Abstract The accuracy of cylinder air charge direct measurement affects air-fuel ratio (AFR) control performance and exhaust emissions of internal combustion engines, especially during the transient response. In lean combustion control, the combustion variation is higher than the normal combustion. Accurate air charge estimation is crucial in this combustion mode. Therefore, the methodology for the improvement of cylinder air charge estimation is required. With the effective air charge estimation method, the feedforward control of fuel injection can be precisely performed. This paper studies the performance of the adaptive air charge estimation algorithm on the lean operation mode. The results show that the applied algorithm is robust and has high efficiency for the cylinder air charge estimation.