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The main objective of this study was to examine impact of hydrogen addition to the compression ignition engine fueled with either rapeseed methyl ester (RME) or 7% RME blended diesel fuel (RME7) on combustion phases and ignition delay as well as smoke and exhaust toxic emissions. Literature review shows in general, hydrogen in those cases is used in small amounts below lower flammability limits. Novelty of this work is in applying hydrogen at amounts up to 44% by energy as secondary fuel to the compression ignition engine. Results from experiments show that increase of hydrogen into the engine makes ignition delay shortened that also affects main combustion phase. In all tests the trends of exhaust HC and CO toxic emissions vs. hydrogen addition were negative. The trend of smokiness decreased steadily with increase of hydrogen. Amounts of hydrogen addition by energy share were limited to nearly 35% due to combustion knock occurring at nominal load. • Hydrogen as secondary fuel for the diesel engine. • Main combustion phase and ignition delay shorten with hydrogen addition. • Indicated thermal efficiency stays not changed remarkably. • Hydrogen addition decreases smokiness, unburnt hydrocarbons and carbon monoxide. • Knock occurred at hydrogen addition over 35% by energy. [ABSTRACT FROM AUTHOR]