In article, the solution of problem of the combined use of fuel in operation is considered. It is shown that during the operation of the gasoline automobile engine the requirement to octane number of fuel changes depending on engine operation mode: at increase in rotary speed the required octane number of fuel decreases, and at increase in loading increase in octane number of fuel is necessary. The combined use of low-bracket gasoline with high-octane liquefied gas is given. The condition supporting necessary octane number of fuel on various modes for ensuring without detonation work and optimization of combustion procedure is defined. At all engine operating modes in operation on combined fuel, only a high-octane component should be supplied. To control the change in the ratio of the supplied components of the combined power system, it is necessary and sufficient to use any combination of the following parameters: – vacuum in the intake manifold; – is the engine speed; – is the degree of opening of the throttle valve of the carburetor, or others, which are functionally dependent on the above. As a result, the feasibility of choosing one or another option is determined by the design features of the fuel combination implementation system