An exhaust system of an internal combustion engine, comprising: - an engine exhaust conduit, - an aftertreatment system including a main exhaust conduit connected to the engine exhaust conduit and at least one catalytic converter, wherein the exhaust system further comprises - a bypass conduit connected to the engine exhaust conduit upstream of the aftertreatment system, the bypass conduit allowing exhaust gases to bypass the aftertreatment system, - an adjustable bypass port valve positioned in an inlet to the bypass conduit by means of which a flow of exhaust gases carried via the bypass conduit can be controlled, - an adjustable main port valve by means of which a flow of exhaust gases carried via the aftertreatment system can be controlled, - an upstream temperature sensor configured to measure an upstream temperature (T_up) of the exhaust gases immediately upstream of the at least one catalytic converter, wherein a control system is further configured to control the main port and the bypass port based on signals from said upstream temperature sensor, characterized in that - the control system configured to determine an amount of hydrocarbons currently stored in the catalytic converter based on the upstream temperature (T_up) and on data relating to current operating conditions of the internal combustion engine and control the main port valve and the bypass port valve in independence of each other to allow exhaust gases to bypass the aftertreatment system if an amount of hydrocarbons stored in the cataylic converter is such that this is suitable in order not to cause choking or degeneration of the flow of exhaust gases, or an exothermic reaction, based on at least said amount of hydrocarbons and based on the upstream temperature (T_up) within the aftertreatment system. - an engine exhaust conduit, - an aftertreatment system including a main exhaust conduit connected to the engine exhaust conduit and at least one catalytic converter, - a bypass conduit connected to the engine exhaust conduit upstream of the aftertreatment system, the bypass conduit allowing exhaust gases to bypass the aftertreatment system, - an adjustable bypass port valve positioned in an inlet to the bypass conduit by means of which a flow of exhaust gases carried via the bypass conduit can be controlled, - an adjustable main port valve by means of which a flow of exhaust gases carried via the aftertreatment system can be controlled, - an upstream temperature sensor configured to measure an upstream temperature (T_up) of the exhaust gases immediately upstream of the at least one catalytic converter, wherein a control system is further configured to control the main port and the bypass port based on signals from said upstream temperature sensor, characterized in that - the control system configured to determine an amount of hydrocarbons currently stored in the catalytic converter based on the upstream temperature (T_up) and on data relating to current operating conditions of the internal combustion engine and control the main port valve and the bypass port valve in independence of each other to allow exhaust gases to bypass the aftertreatment system if an amount of hydrocarbons stored in the cataylic converter is such that this is suitable in order not to cause choking or degeneration of the flow of exhaust gases, or an exothermic reaction, based on at least said amount of hydrocarbons and based on the upstream temperature (T_up) within the aftertreatment system.