Perspectives of usage natural gas in feeding systems of internal combustion engines and development of electronic control systems have resulted in survived interest in dual fuel system of engine feeding. Compression ignition, dual fuel engine enables to preserve majority of positive features of compression ignition engine and shows many advantageous features comparing with gaseous engine with spark ignition system. The paper shows an impact of gaseous mixture composition on operational parameters of the engine, with special consideration of combustion parameters and toxicity of exhaust gases. It has been confirmed that in range of gaseous mixture composition change in scope of λo = 1.4–6.0 worsening of combustion parameters is negligible and can be accepted for medium size traditional engines. Simultaneously, in spite of implementation of traditional injection system, leaning of the mixture up to λo < 6.0 enables significant change of engine load. Limitation of leaning of the mixture up to λo < 4.5, restricts harmful phenomena connected with combustion of lean gaseous mixtures. It requires, however, fuel injection electronic systems such as common rail. Results presented in the paper can be useful in adaptations of compression ignition engine to gaseous feeding and in stage of development of control systems to such engines.