Due to declining fossil energy resources and rising fuel prices, as well as more stringent emission guidelines mobile machine manufacturers are being forced to increase the energy efficiency of their products. Investigations concerning a valve controlled constant pressure system with an intermediate pressure rail (STEAM-system) are introduced in this thesis. The idea of this system is to keep the load torque on the internal combustion engine of mobile working machines constant, so that it is forced to operate in an energy efficient region. The additional intermediate pressure rail is used to efficiently adapt the load pressure to the constant pressure of linear actuators in a digital way. The remaining difference between supply and load pressure is throttled using independent metering edges for the STEAM-system. They support the increase of the system efficiency by avoiding throttling of the volume flow to the tank. The three pressure rails, i.e. high, medium and tank pressure, are connected to each cylinder independently using simple 2/2 way switching valves. In this manner, only a low pressure difference between supply and load pressure has to be throttled. Two algorithms are implemented to control the STEAM-system. A pressure coupled operating mode connects the three pressure rails individually to the cylinder in such a way that only a minimum pressure difference has to be throttled. The volume flow coupled mode is enabled, when only one actuator is connected to a pressure rail. This mode corresponds to a displacement control. Using system simulation the system behaviour is investigated regarding the switching between pressure rails. The second topic of this thesis is a systematic research of different possibilities to recapture the potential energy of an excavator boom in combination with the STEAM-system. As a third topic, design and functionality of a Digital Flow Pump (DFP) is introduced. Several fixed displacement pumps of different sizes are mechanically coupled. The single output volume flows can be individually switched on and off via simple 2/2 way switching valves. They can be combined to one oil flow. The last topic of this thesis is an efficiency analysis of the STEAM-system and its modifications in comparison to a Load-Sensing system using two full load and two partial load cycles.