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Abstract: The investigation presented in this paper concerns both pure hydrogen combustion under HCCI (homogeneous charge compression ignition) conditions and hydrogen–diesel co-combustion in a compression ignition (CI) engine. The investigation on the simultaneous combustion of hydrogen and diesel fuel was conducted with various hydrogen doses in the range from 0% to 17% with respect to energy percentage. With hydrogen of 17% the hydrogen–diesel–air mixture was stoichiometric and provided favorable conditions for generating combustion knock. Small amounts of hydrogen (about 5%) when added to a diesel engine shorten the diesel ignition lag and, in this way, decrease the rate of pressure rise. It provides better conditions for soft run of the engine and can increase engine durability. The final conclusions concerning hydrogen impact on combustion knock intensity, mass fraction burned (MFB) and heat release rate of the engine are detailed. [Copyright &y& Elsevier]