In general most engine models for control applications have been constructed using regressions fitting and measured engine data. Such techniques have also been used to model the dynamic performance of engines. Unfortunately regression equation models are very complex and do not show directly the physical reality from which they emerge. This has for example made it impossible to write down explicitly the dymanic equations for, for example, the air exchange process in an SI engine in any form other than as the manifold pressure state equation.In recent a publication a Mean Value Engine Model (MVEM) has been constructed for an SI engine which is physically based and which has a simple physical form which can be immediately understood and manipulated. In this paper this simple model is mathematically reformulated in such a way that it describes directly not only the dynamic behavior of the manifold pressure but also the port and throttle air mass flows directly, without involving other variables than engine input variables. In this way a new family of nonlinear closed loop observers is derived which is suitable for accurate air/fuel ratio control which immediately can be used with not only Manifold Absolute Pressure (MAP) sensors, but also Mass Air Flow (MAF) sensors and a newly developed port air mass flow sensor (or all possible combinations of them). Accurate air/fuel ratio control has been demonstrated experimentally with a number of the members of this family with different sensors during both steady state and transient operation. It is thought that such built-in redundancy will be more and more important as OBD legislation becomes tighter and more far reaching.