Scotch yoke engine

A scotch yoke fluid device which includes:

• a crank including a big end having an axis which orbits about a main axis for the crank;
• connecting means mounted on the big end axis;
• at least one piston mounted for reciprocal motion in a cylinder along a piston axis, the piston having a cross-sectional area perpendicular to the piston axis, the piston having guide means including a linear surface transverse to the piston axis, the guide means engaging engagement means on the connecting means;
• and
• at least one restraining means for constraining the piston to move along the piston axis;
• wherein the piston guide means bisects the piston cross-sectional area and at least part of each restraining means is located within a volume defined by the piston cross-sectional area projected along the piston axis, but is not located along the center line of the bisection formed by the piston guide means,
• characterised in that at least one of:
• i) the or at least one of the restraining means;
• and
• ii) a constraining means for the or at least one of the restraining means always extends into a volume defined by the swept area of the crank projected along the main axis of the crank.