The article addresses some questions concerning the exhaust gas carbon dioxide emissions as a result of applying different kinds of engine fuels. The current exhaust emission regulations are provided in the article together with the new standards that have been proposed. In addition, the paper discusses some present methods applied to decrease carbon dioxide emissions from combustion engines, including the use of new fuels. The following part of the article provides a calculation procedure of carbon dioxide emissions and proposes a new comparative method of evaluating carbon dioxide emissions with respect to different fuels. The criterion for the comparison is the value of energy transfer as an air-fuel mixture to the engine. This coefficient of carbon dioxide emissions allows a comparison between emissions in the case of various fuels with reference to gasoline and to draw some conclusions.