Exhaust gas recirculation system having means to estimate actual recirculation rate based on intake and exhaust gas temperatures

A method of estimating the exhaust gas recirculation rate in an internal combustion engine having a recirculation passage for recirculating a portion of the exhaust gas from the exhaust line to the induction passage of the engine, so that the recirculated exhaust gas is mixed with a fresh gas which is either air or an air-fuel mixture flowing through the induction passage at a section where the recirculation passage joins the induction passage, the recirculation rate being defined by the ratio of the mass of the recirculated exhaust gas per unit time to the mass flow rate of said fresh gas in the induction passage, the method comprising the steps of:

• measuring the respective temperatures of said fresh gas, the recirculated exhaust gas and a resulting mixture of said fresh gas and the recirculated exhaust gas;
• and computing the recirculation rate by the following equation, ##EQU6## where q is the mass of the recirculated exhaust gas per unit time, Q is the mass flow rate of said fresh gas at said section of the induction passage, T is gas temperature, C is specific heat, and the subscripts a, e and m represent said fresh gas, recirculated exhaust gas and a resulting mixture, respectively.