In bi-fuel engines, in which thermodynamic and mechanical design concepts are entirely based on gasoline mode, torque and power values are lower in natural gas mode than gasoline one. This problem is directly related to volumetric efficiency reduction in natural gas (NG) mode. Volumetric efficiency in natural gas is lower because of gaseous form of natural gas entering the engine and differences between thermo-chemical properties of natural gas and gasoline; e.g. stoichiometric air-fuel ratio and heat of vaporization. On the other hand, natural gas stoichiometric air-fuel ratio is generally greater than gasoline one and theoretically, more air should be supplied in NG mode. Although the effect of volumetric efficiency reduction is more important than another, both of them state the amount of charged air should be increased in NG mode to prevent power fall in this mode.

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