Transient A/F Ratio Errors in Conventional SI Engine Controllers

In an earlier paper one of the authors of this paper (E. Hendricks and co-authors) treated the question of obtaining correct steady state and transient control of the air/fuel (A/F) ratio of an SI engine. This study was based in part on simulations conducted with a dynamic engine model developed earlier and in part on experimental results. The main conclusions were that conventional control strategies (Speed-Throttle, Speed-Density and Mass Air Flow (MAF)) cannot give proper A/F control because of 1. sensor and anti-aliasing filter time constants and 2. improper or lacking compensation for manifold fuel film and (air) filling dynamics.

In this paper, the results of a long series of experiments conducted with the control systems above are to be presented. Both central fuel injection (CFI) (or throttle body (TBI)) and electronic fuel injection (EFI) (or multipoint (MPI)) manifolds have been investigated. These results completely support the conclusions of the original paper and show that in practical systems, other important problems arise. In particular, the details of fuel film compensation used, the method of treating air mass flow (or related) measurements and time delays are important. It is made clear that these difficulties can only be completely solved by using advanced modelling, control and signal analysis techniques. Nevertheless it is possible to make some useful improvements of conventional systems using simple elementary methods.