Study On Lean Combustion And NOx Emission Control On GDI Engine

The development of the automobile industry has been greatly affected by more and more severe problems of energy source lacking and environment pollution. The further development of internal combustion engines has to meet the demands of low fuel consumption and low emissions. Lean combustion techniques in gasoline engines can both reduce fuel consumption effectively so as to reduce the emission of greenhouse gas and lower the emissions of HC and CO significantly. However, the problem of controlling NOx emissions at lean combustion conditions constrains the development of lean combustion gasoline engines. In this paper, the lean combustion and NOx emissions control technology on 4G15 gasoline direct injection (GDI) engine which is researched and developed by our own are studied.Firstly, this paper studies the components, structure and control of the tumble-stratification lean combustion system on GDI engine. The intake flow tumble characteristics of 4G15 GDI engine are analyzed by using both flow rig and CFD transient numerical simulation. The results show that a rather strong tumble motion exists in this GDI engine, which is beneficial to the formation of stratified mixture and lean combustion. Then, the lean combustion is achieved successfully by adopting reasonable control strategies, and the lean combustion limit can reach 25~27. Formation mechanisms and characteristics of the gasoline engine emissions at lean combustion conditions and the methods to reduce NOx emissions in the cylinder, such as increasing air-fuel ratio, optimizing spark timing and injection timing and applying EGR system, are examined.The mechanism of adsorbing NO in three-way catalyst is analyzed. Based on the characteristics of emissions on GDI engines at lean combustion conditions, combining leaner combustion and NOx emissions control experiments by using three-way catalyst, a new method is proposed, which utilizes the mechanism of NO adsorption and reduction in three-way catalyst. It can reduce the NO concentration in the exhaust significantly and make full use of the reductive gas such as carbon monoxide in the rich period to increase the NOx conversion efficiency of three-way catalyst.With CHEMKIN 4.0 chemical kinetic software, the rationality of controlling lean-combustion NOx emissions on GDI engine by utilizing the mechanism of NO adsorption and reduction in three-way catalyst is studied from the perspective of transient simulation. The impact on emissions and the causes of key parameters in the conversion strategies of air-fuel ratio from rich to lean, such as conversion ratio and frequency, exhaust flow and exhaust temperature, are analyzed deeply.A large number of experiments on NOx emissions control by utilizing the mechanism of NO adsorption and reduction in three-way catalyst are carried out. The results show that the conversion ratio and frequency of air-fuel ratio from rich to lean and the rich air-fuel ratio have a great relationship with air pollution emissions and fuel economy of lean combustion GDI engine. The selection of these control strategy parameters should consider the NOx, HC and CO emissions and the fuel consumption rate of the engine comprehensively. Simultaneously, the speed and load conditions of the engine also have a influence on NOx emissions and catalytic conversion efficiency. The bench test results show that the NOx catalytic conversion efficiency on leaner combustion conditions can be increased to 50% or more and the NOx emissions can reach a lower level by taking this method. Because the lean-combustion catalyst can not be promoted and popularized currently, so this method in which the relatively mature commercial three-way catalyst at present is used has important practical significance.