2. Complex Catalytic Conversion System Meeting the future requirements on the toxic emissions of a natural gas engine is possible by using an integrated system of catalytic conversion, able to work efficiently with different air-fuel ratios. Table 2 shows the influence of air excess ratio λ in the mixture on the choice of technologies that may be used for efficient treatment of exhaust gases. Engine operation mode and air excess ratio λ determine the exhaust composition of the natural gas fueled ICE's and, consequently, the type or set of catalysts, which can be used in the process of aftertreatment. The exhaust gas treatment technologies, used for ICEs running on natural gas, may be divided into oxidation, reduction, oxidation-reduction and complex conversion systems, depending on the concentrations of toxic components, oxygen and temperature. In the reviewed version of a gas engine operating with both stoichiometric and lean mixtures a complex system of catalytic conversion is necessary. The system shall consist of a three-way catalyst to ensure treatment of gases when the engine runs with stoichiometric mixtures and a selective catalytic reduction (SCR) system of nitrogen oxides for lean-burn operation. A scheme of an integrated system is shown in Figure 1.