Transient Air to Fuel Ratio Response in a Fuel Injected S.I Engine
Abstract: Future emission standards make it necessary to focus on the conditions under which engine emissions are produced, especially before the catalyst light off. During practical driving conditions, such as those represented by the FTP 75 test cycle, the engine is operated very much under transient conditions regarding load and speed. It is, under those conditions, very difficult to ensure the requirements of the three-way catalyst system for a very precise value of the air to fuel ratio (A/F) in the exhaust. Most of the emissions are produced during transient conditions which therefore have a large influence on the exhaust emissions. The main reason for this is the behavior of the liquid fuel in the inlet manifold and the time delay between the injected fuel mass and the air flow mass entering the cylinder.During a steady state there is a balance between the fuel flow from the injector and the fuel flow to the cylinder which is not present during a transient. One of the most important challenges of modern fuel injection systems is to ensure a very constant air to fuel ratio because of the large influence it has on the exhaust emissions of an engine with a catalytic converter. It is, however, difficult to measure and analyze the A/F response of the engine under those conditions. In this thesis a method for measuring A/F under transient conditions and an analysis of the A/F response behavior under transient conditions is developed. Also, a model is developed that can be used in a transient compensation control strategy. The main conclusions of this thesis can be summarized as follows: A transient test method that works very well in describing transient A/F response was developed. A phenomenological model was developed and used for data matching of the measurements. The model was based on a manifold air flow model and a fuel film model, with two deposit parameters and two time constants, together with a time delay between air and fuel supply to the engine during the transient. It was found that a single deposit parameter and one time constant together with a time delay describe the fuel film behavior during a transient very well. It was also found that the peak part of the .gamma. excursion is due to the time delay effect, this applies particularly to throttle opening transients, while the fuel film effect is dominant during throttle closing transients. The time delay in the model consists of two parts, one is the actual time delay between supply of the air flow and the fuel flow determined by the control system the other is due to the delay of fuel caused by physical effects such as back-flow during valve opening overlap and when the inlet valve is closing. Modeling the A/F dynamic response with this kind of time delay includes effects which otherwise have been difficult to model. Fuel properties affect the A/F response greatly which implies that it is necessary to use some kind of adaptation of the calibrated parameters in the control system. The fuel flow dynamics are also very much influenced by temperature and injection timing. The fuel film mass is more than three times larger at 30oC than at 90oC and injection at open inlet valve can reduce the fuel film mass with 50%, compared to injection at closed inlet valve.
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