Hydraulic hybrid system for a work machine and a method of controlling the hydraulic hybrid system

A method for controlling a hydraulic system of a work machine, the hydraulic system comprising:

• a main hydraulic machine connected to an output shaft of an engine of the work machine, the main hydraulic machine operable as a pump to provide power to the working hydraulics of the work machine and as a motor to regenerate power from the working hydraulics;
• a main pressure line connecting the main hydraulic machine to working hydraulics of the work machine;
• a support hydraulic machine connected to the output shaft;
• a hydraulic accumulator;
• a discharge valve connected between the accumulator and an input side of the support hydraulic machine;
• a charge valve connected between an output side of the support hydraulic machine and the accumulator to selectively allow a flow from the support hydraulic machine to the accumulator;
• a flow support valve connected between the output side of the support hydraulic machine and the main pressure line to selectively allow a flow from the support hydraulic machine to the main pressure line;
• and
• a hydraulic tank connected to the input side of the support hydraulic machine, wherein the method comprises:
• controlling the hydraulic system to operate in a flow support mode, a power boost mode or a charging mode,
• wherein the flow support mode comprises opening the flow support valve, closing the discharge valve and charge valve, and operating the support hydraulic machine as a pump to provide a hydraulic flow from the tank to the main pressure line,
• wherein the power boost mode comprises opening the discharge valve and the flow support valve and closing the charge valve, and if the pressure on the main hydraulic line is higher than the pressure of the hydraulic accumulator, operating the support hydraulic machine as a pump, and if the pressure on the main hydraulic line is lower than the pressure of the hydraulic accumulator, operating the support hydraulic machine as a motor,
• wherein the charging mode comprises closing the discharge valve and the flow support valve and opening the charge valve, and operating the support hydraulic machine as a pump to charge the hydraulic accumulator,
• further comprising controlling the hydraulic system to be in the flow support mode if an engine load is lower than a maximum load and when a requested hydraulic flow is higher than a maximum hydraulic flow of the main hydraulic machine.