A diesel-fueled homogeneous charge compression ignition (HCCI) combustion technique with the intake air preheating has been investigated in a single cylinder optical engine. Difficulty in vaporization of a diesel fuel is the main source of low power density of HCCI engine. The auxiliary heating of the intake air could be a solution of this problem. The fuel injection strategy was widely applied. First, the single injection was executed in an extremely early timing as the intake air temperature was varied. A two-stage injection strategy was also applied as the main injection to achieve premixing with air and the second injection to promote the ignition. The optimal timing of the main injection was suggested from the result of combustion images. The optimal second injection timing was also suggested with variation of an intake air temperature.
Results showed that the heat release was increased but the ignition timing was advanced as intake air was preheated. The main injection timing should be advanced earlier than BTDC 100°CA for non-luminous combustion. The second injection timing should be advanced as the ignition timing with high intake air temperature was advanced for maintaining ignition promotion.