Abstract: A method for controlling the air-fuel ratio in an internal combustion engine equipped to operate from a gaseous alternative fuel. The output signal of the engine's exhaust gas oxygen sensor is used to measure the variable engine transport delay. The variable is used to approximate a reciprocal of the delay. The reciprocal is used as a representation of the variable engine mass air-flow. A limit cycle operating in a transport-delay-oscillator mode is modified by extending the duration of the enrichment phase in proportion to the mass air-flow. The resulting exhaust gas concentrations are biased rich and thus shifted within an optimum operating window of the engine's gasoline catalytic converter. A significant reduction of unwanted exhaust emissions is observed.