The addition of hydrogen to diesel engines is increasingly being seen as a promising interim solution to reduce fossil fuel consumption and carbon emissions without changing the basic architecture of the compression-ignition engine. This study experimentally evaluated the effect of hydrogen addition on the combustion, performance and emissions characteristics of a single-cylinder AVL 5402 Common Rail diesel engine when hydrogen was supplied via the intake manifold and diesel injection parameters were kept constant. The tests were performed at engine speeds of 1200, 1500, 2000 and 2500 min⁻¹ and loads of 5, 10, 15, 20 and 25 Nm, using a hydrogen mass replacement level of up to 10%. It was found that the addition of hydrogen in many modes increased the maximum cylinder pressure, accelerated the pressure rise, brought the pressure peak earlier and increased engine power, especially at higher loads and speeds. At the same time, fuel consumption decreased in all modes, and CO₂ emissions decreased by 3–22.7%. However, it was found that the addition of hydrogen usually increased NOx emissions; in some modes, it increased particulate and hydrocarbon emissions and extended the total combustion duration due to the reduced oxygen concentration and the formation of a richer mixture. The results showed that the effect of hydrogen on the operation of a diesel engine is strongly dependent on the load and speed; therefore, the benefits of its application can be achieved only under carefully coordinated operating conditions that ensure a favorable compromise between combustion intensification, fuel economy and emission control.