To thermally insulate a cylinder bore wall at a lower part of a water jacket, to suppress the circulation of cooling water in the water jacket caused by the intrusion of the cooling water into the lower part of the water jacket, to prevent the deterioration of thermal insulation performance at the lower part of the water jacket, and to prevent the deterioration of cooling performance caused by a rise of a temperature of cooling water in an upper part of the water jacket.SOLUTION: A water jacket spacer comprises a partitioning part 21 which is formed so as to surround a cylinder bore wall, partitions the inside of a water jacket into an upper part and a lower part in a bore axial line direction of a cylinder, and partitions the water jacket into an upper region and a lower region. The water jacket spacer also has an upper spacer 27 which is formed in the upper part of the partitioning part, makes cooling water circulate to the upper region in the upper part rather than the partitioning part, and forms a cooling part for cooling the cylinder bore wall, and a lower spacer 29 which is annularly formed so as to surround the cylinder bore wall at the lower part of the partitioning part, suppresses the circulation of the cooling water to the lower region rather than the partitioning part, and thermally insulates the cylinder bore wall.SELECTED DRAWING: Figure 9
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