the left by the elastic force of the coil spring 158 this causes the piston rod 154 to be fully protruded out of the housing 152 such that the length u201c b u201d is shortened to cause the second link 140 to sensitively rotate in a counter clockwise direction in response to the rotation of the first link 130 in a clockwise direction during the intake stroke of the engine accordingly the exhaust valve 120 opens so as to let in a relatively large amount of the exhaust gas remaining in the exhaust manifold it is preferred that a ratio among the distances u201c a u201d between the second pivot 142 and the contact point of the second link 140 with the upper end of the exhaust valve 120 u201c b u201d between the second pivot 142 and the contact point of the second link 140 with the distal end of the piston rod 154 u201c c u201d between the contact point of the second link 140 with the distal end of the piston rod 154 and the first pivot 132 of the first link 130 and u201c d u201d between the first pivot 132 and the contact point of the first link 130 with the upper end of the intake valve 110 is 1 1 1 8 when the air temperature from the air port 160 is higher than a first predetermined threshold temperature i e when the piston rod 154 is fully withdrawn into the housing 152 such.