Internal combustion engine and control system

The exhaust gas discharged from the combustion chamber 20 flows through the exhaust flow passage 42 of the internal combustion engine 1. The reducing agent supply unit supplies the reducing agent to the exhaust flow passage 42 via the nozzle 61 attached to the exhaust flow passage 42 and mixes it with the exhaust gas. The reactor 66 brings the exhaust flowing in from the exhaust flow passage 42 into contact with the catalyst to carry out the NOx removal treatment. The exhaust gas that has passed through the reactor 66 is led to the flue 81. The bypass passage 681 bypasses the reactor 66 and connects the exhaust passage 42 and the flue 81. The bypass valve 682 opens and closes the bypass passage 681. When the valve opening process of the bypass valve 682 is detected, the supply control unit controls the reducing agent supply unit to stop the supply of the reducing agent to the exhaust flow path 42. Thereby, the inflow of the reducing agent to a part downstream of the reactor 66 can be suppressed. A control sys