An experimental investigation was conducted on the combustion, performance, regulated and unregulated emissions of a diesel engine with naturally aspirated hydrogen at the engine speed of 1800 rev min(-1) under five engine loads. Hydrogen was added to provide 10%, 20%, 30% and 40% of the total fuel energy. Improvement of engine performance can be achieved at medium to high loads. At 90% load, abnormal combustion occurs with more than 30% hydrogen addition as indicated by the drastic increase of peak heat release rate, shortened ignition delay and combustion duration. CO/CO2 and seven kinds of unregulated emissions (except for formaldehyde) can be efficiently reduced. More than 30% of hydrogen addition can reduce the HC emission at low to medium loads. The percentage reduction of NO emission is engine load dependent, being positive at low loads and negative at high loads. Drastic increase of NO2 is observed at low to medium engine loads. Reductions on particulate mass concentration, particle number concentration and diesel thermal pyrolysis intermediates (olefins and benzene) are associated with the inhibition of polycyclic aromatic hydrocarbons (PAHs) formation. All the unregulated emissions increase at 90% load. (C) 2014 Elsevier Ltd. All rights reserved.