Abstract: The film boiling state of a refrigerant is maintained for a time longer than in the prior art by t to lower the nuclear boiling start temperature to a temperature at which the material proof strength of a work exceeds thermal stress, thereby lowering the cooling rate of the work. In addition, by controlling the cooling rate in hardening, the film boiling state of the refrigerant is maintained to a temperature at which the material proof strength exceeds thermal stress, thereby cooling the work at a temperature equal to or higher than the critical cooling rate of the work.

Abstract: An engine cylinder block structure includes: a cylinder block integrally formed to include skirts projecting downwardly from both sides of the lower surface of the cylinder block; an oil pan attached to lower surfaces of both skirts of the cylinder block, the oil pan having a deep first portion on a front side and a shallow second portion on a rear side, the first and second portions being connected to each other; flanges integrally formed on lower ends of the skirts for having the oil pan attached thereto; and reinforcing mechanism for reinforcing the strength of both skirts.