SPARK IGNITION INTERNAL COMBUSTION ENGINE

A spark ignition-type internal combustion wherein a first mechanical compression ratio control and a second mechanical compression ratio control are alternatively carried out, the first mechanical compression ratio control makes the mechanical compression ratio in the low engine load side operation area higher than that in the high engine load side operation area to increase the expansion ratio in the low engine load side operation area, the second mechanical ratio control makes the expansion ratio in the low engine load side operation area lower than that in the low engine load side operation area made by the first mechanical compression ratio control and a boost pressure control increases the boost pressure like a step change when the engine load becomes a set engine load in the low engine load side operation area or higher, is provided. The set engine load of the boost pressure control when the second mechanical compression ratio control is carried out is set higher than that when the first mechanical compression ratio control is carried out.