Many self-propelled harvesting machines have extremely high performance requirements, not just for the drive, but also during the processing of harvested goods. Although the tried and tested drive system representing the status quo is a hydraulic system, alternative drives are being investigated due to the frequently poor efficiency factors shown. The general trend to electrification would appear to open up a major savings potential, in particular in case of high performances. The proof for the correctness of this theory has now been provided by the companies Sensor-Technik Wiedemann (STW) and ROPA together with the TU Dresden within the scope of a project supported by the Bavarian State.
The starting point was the tried and tested hydraulic and hydrostatic drive systems, which, due to decades of development, feature a comprehensive modular system. Over these decades of development, the technology was adapted to the vehicles, and the concepts were harmonized from diesel via pumps right up to the drives. In spite of these efforts, the drives as a whole achieve only moderate efficiency factors. Electric drives, on the other hand, score points through their high efficiency factors in almost all operating conditions and are extremely easy to control. They offer decisive advantages for this reason alone. The aim of the State support was to provide both theoretical and practical verification of an increase in efficiency through the use of electric drives and to make the experiences and methods obtained also available for the evaluation of other machines. A further objective was to develop a system environment, which permits reliable operation of the drives in the frequently adverse ambient conditions and allows development of the full potential of control, build-up, power transfer, storage etc. for the machine manufacturer.
Sensor-Technik Wiedemann (STW) started with the development of such drives in 2001 and has been able to gather a lot of experience in extreme ambient conditions, such as for example the use of their products in the Pistenbully by Kässbohrer. This created the optimum prerequisites for the definition, together with the research conducted by the TU Dresden, of a system environment which permitted mobile machines in all branches such as construction machines, passenger and load transporters and special applications such as garbage trucks, aircraft towing vehicles and much more to operate using this technology.
Each of the development steps was checked with ROPA regarding the requirements placed on beet harvesters. In order to retrofit the hydrostatic vehicle drive to a diesel-electric version with comparable performance data, the actual condition is first recorded, and this not only in normal harvesting operation, but also under peak loads. The measurement data from a real field operation was assessed, and load collectives formulated via class frequency and average length of stay distributions.
Parallel to the retrofitting of the vehicle with an electric drive system, the simulation of the conventional hydraulic and the alternative electric drive took place. In the models, the complete drive line from the diesel engine via the generators and the electric motors (electrical drive line) or via the pump and hydro motors (hydraulic drive line) were depicted via the mechanical central drive right up to the wheel-ground contact. The simulation can supply comparative statements on the performance capability, the efficiency factor and the expected fuel consumption between the hydraulic and the electric drive.
The increase in the efficiency factor of the drive lies within a range from 20 to 30 percent points in accordance with the simulation results. The fuel consumption could be lowered in the simulation by up to 30 %. The initial field tests under realistic conditions confirmed the results. Under normal harvest conditions, with both versions of the beet harvester in a direct comparison, the electrically-powered Euro Tiger displayed surprisingly agile characteristics and did not display any weaknesses even under full load.
This highly promising approach now had to be confirmed using suitable measurement methods. For this purpose, the TU Dresden developed procedures in order to obtain reliable results under comparable conditions. The necessity of remaining independent of non-measurable loads occurs in almost all comparison journeys, as in the hydraulic version the exact amount of power emitted cannot be measured or can only be measured with a great deal of effort. In this specific case, the digging unit is pushed through the earth in order to dig out the beets. Even minor fluctuations in submersion depth can cause the performance requirements of the drives to fluctuate by 10-20%. The measuring procedure did not utilize the digging unit, and simulated the harvesting operation through a braking vehicle, which accurately determined the load through traction force measurements.
This highly promising approach now had to be confirmed using suitable measurement methods. For this purpose, the TU Dresden developed procedures in order to obtain reliable results under comparable conditions. The necessity of remaining independent of non-measurable loads occurs in almost all comparison journeys, as in the hydraulic version the exact amount of power emitted cannot be measured or can only be measured with a great deal of effort. In this specific case, the digging unit is pushed through the earth in order to dig out the beets. Even minor fluctuations in submersion depth can cause the performance requirements of the drives to fluctuate by 10-20%. The measuring procedure did not utilize the digging unit, and simulated the harvesting operation through a braking vehicle, which accurately determined the load through traction force measurements. The results impressively confirm the simulations. At an uprooting speed of 6 km/h, the efficiency increase in the simulation totaled 24.8%, and 24.1% in reality. The measurements continued to show that the electric drive is superior in all operating conditions and that the distance continues to increase at high speeds. As the hydrostatic drive reaches its limits substantially earlier, the design of the engine and transmission can also be revised. Subject to a transmission adaptation, the electric drive can be reduced by approximately one quarter in order to reveal comparable traction forces.
The measurements of the diesel savings lay at approximately only 15%. This result was under no circumstances a surprise, although the expectations lay at 20%. The cause of the low diesel savings was the increased consumption required for the additional cooling of the electrical system and the differences reasoned in the efficiency of the two diesel engines. This includes the fact that the measurement of the consumption revealed many errors, as only short measurement cycles with very low consumptions were possible. The losses of the cooling system can be compensated for through a consistent transfer to electric drives through the loss of the hydraulic system. Due to the higher efficiency factor of the electrical systems, less cooling power is required overall, which then leads to further savings.
In the current stage of development, many compromises have had to be made in the mechanical structure. In a series machine, further improvements would be added which would then make diesel savings of up to 30% possible as a result.
Summary:
The complex investigations of the TU Dresden with their scientific precision clearly proved that the use of electric drives in harvesting machines is expedient in spite of the limited number of operation days per year. Although the full savings potential has not yet been exhausted and the electric drives have certainly not yet reached their final possible price in this stage of development, a positive cost/benefit balance can already be achieved within only a few years.
Within the scope of the State support, it was also possible to show that electric power transmission can open up further potentials due to its outstanding control characteristics, if an adaptation of the machines and devices is conducted based on this technology.
