Research status and technical difficulties of in c

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Research status and technical difficulties of gasoline engine direct injection technology

1 technical status of GDI Engine

1.1 fuel supply and injection system

modern GDI engine fuel supply system adopts flexible electronic control means with high precision and fast response. The high-pressure common rail injection system plus electromagnetic drive on January 23 is considered to be one of the injection systems that meet the requirements of flexible injection in the cylinder. The system consists of a low-pressure fuel transfer pump, a fuel pressure sensor, an injection pressure control valve, a high-pressure oil pump, a pressure storage fuel rail, and an injector

in the fuel injection system, the structural form of the injector has a great influence on the quality of spray. At present, the internally opened swirl injector is widely used in GDI engines. It has only one orifice and the working oil pressure is 5 MPa, with fuel swirl chamber inside, it can achieve better spray shape and proper penetration through the selection of swirl ratio, and the spray direction is easy to adjust, which facilitates the layout in the cylinder

1.2 injection mode

gdi engine fuel injection mode can be divided into single-stage injection mode and multi-stage injection mode. The single-stage injection mode refers to that when the load is small and medium, the fuel is injected at the later stage of the compression stroke to realize the stratified lean combustion of the mixture, and the quality regulation is adopted to avoid the throttling loss of the throttle valve, so that the GDI gasoline engine can achieve the same economy as the diesel engine; At high load and full load, fuel is injected into the cylinder during the intake stroke to realize homogeneous premixed combustion, so as to maintain the characteristics of high power rise of gasoline engine. The multi-stage injection mode means that 1/4 of the required fuel is injected first in the intake stroke to form a very thin homogeneous mixture, and the rest of the fuel is injected again in the later stage of the compression stroke to form a stratified mixture. The application of this technology can realize the smooth transition of the engine from small and medium load to large load, reduce the gas temperature in the cylinder and inhibit the generation of deflagration

1.3 combustion system

gdi engine combustion system can be divided into three types according to the relative position of fuel injector and spark plug and the organization form of mixture: (1) injection beam guidance method. The fuel nozzle is arranged close to the spark plug, which is located at the edge of the fuel spray beam. The advantage of this method is to ensure that when the entire combustion chamber is a thin mixture, the mixture concentration that can be ignited can still be formed around the spark plug. Some models produced by Ford and Honda adopt this combustion system. (2) Wall guiding method. The fuel nozzle is arranged far away from the spark plug, and the piston surface with special shape is used to cooperate with the airflow movement to guide the fuel vapor to the spark plug and form a mixture of appropriate concentration in the spark plug gap, such as the models developed by Mitsubishi, Toyota, Nissan and other companies. (3) Airflow guidance method. Similarly, the fuel nozzle is far away from the spark plug, and the organized air flow in the cylinder is used to achieve the above purpose. The scheme developed by FEV and AVL adopts such a combustion system

1.4 organization of air movement in cylinder

gdi engine cylinder air movement includes swirl, tumble and squeeze. At present, most GDI engines use vortex as the main form of air movement in the cylinder, which is characterized by long duration and less radial divergence in the cylinder, which is beneficial to maintaining the relative concentration and stratification of the mixture. It can be fully used to maintain the stratification of the mixture in the compression stroke. However, there is a limitation in the operating range of using eddy current to promote oil-gas mixing. In addition, high eddy current ratio will also cause oil droplets to throw towards the cylinder wall due to the effect of centrifugal force, resulting in the increase of wet wall phenomenon. In recent years, Mitsubishi Motors Corporation of Japan has done a lot of experimental research on the application of tumble in GDI engines. The results show that in the late stage of the compression stroke, tumble can effectively increase the gas flow velocity near the cylinder wall, which can promote the rapid evaporation of oil droplets adhering to the cylinder wall. It can also use the matching of tumble and the pit on the top of the piston to guide the stratified mixture to the spark plug to control the oil beam collision and flame propagation, The squeeze flow from the exhaust side to the intake side can also improve the combustion speed

2 existing problems and prospects

the main technical difficulty restricting the development of GDI engines is emissions

it is mainly manifested in: (1) there are more emissions of unburned hydrocarbons (ubhc) under medium and small loads. The main reasons are that the flame extinguishes from the rich area to the lean area when stratified mixture is used, and the working conditions of lean air-fuel ratio cause the temperature in the cylinder to be low, which is not conducive to the subsequent continuous oxidation of unburned hydrocarbons. The combustion system organized in a remote way has more spray hitting the wall, while the temperature of the piston crown and cylinder wall is low, resulting in more ubhc. Inadequate mixture and flame delay caused by other improper designs will also reduce the flame propagation speed and increase ubhc emissions. (2) Because the air-fuel ratio is not near the equivalence ratio, the current mature three-way catalytic technology can not be effectively used, so the NOx emission is high. In addition, GDI engine hydraulic testing machine is a non-standard customized experimental equipment. Higher compression ratio and faster reaction heat release rate will also cause NOx to rise. (3) Under the conditions of low load, transition conditions and cold start, the particulate emission of GDI engine is significantly higher than that of inlet injection engine

in addition to the emission problems, the development of GDI engine also has the following technical difficulties: it is difficult to control the organization and combustion of stratified mixture within the scope of theoretical requirements; The injection strategy required by the smooth transition of load is more complex when the working condition changes; More combustion deposits in the cylinder cause spark plug pollution; High pressure oil supply system parts wear faster and cost higher

reducing GDI engine emissions to meet increasingly stringent emission laws, especially in the field of dry products, is the focus of national researchers' efforts. Tests have proved that the harmful substances emitted during the starting process and after the starting stage can reach 90% of the total emissions of chlorine and caustic soda produced in Shanghai chlor alkali electrolysis plant. Therefore, the combination of "stratified combustion starting" and "two injection heating" is adopted, It is a very effective emission countermeasure for GDI engine, which has been confirmed by a large number of engine bench tests and real vehicle tests. It can be expected that with the continuous progress of injection technology and exhaust after treatment technology, the performance of GDI engine in emission and other aspects will be further improved, and GDI gasoline engine will undoubtedly occupy more market share of vehicle engines. (end)

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