Nowadays, the even increasing stringent environmental legislations have promoted interest in alternative fuels for internal combustion engines. Particularly, hydrogen is becoming a promising fuel due to its high specific energy and low emissions production. Environmentally, the main disadvantage of hydrogen is the high level of nitrogen oxides (NO.) which produces. In this regard, this work proposes a NO reduction method which consists on direct injection of ammonia (NH3) into the combustion chamber. A numerical model validated with experimental measurements was carried out to analyze emissions and brake specific consumption in a commercial engine operating with diesel -hydrogen blends. Comparing to diesel operation, a 10% hydrogen content increases a 5.3% the peak pressure and 5.7% the maximum temperature. The CO2, CO and HC emissions are reduced but NO. emissions increase up to 18.3%. Several injection instants and ammonia flow rates were analyzed, obtaining more than 70% NO. reductions with a negligible effect on other emissions and brake specific consumption. It was found that the start of ammonia injection is too critical since the maximum NO. reduction takes place when the temperature is around 1200 K. The NO. reduction increases with the ammonia flow rate but an excessive quantity of ammonia can lead to un-reacted ammonia slip to the exhaust. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.