In some examples, a method for estimating the cetane number of a fuel includes receiving current engine operating condition measurements from a compression ignition (CI) engine using the fuel, and determining a value for a current engine performance indicator. The method further includes determining a projected engine performance indicator value for the CI engine based on the current estimated cetane number, current engine control signals and the current engine operating condition measurements. The method also includes determining that a difference between the value of the current engine performance indicator and the projected engine performance indicator value is greater than a threshold amount. If the difference is greater than the threshold amount, the method also includes determining a new estimated cetane number.