System and method for determining the mass fraction of fresh gas in the intake manifold of an internal combustion engine of a motor vehicle

System for determining the mass fraction of fresh gas in the intake manifold of an internal combustion engine of a motor vehicle, equipped with a turbocharger and at least one partial exhaust gas recirculation circuit at low pressure connected between the exhaust line and the compressor inlet, the system comprising: a subtractor (12) receiving as input a value of the mass fraction of fresh gas in the exhaust manifold as well as a value of an observer of the mass fraction of fresh gas in the exhaust manifold, a means for determining (13,14,15,16) the vector of time derivatives of the observed variables as a function of a predetermined model, an integration means (17) of the signal received from the means for determining (13,14,15,16) the vector of time derivatives of the observed variables, and a third calculation means (18) of the value of the mass fraction at the level of the intake manifold as a function of the vector of the observed variables.