As is generally known, in the case of a turbocharger driven by exhaust gas of an internal combustion engine, a delay of response to a rise in supercharging pressure, commonly called “turbo lag” takes place in an acceleration-transient state from a non-supercharged state to a supercharged state, on an acceleration condition accompanying a rise in required load for the internal combustion engine. Patent document 1 discloses a technology in which a turbo lag is suppressed by the use of a compression ratio changing mechanism that can change an engine compression ratio. In the above-mentioned document, in the presence of an acceleration requirement that the influence of a turbo lag becomes remarkable, the compression ratio is changed to a compression ratio lower than a reference compression ratio at which a prescribed thermal efficiency can be achieved, thereby increasing exhaust energy and consequently suppressing the turbo lag.
On the other hand, Patent document 2 discloses a technology in which an engine compression ratio is set to raise a thermal efficiency as much as possible during a time period that a turbo lag may occur, and therefore torque is increased even in a transient state wherein the turbo lag occurs.