A technique to estimate the in-cylinder air-fuel ratio (AFR) of a spark-ignition engine has been developed. This method uses cylinder pressure and temperature measurements taken during a single engine cycle to calculate the ratio of molecular weights before and after combustion (G-ratio). The G-ratio is uniquely related to the AFR of the cylinder charge. This method is suitable for use in closed-loop AFR control strategies.A G-ratio based AFR estimator was constructed using experimental measurements of cylinder pressure and intake and exhaust manifold gas temperatures. Its AFR estimation accuracy over the engine's steady-state operating range was 4.85% RMS based on a ten cycle average. The accuracy will improve substantially if in-cylinder temperature measurements can be made.An engine simulation was used to predict that a G-ratio based AFR estimator using in-cylinder temperature measurements would be insensitive to inlet pressure, inlet temperature, heat transfer and compression ratio variations. The estimator would have deterministic sensitivities to unknown EGR ratio and fuel H:C ratio. However, if an engine's EGR ratio characteristics and the fuel's H:C ratio are known, the AFR estimator can be applied without requiring explicit knowledge of the engine's construction or operating condition.G-ratio based AFR estimation will provide higher bandwidth AFR control than exhaust sensor based techniques and will require less calibration effort than cylinder pressure-only based AFR estimators.