A first pressure upstream of a throttle is received. A temperature upstream of the throttle is received. A throttle position is received. A second pressure at a convergent end of a convergent nozzle positioned downstream of the throttle is received. An air flow is estimated based on the received first pressure, the received temperature, the received throttle position, and the received second pressure, wherein estimating the air flow includes determining one or more models to use for calculating air flow based on the throttle position, the models including a throttle flow model, a convergent nozzle flow model, or both.