New diesel combustion modes face difficult challenges with respect to engine-out emissions and transient behavior. Transitions between lean conventional diesel combustion and lean, premixed, low-temperature diesel combustion are investigated with an automotive diesel engine. Effects of fuel pressure on transitional cycles are investigated. Cycleby- cycle heat release analysis is performed and an exhaust mass flow model is used to obtain cycle-averaged NO concentrations. The behavior of combustion progression and NO emissions during the transitions is discussed. Observed cool-flame behavior is identified and explained.