To produce enough NO(Nitrogen Oxide) even at the time of a low load in a diesel engine and to keep the exhaust gas at a high temperature in order to activate the catalyst for NOX.
Two or more of the following configurations are to be combined to secure and maintain a high temperature of exhaust gas: a configuration in which a multi-cylinder diesel engine 1 is equipped with a first throttle valve 7 at the entrance of an air-intake passage 6 and a second throttle valve 8 at the passage that belongs exclusively to a group of cylinders, and the exhaust valve is opened to a prescribed degree when the piston's air-intake stroke is at around the bottom dead center so that the temperature of exhaust gas may be raised; a configuration in which the second throttle valve is closed to stop sending fuel to a group of cylinders while fuel supply to the other group of cylinders is increased, thereby raising the temperature of the group's exhaust gas; a configuration in which a continuous regenerative trap 12 is installed at an exhaust pipe 10 together with a third throttle valve 13, and the throttle valve is closed to raise the temperature of exhaust gas; and a configuration in which a turbocharger with a bypass pipe and a bypass valve is installed, and the turbocharger is driven to raise the temperature of exhaust gas.
