Hypotrochoid positive-displacement machine

A displacement device comprising:

• a housing;
• an inner rotor with an inner rotor projection number of outward-facing projections, the inner rotor being fixed for rotation relative to the housing about a first axis;
• an outer rotor with an outer rotor projection number of inward-facing projections, the outer rotor being fixed for rotation relative to the housing about a second axis parallel to and offset from the first axis;
• and the outward-facing projections of the inner rotor and the inward-facing projections of the outer rotor intermeshing, the outer rotor and the inner rotor configured to rotate at a relative ratio of rotation speeds defined by a ratio of the inner rotor projection number to the outer rotor projection number;
• the inward-facing projections of the outer rotor having inward-most tips defining, during respective rotation of the inner rotor and the outer rotor, a hypotrochoid path relative to the inner rotor;
• the inner rotor comprising tip sealing zones at tips of the outward-facing projections and trough sealing zones at troughs between the outward-facing projections, the tip sealing zones and the trough sealing zones being arranged to seal against the inward-most tips of the inward-facing projections of the outer rotor as the inward-most tips movingly trace along the hypotrochoid path during the respective rotation of the inner rotor and the outer rotor and form respective engagements with the tip sealing zones and with the trough sealing zones along the hypotrochoid path;
• and
• during at least part of each of the respective engagements with the trough sealing zones, the movingly tracing inward-most tips have the same sense as the rotation of the inner rotor;
• and
• during the entirety of each of the respective engagements of the inward-most tips of the outer rotor with the tip sealing zones, the movingly tracing inward-most tips have the opposite sense as the rotation of the inner rotor.