Abstract: Intake air is severely restricted during cold start of a diesel engine to provide high engine-out exhaust gas temperatures and quickly heat exhaust treatment devices. Cylinder pressures are maintained substantially below the pressure of the ambient atmosphere during a principal portion of the intake stroke of each combustion cycle to promote rapid fuel vaporization and higher combustion temperatures. The engine is operated in a premixed charge compression ignition combustion mode to provide a high engine-out exhaust temperature, stable combustion, and low emissions during the cold start period.