HCNG is a fuel, which as a mixture of hydrogen (H₂) and compressed natural gas (CNG), combines the advantages of both gases. The addition of hydrogen shortens the combustion time, increases the flammability range and enables greater exhaust gas recirculation (EGR) while maintaining cyclic stability and low NOx emissions. As a result, HCNG can be used in most classic diesel engines. This fuel has a higher energy volumetric storage density than pure hydrogen, which reduces the need to transport large amounts of fuel. Disadvantages include the lack of appropriate infrastructure, fuel and vehicle standards and the need for additional safeguards. The aim of the study was to check the emission levels of a single-cylinder diesel engine powered by HCNG compared to conventional fuel. It has been shown that the use of HCNG effectively reduces CO emissions due to the lack of carbon in hydrogen and its high combustion rate, which intensifies the process and improves its efficiency. The higher hydrogen content further stabilizes methane combustion and increases the homogeneity of the mixture. HCNG also reduces hydrocarbon (HC) emissions, compensating for the ignition delay and slow speed of the CNG flame. At higher engine loads, these effects are more visible, leading to more efficient combustion. Particulate matter (PM) emissions are reduced due to the homogeneous mixture resulting from the high diffusivity of hydrogen. PM reduction is more visible at lower loads. Higher combustion temperatures in HCNG promote the formation of NOx, which results from the intensification of combustion by hydrogen and excess oxygen, according to the Zeldovich mechanism. By analyzing emissions, the authors showed the great potential of using HCNG in a diesel engine. At the same time, the main challenges that should be faced in order to popularize this solution were indicated.