The aim of this study was to perform a comparative analysis of the unit gas emission value in the exhaust of a dual fuel diesel engine. The results of the effects of a diesel engine’s applications in biogas plants and the method for calculating mass gas emissions per unit of produced electricity are shown. The test was performed using a two-cylinder, naturally aspirated, liquid-cooled diesel engine. The diesel engine powered a generator connected to the grid. The engine was fed with liquid fuels—waste cooking oil methyl ester (UCOME) and diesel fuel (DF)—and with a gas fuel, biogas (BG). The engine ran at a constant rotational speed (2000 rpm ± 30 rpm) with variable load. The gas analyzer measured the amount of CO, NO, NO2, and PM (particulate matter) in exhaust gas. This gas content share was then converted to mass per engine generated energy unit. This experiment showed the effect of BG introduced to the intake manifold on fuel combustion, as well as an increase in CO and NO2 emission and decrease in NO and PM. In terms of dependence of exhaust emissions on the type of liquid fuel used, the use of UCOME as opposed to diesel fuel resulted in PM reduction and increase of NO emissions.