A rotary expansible chamber device, comprising: an outer rotary component having a machine axis; an inner rotary component located relative to said outer rotary component so as to define a fluid zone (116, 224) between said inner and outer components, said fluid zone (116, 224) comprising a plurality of fluid volumes for receiving a working fluid during use, wherein said inner and outer rotary components are designed and configured to engage one another so that, when at least one of said inner and outer rotary components is continuously moved relative to the other and about an axis parallel to said machine axis, said inner and outer rotary components continuously define at least one shrinking arc, at least one expanding arc, and at least one zero volume arc within said fluid zone (116, 224); a first working-fluid port in fluid communication with said fluid zone (116, 224) and having a first circumferential extent and a first angular position about said machine axis; a first mechanism designed and configured to controllably change at least one of said first circumferential extent and said first angular position; a second working-fluid port in fluid communication with said fluid zone (116, 224) and having a second circumferential extent and a second angular position about said machine axis; a second mechanism designed and configured to controllably change at least one of said second circumferential extent and said second angular position; characterized by an arc of inaccessibility over which said fluid volumes do not have access to any working fluid port, including said first and second working-fluid ports, said arc of inaccessibility having a circumferential location and circumferential size, wherein changing any one of said first circumferential extent and said first angular position with said first mechanism changes at least one of said circumferential location and said circumferential size of said arc of inaccessibility, and changing any one of said second circumferential extent and said second angular position with said second mechanism changes at least one of said circumferential location and said circumferential size of said arc of inaccessibility. an outer rotary component having a machine axis; an inner rotary component located relative to said outer rotary component so as to define a fluid zone (116, 224) between said inner and outer components, said fluid zone (116, 224) comprising a plurality of fluid volumes for receiving a working fluid during use, wherein said inner and outer rotary components are designed and configured to engage one another so that, when at least one of said inner and outer rotary components is continuously moved relative to the other and about an axis parallel to said machine axis, said inner and outer rotary components continuously define at least one shrinking arc, at least one expanding arc, and at least one zero volume arc within said fluid zone (116, 224); a first working-fluid port in fluid communication with said fluid zone (116, 224) and having a first circumferential extent and a first angular position about said machine axis; a first mechanism designed and configured to controllably change at least one of said first circumferential extent and said first angular position; a second working-fluid port in fluid communication with said fluid zone (116, 224) and having a second circumferential extent and a second angular position about said machine axis; a second mechanism designed and configured to controllably change at least one of said second circumferential extent and said second angular position; characterized by an arc of inaccessibility over which said fluid volumes do not have access to any working fluid port, including said first and second working-fluid ports, said arc of inaccessibility having a circumferential location and circumferential size, wherein changing any one of said first circumferential extent and said first angular position with said first mechanism changes at least one of said circumferential location and said circumferential size of said arc of inaccessibility, and changing any one of said second circumferential extent and said second angular position with said second mechanism changes at least one of said circumferential location and said circumferential size of said arc of inaccessibility.