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 .lamda. above 1.0, and in some cases above the upper range (.lamda.=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.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH AND DEVELOPMENT
 This invention was made with Government support under contract DE-EE0003533 awarded by the Department of Energy. The Government has certain rights in this invention.