Reduction of greenhouse gases emissions into the atmosphere, as well as increasing the share of renewables in the overall energy balance, forces the search for new, alternative energy sources. One of the fuels, which presents high potential for combustion engines are biomethane or biogas, with methane as the main flammable component. Biogas can be obtained from different products and using a variety of technologies which results in its wide availability and relatively easy manufacture. The largest sources of biogas can be animal farms or sewage treatment plants and waste dumps in which significant quantities of biogas are obtained as a result of naturally occurring processes. Biogas can also be obtained from processing of energy crops or waste processing in agricultural, food and meat processing plants. In this article, the possibility of using biogas as a fuel for CI engines has been examined. In such engine, combustion of biogas (methane) requires the use of dual fuel supply system, in which in addition to methane, liquid fuel is injected into the combustion chamber, in order to initiate the self-ignition of gaseous fuel. The paper presents exemplary results of the impact of the proportion of different fuels and biogas composition on the efficiency of the engine work cycle.