The paper presents results of SI engine tests, carried on for different gaseous fuels. The analysis carried out made it possible to define the correlation between fuel composition and engine operating parameters. Tests cover various gaseous mixtures: methane with hydrogen and LPG with DME. The first group, considered as low carbon content fuels, can be characterized by low CO2 emissions. The flammability of hydrogen added in those mixtures realizes the function of combustion process activator and improves the energy conversion. The second group of fuels is constituted by LPG and DME mixtures. DME mixes perfectly with LPG and differently than in the case of other hydrocarbon fuels also consisting of oxygen, which makes the stoichiometric mixture less oxygen demanding. In the case of this fuel, improvement in engine volumetric and overall engine efficiency has been noticed, compared to LPG. During the tests, standard CNG/LPG feeding systems have been used, which underlines the utility value of the research.