Fuel Dynamics Model for Engine Coldstart

Different strategies have been used in the past to improve the performance of hydrocarbon emissions controllers in SI engines. One of them relies on the use of the model-based control design scheme, which offers the possibility of automating the controller design-to-implementation phase. It also gives the chance to update the model with a potential reduction in required experiments if physical changes are made to the plant. Under the model-based scheme, an accurate plant model can greatly enhance the development of an effective control systems. In particular, acquiring a correct fuel-dynamics model can be crucial in developing a good hydrocarbon emissions controller for coldstart. During this period, the factory AFR (air-fuel ratio) sensor is not active and the engine experiences an abrupt transient, which makes modeling difficult. In this paper, a model that describes fuel dynamics for coldstart is developed. The model uses mainly two parameters: one of them accounts for the fraction of injected fuel that runs directly into the cylinders and the other represents the time constant of fuel vaporization from the wall-wetting film formed in the intake port. Two discretized versions of the model are evaluated and compared. The parameters are calculated at different intake port temperatures. To implement the proposed model, experiments are carried out using a 4-cylinder SI MPFI combustion engine connected to a Simulink interface with capabilities of automatic code generation. Factory controller measurements and other external measurements such as intake port temperature and broadband AFR sensor measurements are used to fit the model parameters. Parameter identification techniques are used to identify the values of the parameters. Both versions of the fuel dynamics model are then tested during coldstart cycles and the results are analyzed.