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The present study experimentally investigated cyclic variation of combustion characteristics of a diesel engine with hydrogen added to the intake air in detail. As the result, there were three ignition modes: (1) hydrogen ignition mode, (2) hydrogen-assisted ignition mode, and (3) diesel-fuel ignition mode. Ignition timing fluctuated from cycle to cycle in each ignition mode and between one ignition mode and another mode. As the coolant temperature was increased, the number of cycles in diesel-fuel ignition mode decreased, and indicated thermal efficiency and cyclic variation was improved. In the case with the blow-by gas introduced to intake port, preflame reaction of blow-by gas first occurred, ignited hydrogen, and then diesel-fuel was ignited by hydrogen combustion in hydrogen ignition mode and hydrogen-assisted ignition mode.