A METHOD OF OPERATING A FOUR STROKE INTERNAL COMBUSTION ENGINE SYSTEM

The invention relates to a method of operating a four stroke internal combustion engine system (), the engine system () comprising:—a four stroke internal combustion engine () provided with an intake duct (),—a turbo compressor (″) arranged to compress intake flow in the intake duct (), and—a displacement machine () provided in the intake duct () downstream the turbo compressor (″), wherein the displacement machine () is arranged to displace intake flow from an inlet to an outlet thereof. The method comprises the step of: operating the displacement machine () in a first mode where a pressure ratio (PR) given by a pressure at the outlet of the displacement machine () divided by a pressure at the inlet of the displacement machine () is substantially equal to 1. The invention also relates to a four stroke internal combustion engine system arranged to be operated by the above method.