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 (6") arranged to compress intake flow in the intake duct, and - a displacement machine provided in the intake duct downstream the turbo compressor (6"), wherein the displacement machine is arranged to displace intake flow from an inlet to an outlet thereof, characterized in that the method comprises the step of: - operating the displacement machine in a first mode where a pressure ratio 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, wherein operating the displacement machine in the first mode is carried out in any of the following operation modes of the internal combustion engine: i) a normal steady state-like operation mode of the engine where there are no significant transients with regard to desired changes, ii) when the engine system is operated in a throttling mode where a desired magnitude of an intake flow to be fed to the engine is significantly smaller than the intake flow fed to the engine during a normal steady state-like operation mode of the engine where there are no significant transients with regard to desired changes in e.g. engine speed or load so that there are no significant changes desired with regard to magnitude of the intake flow fed to the engine, and iii) when the engine system is operated in a boosting mode where a desired magnitude of an intake flow to be fed to the engine is significantly larger than the intake flow fed to the engine during the normal steady state-like operation mode. - a four stroke internal combustion engine provided with an intake duct, - a turbo compressor (6") arranged to compress intake flow in the intake duct, and - a displacement machine provided in the intake duct downstream the turbo compressor (6"), wherein the displacement machine is arranged to displace intake flow from an inlet to an outlet thereof, characterized in that the method comprises the step of: - operating the displacement machine in a first mode where a pressure ratio 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, - operating the displacement machine in a first mode where a pressure ratio 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, wherein operating the displacement machine in the first mode is carried out in any of the following operation modes of the internal combustion engine: i) a normal steady state-like operation mode of the engine where there are no significant transients with regard to desired changes, ii) when the engine system is operated in a throttling mode where a desired magnitude of an intake flow to be fed to the engine is significantly smaller than the intake flow fed to the engine during a normal steady state-like operation mode of the engine where there are no significant transients with regard to desired changes in e.g. engine speed or load so that there are no significant changes desired with regard to magnitude of the intake flow fed to the engine, and iii) when the engine system is operated in a boosting mode where a desired magnitude of an intake flow to be fed to the engine is significantly larger than the intake flow fed to the engine during the normal steady state-like operation mode.