A vehicle includes an engine and an exhaust system which ignites fuel from fuel injectors to purify exhaust gas. An air compressor delivers compressed intake air to the engine, and a turbine energizes the compressor. A controller calculates an engine thermal efficiency value using temperature and mass flow rate values from various sensors. The controller maintains a temperature of the exhaust gas downstream of the turbine using the thermal efficiency value. A control system includes the sensors and a host machine operable for maintaining the temperature of the exhaust gas above the threshold using the thermal efficiency value. A method for maintaining the temperature of the exhaust gas includes measuring the inlet and outlet temperatures of an air intake system, measuring the mass flow rate of compressed intake air, and using the host machine to maintain the temperature of the gas using the thermal efficiency value.