Abstract The present research study the emissions, the engine performance, and the maximum substitution of diesel by hydrogen in a turbocharged four-cylinder direct injection diesel engine. Moreover, the relationship between the hydrogen flammability limit, engine knock and hydrogen energy substitution was explored. Measurements were done at three engine loads (30%, 60%, 100%) while maintaining constant the engine speed (2400 rpm) whereas the engine knock was detected through an in-house non-invasive system. According to the experimental measurements, backfire took place at hydrogen concentration higher than the downward flammability limit. The maximum hydrogen energy substitutions were 80%, 60%, and 40% corresponding to engine loads of 30%, 60%, and 100%, respectively. The maximum reduction of diesel consumption was 54.2% with respect to 100% diesel operation at 30% of engine load and 80% of hydrogen energy substitution. Brake fuel conversion efficiency decreased as hydrogen addition increased for all the engine loads tested. This decrease may be mainly caused by the steam formation during the hydrogen combustion. Brake specific CO2 carbon dioxide emissions were reduced in most cases whereas the effect on brake specific NO emissions was dependent on engine load. At low engine load, a reduction in NO emissions was observed as hydrogen addition increased. Opacity was reduced in all cases and demonstrated the instability of operation near to the backfire region at 100% of engine load.