustion engine shaft.

This concept is the force behind the new start-stop solution. Electronics assess whether enough pressure is available to restart the diesel aggregate once it has been switched off. If the internal-combustion engine is off, the electronics evaluate the operator’s potential energy needs. If the travel drive or work hydraulics require energy, the start-stop system immediately restarts the diesel engine with energy stored earlier on. The engine speed needed for travel and working functions is reached practically instantaneously with no time lag.

The start-stop function is also possible without the additional pump/motor unit required for the HFW approach. Here, the only prerequisite is a mooring version of the working hydraulics pump already available. This mooring pump can also serve as a hydraulic motor that can restart the diesel engine if needed.

Bosch says the ease of operation and work efficiency of equipment such as wheel loaders and dump trucks would be increased, meeting customer needs for fuel-efficient, high-performing machinery while at the same time fulfilling ever stricter emissions standards.

To view a video of the system at work, click here.