The prime intention of this work is to provide a maximum replacement for diesel using hydrogen in a common rail direct injection equipped diesel engine. The experiment was conducted upto 5.2 kW brake power constant speed water-cooled engine. In the combustion chamber, diesel fuel was injected at a crank angle of 23 degrees bTDC, making it an ignitor for the premixed mixture of hydrogen and air. Hydrogen is injected at 6 different proportions ranging from 6 to 36 liter per min (LPM). The air and hydrogen gas were mixed homogeneously using the timed manifold injection technique, which was controlled through the in-house PC based data acquisition (DAQ) program developed on data factory. The electronic control unit helps to induct the hydrogen for a period of 211 degrees CA during the suction stroke. Performance, emission and combustion studies were made with the different levels of hydrogen injection, which proves that the 30 LPM of hydrogen provide the best results. Further, 30.65% improvement was achieved in brake thermal efficiency with 23.48% decreased brake specific energy consumption. This also helped to reduce the harmful emissions like CO, CO2, UHC and smoke by 22.3%, 14%, 32.74% and 43.86%, respectively. However, oxides of nitrogen emission level was increased by 7.3% compared to that of the diesel fuel at its maximum power output setting. The duration of the combustion also reduced due to the higher flame speed character of hydrogen. Thus, the overall results conclude that the addition of hydrogen improved the performance factors and reduced all the emission values of the common rail direct injection diesel engine at an optimum level of 30 LPM. (C) 2020 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.