Described here are systems and methods for controlling IC engines. In one aspect, a method for controlling a fuel processor is provided, the method including i) determining a temperature of an exhaust flow to the fuel processor, the fuel processor including a fuel processor catalyst; ii) determining a concentration of O2 in the exhaust flow upstream of the fuel processor catalyst; iii) determining a rate of the exhaust flow; and iv) adjusting a fuel flow rate to the fuel processor based on i), ii), iii) and a heat capacity value associated with the fuel processor. In other aspects, a system comprising logic operable to control a fuel processor is provided.