Hydraulics for Electrically Powered Mobile Machines

Reduce C0₂ emissions through more efficient components and systems

Interview with Dierk Peitsmeyer

Dierk Peitsmeyer, Product Portfolio Manager at Bucher Hydraulics talks in an interview with "Fluid Power World" about which drive systems will be useful in the future to meet the demands for high power density and efficient, economical operation with minimized pollutant emissions.

This implies that in the future, not only passenger cars but also mobile machinery will be driven hybrid or even fully electric, for example, earthmoving, forestry, and agricultural applications. What do operators essentially demand from these "alternatively" driven machines?

Dierk Peitsmeyer: An important factor is undoubtedly a sufficiently long operation time without recharging the batteries, which takes a lot of time. Besides, the operators naturally keep an eye on the economic operation. They expect low costs and little effort in energy consumption, maintenance, service, and repair. In addition, alternatively driven machines must also function very reliably and maintenance-free over long periods of time. The operating company can only then work economically. Another aspect related to environmental protection is definitely noise emission. Mobile machines should be as quiet as possible. It will certainly be crucial that the machines are at least up to the usual standard or better for daily operation and acceptance of the operator. To achieve high performance, short reaction times are of vital importance when controlling the functions.

What options are available to meet these requirements?

Dierk Peitsmeyer: Sufficient operating time can be achieved with a large dimensioned energy source or low power dissipation. However, a large battery or fuel cell is relatively expensive. Efficient components and an effective system can prove to be a more sensible approach depending on the load spectrum, as the energy costs are also lower. Electric drives already offer high efficiency over a wide operating range. Nevertheless, the hydraulics used today in mobile machines powered by diesel engines are not efficient enough.

LS (Load Sensing) systems with variable displacement pumps do not optimally meet the criteria for electrified machines. The noise emissions of conventional variable displacement axial piston pumps are too high. They will be particularly prominent in electrified machines due to the absence of diesel engine noise. The level of efficiency is not optimal throughout the entire operating range. The LS control causes unnecessary losses in most operating points. Significant damping is occasionally necessary to operate the LS system without oscillation. This extends the response time considerably.

Consequently, it is not advantageous to replace the diesel engine with an electric motor and leave the existing hydraulic system unchanged. Replacing hydraulic cylinders with electromechanical linear drives has proven to be impractical. The mechanical lineargears are not suitable for the high loads of a construction machine. Furthermore, every electromechanical linear drive requires an electric motor with correspondingly high power. As a result, the installed power is relatively high, even though it is never driven simultaneously. An optimized hydraulic system is a compromise between efficiency and cost. For high-performance functions with a high energy recovery potential, closed-circuit linear drives are an excellent choice. They have the lowest losses. Each linear function requires a combination of electric motor and pump, comparable to electromechanical linear drives. It does not make sense for all functions.

Therefore, you consider the technical options shown to be insufficient or a compromise where you have to accept too many restrictions. What alternatives are there to meet the partly contradictory demands much better?

Dierk Peitsmeyer: For precision movements, valve controls with “Flow on Demand” (FoD) are an economical alternative. Hydraulic control blocks with separate control edges, such as the LVS12 from Bucher Hydraulics, reduce power loss, and enable movements that are more dynamic; therefore, the effectiveness of the machine is further increased. It is possible to combine variants of electric drives and hydraulic systems optimally in an electrical system with a DC intermediate circuit.

The pump is operated as a fixed displacement pump with variable speed. The electric motor speed is calculated by the Bucher Hydraulics FoD software using the valve actuation characteristics. The control pressure difference required in the LS system is eliminated. The result of the FoD system is always the lowest possible pressure at the pump. It depends on the current load and pressure losses. The pressure losses can be minimized by an optimized system design.

In the FoD system, the valve opening and pump speed are controlled simultaneously. As a result, the responding behavior is very fast and direct, comparable to the performance of electric travel drives. The control system works stable without oscillations. This allows the operator to drive the machine more effectively. The FoD also offers additional advantages for controlled functions, such as assistance control systems. Our FoD software features additional options that are not available with a traditional LS system.

This certainly requires a much greater effort for control technology as well as hardware and software development by the machine manufacturer. To what extent are hydraulics manufacturers able to offer economic solutions in this area?

Dierk Peitsmeyer: With Bucher Hydraulics' FoD software, the user has no trouble generating the optimal electric motor speed. In variable speed operation, the pump is of particular importance to meet the specific requirements of mobile hydraulics. The pump is the heart of the system. Comparable to the efficiency of high-quality electric motors, it must be operable at 92 to 94 % efficiency over a wide operating range. This minimizes the need for costly battery capacity. There is a significant difference whether 5 kW or only 2.5 kW power loss is generated by the pump. The efficiency chain further enhances this effect.

At Bucher Hydraulics, we offer our AX pumps as the optimum solution. Even at high performance, the noise level is low and pleasant, as expected by end users. The AX pumps can be started at high pressure and operated at very low speed without premature and excessive wear. Due to low leakage and the high number of pistons, smooth cylinder movement can be achieved even at low speeds. This is advantageous for precise positioning or when large booms are involved. The design principle with consistent hydrostatic balance of the components makes these pumps particularly reliable.

With AX pumps and FoD software, the hydraulic systems of Bucher Hydraulics perfectly meet the requirements of electrified machines and thus make a vital contribution to the future economical and productive use of electrified machines.

Mr. Peitsmeyer, thank you very much for your detailed information on efficient, economical, and powerful mobile hydraulics for electrified working machines.

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Further information

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