Patent ID: 8177533

Claim:
A scroll expander comprising: a first scroll having a first scroll lap; and a second scroll having a second scroll lap that is meshed with the first scroll lap, wherein the first scroll and the second scroll form an inner wall side expansion chamber on the inner wall side of the first scroll lap and an outer wall side expansion chamber on the outer wall side of the first scroll lap, the inner wall side expansion chamber and the outer wall side expansion chamber move from the center toward the outer circumference of the first and second scrolls while increasing their volumetric capacities along with a revolution of the first scroll relative to the second scroll, the first scroll lap and the second scroll lap have such shapes that an expansion ratio of the inner wall side expansion chamber and an expansion ratio of the outer wall side expansion chamber are different from each other, the volumetric capacity of the inner wall side expansion chamber and the volumetric capacity of the outer wall side expansion chamber are equal to each other when suction is completed, and different from each other when discharge starts, along with the revolution of the first scroll relative to the second scroll, a first contact surface and a second contact surface are created at the same time in the center of the first and second scrolls, the first contact surface being a contact surface between the inner wall of the first scroll lap and the outer wall of the second scroll lap, and the second contact surface being a contact surface between the outer wall of the first scroll lap and the inner wall of the second scroll lap, and an involute step is provided on the inner wall of the second scroll lap in such a manner that a second disappearance position is displaced from a first disappearance position by a predetermined angle of more than 0 degree but less than 180 degrees or a predetermined angle of more than 180 degrees but less than 360 degrees toward a volute tongue side of the second scroll lap, the first disappearance position being a position where the first contact surface disappears after moving from the center toward the outer circumference of the first and second scrolls, and the second disappearance position being a position where the second contact surface disappears after moving from the center toward the outer circumference of the first and second scrolls.