The aim of the study is to evaluate the influence of hydrogen addition on the combustion process, the specific fuel consumption, brake thermal efficiency and emissions, especially particle emissions in the range 5.6-560 nm in size distribution and number of a diesel engine. The tests were performed on a 2.0 TDI diesel engine VW Euro 4, in nine stable conditions (2000, 2500, and 3000 min (1) at 15%, 30% and 45% engine load) with 0% of H-2 and substituted 25% of energy from diesel fuel by H-2. Experimental result shows that if we substitute 25% of the energy released in the combustion process with energy from hydrogen, the rate of decrease in the emission of CO2 reaches up 22% for a load of 15% and 2500 min (1), the expected 25% reduction is not achieved due to a slight decrease in brake thermal efficiency in many of the conditions experienced. No significant increase of NOx is observed in any condition. The number of particles decreased up to 63% when engine works at 3000 min (1) and 15% of maximum engine torque with hydrogen injection, in relation to the engine operating with commercial diesel. Moreover, the 25% of diesel fuel substitution by hydrogen causes a reduction in the maximum in-cylinder pressure, decreasing up to 16% at most demanding load condition (3000 min (1) with 45% of maximum engine torque). This is due to the water generated during hydrogen oxidation, which absorbs heat to evaporate and cause a temperature and in-cylinder pressure decrease in the combustion process, that ensures the physical motor integrity. (C) 2017 Elsevier Ltd. All rights reserved.