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Considering the current environmental restrictions, particularly those imposed on fossil fuel exploitation, hydrogen stands out as a very promising alternative for the power and transportation sectors. This paper investigates the effects of the employment of hydrogen in a K9K automotive diesel engine. Experiments were conducted at a speed of 2000 min−1 with various engine load levels of 40%, 55%, 70%, and 85%; several quantities were monitored to evaluate the performance with hydrogen use in terms of brake-specific energetic consumption (BSEC), fuel economy, maximum pressure, and heat-release characteristics. It was found that at 55% engine load, the engine efficiency increased by 5.3% with hydrogen addition, achieving a diesel fuel economy of 1.32 kg/h. The rate of increase of the peak pressure and maximum pressure started to increase as a consequence of the higher fuel quantity that burned in the premixed combustion phase, while still remaining within reliable operational limits. The accelerated combustion and augmented heat release rate resulted in a combustion duration that was reduced by 3° CA (crank angle degree), achieving a mass fraction burned percentage of 10% to 90% earlier in the cycle, and the combustion variability was also influenced. Hydrogen use assured the decrease of CO2, HC, NOx, and smoke emission levels in comparison with classic fueling.