Abstract: A method for switching between low- and high-dilution combustion modes in an internal combustion engine having an intake passage with an exhaust-driven turbocharger, a crankshaft-driven positive displacement supercharger downstream of the turbocharger and having variable boost controllable with a supercharger bypass valve, and a throttle valve downstream of the supercharger. The current combustion mode and mass air flow are determined. A switch to the target combustion mode is commanded when an operating condition falls within a range of predetermined operating conditions. A target mass air flow to achieve a target air-fuel ratio corresponding to the current operating condition and the target combustion mode is determined. The degree of opening of the supercharger bypass valve and the throttle valve are controlled to achieve the target mass air flow. The amount of residual exhaust gas is manipulated.

Abstract: An internal combustion engine includes a turbocharger and a selectively operable positive displacement supercharger downstream of the turbocharger in the intake path. An input shaft of the supercharger is mechanically coupled to and driven by a piston-driven crankshaft. A bypass valve is configured to selectively bypass at least a portion of the air from the turbocharger air outlet around the supercharger. Power and fuel consumption are optimized by operating the supercharger to dilute the fuel-air mixture to achieve an excess air factor ? above 1.0, and in some cases above the upper range (?=1.3) of normal stoichiometric operation. The engine can operate in a lean low temperature combustion mode (e.g., HCCI) with the supercharger operating, and in a stoichiometric spark ignition combustion mode with the supercharger bypassed.