Abstract: An exhaust pressure control valve to be placed in an exhaust gas passage of a diesel engine for controlling pressure of exhaust gas to be discharged from the engine comprise: a housing formed with a main passage and a bypass passage; a main valve arranged to open and close the main passage; a bypass valve arranged to open and close the bypass passage; and an opening/closing drive device for opening and closing the bypass valve. The opening/closing drive device includes: a positive pressure actuator for opening and closing the bypass valve by pressure upstream of the main passage; and a negative pressure actuator for changing valve opening pressure of the bypass valve.

Abstract: In a sequential turbocharger system having a first stage large volume turbo-charger and a second stage small volume turbocharger, a diaphragm actuator is provided for an exhaust bypass valve bypassing a turbine of the small volume turbocharger. The diaphragm actuator has a two step characteristic in relation to an opening of the exhaust bypass valve, so that an increased speed of opening of the exhaust bypass valve is obtained when the pressure downstream of the compressor of the first turbocharger is equal to the pressure downstream of the compressor of the second turbocharger.