A Control-Oriented Carbon Canister Model

Carbon canisters have been adapted for automobile use since the early 1970s to control evaporative emissions. Stringent emission regulations and the requirement for an enhanced evaporative emissions test procedure, make this an important issue. The air and evaporative fuel from the carbon canister therefore need important consideration with respect to air to fuel ratio (AFR) control and idle by-pass air control. Although a few complex models of the activated carbon canister have been developed, a control-oriented, simplistic carbon canister model needs to be developed. This paper explores the control-oriented modeling of a canister purge air system along with the on-line estimation of evaporative fuel loading of the activated carbon. An attempt was made at providing an analytical expression for the evaporative fuel and air entering the intake manifold. In order to allow the model to have the capability of direct industrial application, modeling and estimation were carried out with the aid of only the stock sensors available on a production vehicle. No additional sensors were used. The estimation of hydrocarbon loading level was performed using the long-term fuel trim value that is readily available from the production engine controller. The model was validated experimentally in a test-cell equipped with an engine and a gas analyzer.