In a conventional diesel combustion mode, one or more cylinders (12) of an engine (10) is or are fueled to cause conventional diesel combustion in each such cylinder. The resulting exhaust gas is treated by a diesel particulate filter (36) in an exhaust system (34) before exiting the exhaust system. In an alternative diesel combustion mode, such as HCCI, one or more cylinders of the engine are fueled to create in each such cylinder an in-cylinder fuel-air charge that ignites by auto-ignition as the charge is increasingly compressed. Also a bypass valve (38) is opened so that the resulting exhaust gas bypasses the DPF as the exhaust gas passes through the exhaust system.