New computational procedures are proposed for evaluating the exhaust brake specific mass emissions of each pollutant species in internal combustion (IC) engines. The procedures start from the chemical reaction of fuel with combustion air and, based on the measured exhaust raw emissions THC, $CH4,$$NOx,$ CO, $O2,$$CO2,$ calculate the volume fractions of the compounds in the exhaust gases, including those that are not usually measured, such as water, nitrogen and hydrogen. The molecular mass of the exhaust gases is then evaluated and the brake specific emissions can be obtained if the exhaust flow rate and the engine power output are measured. The algorithm can also be applied to the evaluation of air-fuel ratio from measured raw volume emissions of IC engines. The new procedures take the effects of various fuel and combustion air compositions into account, with particular reference to different natural gas blends as well as to the presence of water vapor, $CO2,$ Ar and He in the combustion air. In the paper, the algorithms are applied to the evaluation of air-fuel ratio and brake specific mass emissions in an automotive bi-fuel Spark Ignition (SI) engine with multipoint sequential port-fuel injection. The experimental tests were carried out in a wide range of steady-state operating conditions under both gasoline and compressed natural gas operations. The specific emissions calculated from the new procedures are compared to those evaluated by applying Society of Automotive Engineers (SAE) and International Standards Organization (ISO) recommended practices and the air-fuel ratio results are compared to those obtained either from directly measured air and fuel mass flow rates or from Universal Exhaust Gas Oxygen (UEGO) sensor data. The sensitivity of the procedure results to the main engine working parameters, the influence of environmental conditions (in particular the effect of air humidity on $NOx$ formation) and the experimental uncertainties are also determined.

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