This work presents a process for the requirement management and conceptual design phases of hydraulic systems. The aim is to provide a foundation for developing more energy efficient systems for the working hydraulics of construction machines. A framework is developed and applied in a case study of a loader crane. An important part of the framework is to evaluate and compare different designs based on customer usage of the machine. Emphasis has also been put on incorporating the knowledge and experience of both industry and academia. A large number of different designs are evaluated during a concept exploration phase. The design space is then narrowed down to the system design analysis phase. Losses from simultaneous operation are significant, systems with two and four pumps respectively are therefore selected for analysis by demonstrators. In order to reach equal energy performance, two-pump systems need to be more complex, i.e., they require a more sophisticated control and more components, than four-pump systems. When comparing demonstrator results, the conclusion is that for two-pump systems, it is more challenging to reach the theoretical performance, both in terms of energy efficiency and of drivability, than for the less complex systems made possible with four-pumps.

The work of this thesis is focused on creating the base for developing new hydraulic systems for mobile machinery tailored for an electric drive system. The loader crane is used as the principal application. With batteries being costly to invest in and having a much lower energy density than diesel fuel, the new systems must significantly reduce the energy consumption compared to the conventional system while keeping the performance in order to enable an electric drive. Exactly how much more energy efficient a new system must be will differ with the use case of the application, and with time, as components and the market develop. The aim is therefore to present a large number of new system concepts with different levels of energy reducing capabilities and complexity rather than proposing a single solution. 

In order to be able to evaluate and compare different system solutions, a drive cycle for the working envelope of the loader crane is developed. The drive cycle includes as much variance of the movement as possible in a short time, which is of practical use when testing prototypes. 

The task of finding new energy efficient solutions is started by performing a loss analysis on the reference crane. The analysis shows that losses due to simultaneously operated functions are significant, and consequently that multi-pump systems are of interest. A concept study that looks deeper into multi-pump systems as well as other loss-reducing concepts is carried out, and a large number of new system solutions with different energy reducing potential are presented. 

A promising concept is the two-pump system with open flow control, which is investigated in more detail by building and testing a prototype. The results from operation of the prototype highlight the challenge of achieving smooth control, but also that the efficiency is indeed improved compared to the refer-ence system. 

As this work is part of a larger research project, studies have been conducted on other interesting concepts as well, but with different applications. Results from these that are relevant to the loader crane case are discussed together with the results from the studies included in this thesis, in order to give a broad view on how a suitable system can be selected for the intended application. 

The load-sensing system has for a long time been the most energy efficient hydraulic system widely used for mobile machines. When replacing the combustion engine drive with an electric drive with a battery as energy source an incentive for using more energy efficient systems arise. Some promising examples of more energy efficient systems are independent metering systems, pump controlled systems and open flow control systems. Before getting to deeply involved in a specific design, an investigation of the intended application is of importance. The objective of this study is to present a large number of energy efficient designs for the hydraulic system of an electrified loader crane by the means of a loss analysis and a high level concept comparison. To be able to cover a large design space all systems are modeled based on static pressure-flow relations for their components.

Based on measurements, the losses from simultaneous operation, backpressure losses and load holding valve losses are found to be the largest loss contributors in the hydraulic system. If an electrical supply system is added to the reference load sensing system, the overall efficiency is found to be 23 %. The hydraulic system it is found to account for 62 % of the losses and the drive system for 38 %.

The concept comparison shows that a two or four pump system with recuperation possibilities can decrease the energy consumption of the complete system on the studied working pattern by about 50 %.

Imagine a system with a pump driven by a speed-controlled electric motor. What and how much can be gained by using a pump with discretely variable displacement instead of a conventional fixed pump in such a system? This question is the focus in this paper, in which a simulation study based on a drive cycle for a loader crane is presented. The results indicate that the system efficiency from inverter input to pump output can increase by a few percentages. This might be considered small in relation to the increasing complexity that comes with discrete displacement. However, the results also show that a system with discrete displacement substantially reduces torque and cooling requirements on the electric motor. The required maximum torque can be reduced by 30 to 50 % and the motor can generate up to 40 % less heat since it can work in more efficient conditions. These potential benefits will be obtained with only a few discrete displacement settings available.

Electrification of hydraulic systems has become an important research topic. The cost of the electric energy storage is high and drives the development of new efficient solutions for the system to be of commercial interest. This study is part of a project where the electrification of a loader crane is the main focus. The crane is a versatile machine and the energy efficiency of the hydraulic system is strongly dependent on the working envelope. A representative envelope can be called a drive cycle and is necessary to evaluate new solutions. This paper describes the method of producing such a cycle. A large amount of data from a field crane has been recorded for several weeks. A pattern of movement is found and used to define parameters of the drive cycle. To extract combinations of the parameters that form a good representation of the measurement data a principal component analysis is carried out. Finally, the drive cycle is defined as a sequence of trajectories with different payloads, corner points and cylinder speeds for movements between corner points. The drive cycle is tested and verified on a crane of the same model as the field crane in a laboratory environment. The methodology is found to produce a drive cycle with good variation and representation of geometrical parameters.