Method to estimate NO2 concentration in an exhaust gas of an internal combustion engine

A method is provided to estimate NO2 concentration in exhaust gas flowing through an exhaust pipe 30 of an internal combustion engine. The exhaust pipe 30 has a first section 30A and a second section 30B, 30C, and is equipped with an aftertreatment device 31, 32 located between the first section 30A and the second section 30B, 30C, and the method includes, but is not limited to determining a parameter related to a NO2 concentration of exhaust gas flowing in the first section 30A, determining an oxidization index expressive of a rate of NO contained in the exhaust gas which oxidizes into NO2 inside the aftertreatment device 31, 32, and a reduction index expressive of a rate of NO2 contained in the exhaust gas which is reduced into NO inside the aftertreatment device 31, 32, and calculating a parameter related to a NO2 concentration in the second section 30B, 30C as a function of the parameter related to a NO2 concentration in the first section 30A and of the oxidization index and of the reduction index of the