Variable Displacement Multi-Stroke Motor, of the compact radial piston design was proposed in HRP20010685A2 HYDRAULIC MACHINE in 2001. Concept is based on two connected pistons where their cinematics is set by two cams that can be rotated. If they are rotated in the way that two connected pistons are synchronized max. displacement will be set. In maximal displacement 100 % flow goes in/out through distribution hole and  50% of the flow will be passed in/out through the connections between two connected pistons. To set min  displacement, actually 0 displacement; cams has to be rotated  in the way that connected pistons moves out of phase so that 100 % of flow is exchanged between connected pistons so that the output input flow is 0 meaning 0 displacement: 
Even basic concept of variable displacement is known; up to now there is no practical realization of this kind of the machine: Comparing to known designs of the fix displacement radial piston multi-stroke motor there are two aspects that has to be solved in order to have successful working designs: One aspect is to have cam profile adapted for variable displacement design. and another aspect is to design of the control system.. On image above, variable displacement control is of the type presented in patent DE10244262A1 from 2002. Technical analysis, based on experience with radial piston machines indicated that more advanced system is needed and recently, one such system is patented. together with advanced cam profile. As those two aspects are essential part that makes difference between existing designs and this new design it will not  be disclosed at moment as it can give crucial advantage for potential industrial partner.. What can bi discussed is characteristic of the new cam profile that brings important advancement over the cam profile presented in  DE10244262A1 .  it will be discussed in more details further in the text in order to show to potential industrial partners that important theoretical work is already done and that project is ready to go for TRL 3. Before explanation of the specific cinematics properties it is important to indicate that work, on this variable displacement concept, resulted with concept of improved piston lubrification indicating that nominal power, of the variable displacement as of the fix displacement radial piston multi-stroke motors; can be doubled meaning that motor can stay close or/at max power permanently:  This is additional reason for fundamental investigation of the proposed concept.
As mentioned above, some additional information will be provided, regarding new cam profile developed for variable displacement radial piston multi-stroke motor.. On image below, that is  fig 23 from patent application, one can see flow profile for the variable displacement radial piston multi - stroke motor, according to DE10244262A1, presented by the line 9. LIne 7 presents flow profile according to new approach recently patented.