| Injector deposits in GDI engines are important reason for poor fuel atomization of injectors and excessive particulate emissions of engines.Reducing the formation of injector deposits and its potential harmfulness is a crucial issue to be solved in the development of GDI engines.In this dissertation,spray test platform,GDI engine bench,optical engine platform and self-designed advanced temperature measuring system of injector were developed,and scanning electron microscope,transmission electron microscopy and transform infrared spectrometer were applied.The aim was to make an in-depth study on the macro/micro spray characteristics and poor spray atomization of injectors with different degree of fouling,physicochemical properties of injector deposits,the source of precipitable components which were fuel and combustion particulates that contribute to deposits formation and the main reasons for deposition,and the heat load characteristics of the injector as well as its influence on the formation of deposits.Based on research mentioned above,the formation mechanism of injector deposits and the measures to inhibit and eliminate deposits were explored.The main conclusions are as follows:1.The spray characteristics of fouled injectors were studied.The results showed that a large quantity of deposits adhered to the nozzle wall,decreasing the diameter,increasing the length/diameter ratio and reorienting the nozzle axis.Ligament and large droplets were the main formats of poor fuel atomization of fouled injectors.With the increase of injection pressure,the number of ligament and large droplets increased.As the degree of deposits fouling(flow loss)increased,the penetration distance of spray plume increased at first and then decreased,and the projected area and the cross section area of the spray plume dropped.However,the spray plume shape of partial nozzles in the injector with mild degree fouling was distorted and expanded,and the projected area and the cross section area increased.The injection orientation of plumes was deflected clockwise along with the injector axis,and the deflection angle increased with the increase of the degree of deposits fouling.The angle between the injection orientation and the injector axis increased,and the angle decreased with the increase of injection pressure.The drop point shape of spray plume was irregular,and the irregularity of the drop point shape decreased with the increase of injection pressure.The starting injection delay was 20μs and the end injection delay was 30-40μs.2.The physical and chemical properties of deposits were studied.The results indicated that the micro-structure of deposits particles had the shape of lacquer,coral and bulk through scanning electron microscope.Transmission electron microscopy showed that the nanoscale carbon particles were formed via the accumulation of 20-50 nm spherical particles and floccule.Under low load,the carbon particles in injector tip were light colored and flocculent.Under the heavy loads,dark black particles were formed.With the increase of the load and time,dark black particles increased.The energy dispersive spectrometer showed that the main elements of the particles were C and O with few elements of N,K,Ca and Fe.Transform infrared spectrometer showed that organic compounds were mainly composed of carboxylic acids,alkanes,aromatic hydrocarbons and other hydrocarbons.The above research indicated that the forming material of deposits was fuel and combustion particles.3.The near-field spray test indicated that during injection and after end of injection fuel film was left in the nozzles and on the protrusion and tip surface.The main reasons were that the step-holes with undersize diameter and oversize depth led to spray impingement on the step-holes wall;the SAC volumes were not discharged well;the ball valves were not sealed well,leading to fuel leakage;the rebound of the needle valve assembly led to after-injection,which caused ligament and large droplets;the outlets of the step-holes were close to the seat base and the droplets dropped on seat base;the deposits interfered the fuel plumes and were soaked with fuel film.The injection pressure had no obvious effect on improving the atomization of after-injection spray.Fortunately,increasing fuel temperature could improve the atomization of after-injection spray and greatly reduce the amount of after-injection.4.The combustion diagnostics of the optical engine illustrated that diffusion combustion and combustion particles were stagnant and aggregated around injector tip after end stage of combustion,it was caused by the combustion of ligament and large droplets formed by after-injection.The injection pressure and injection timing had no obvious effect on improving the diffusion combustion.5.The heat load characteristics of GDI injector with its influence on the formation of deposits were studied.The results indicated deposits on cylinder head were mainly distributed on the intake side,and the most were located close to the injector.It was indicated that temperature was an important factor of the deposits formation.The thermal load characteristics of the injector showed that with the increase of the engine speed and load,the temperature of the sealing ring and its upstream position on the injector decreased;the tip temperature increased dramatically and reached 132-147 ℃ under the maximum load(14bar IMEP)in the test.As the engine load increased,the seat side temperature increased at first and decreased slightly afterwards and become stable;As the combustion phase was delayed,the tip temperature decreased by 4.5-9 ℃.With the increase of excess air coefficient,tip temperature increased firstly and then decreased,and the temperature variation range was 7.5-13.5 ℃.There was a good linear relationship between the injector temperature and the cooling water temperature.As the proportion of direct injection decreased,the tip temperature increased significantly by 27-58.5 ℃.Under the condition of keeping good engine performance,the PFI/GDI dual-injection technology was an effective measure to improve the temperature of the injector tip.With the increase of engine load and injector tip temperature,the number of deposits on the tip was reduced.The tip temperature was increased to 184 ℃through application of dual-injection technology,and endoscopy testing showed that the deposits on the fuel injector tip and cylinder were completely removed.6.The formation mechanism of deposits in GDI injectors was obtained through the above research.After fuel injection,fuel film was left in the nozzles and on the protrusion and tip surface.Combustion particles were gathered around the fuel injector tip after end stage of combustion.As the temperature of the injector tip increases,the amount of deposits on the injector tip decreased.Because the fuel spray could reduce the tip temperature of the injector,there was a big temperature gradient between the tip surface and the gas in the cylinder,and the fuel droplets and combustion particles close to the wall were deposited on the wall of the injector due to the thermophoresis force.The hydrocarbon was condensed and attached to the wall of the injector,forming the viscous fuel film and deposits layer.Therefore,the low temperature of injector tip was able to promote the formation of deposits.According to the temperature test,the chemical reaction of high temperature pyrolysis did not occur on the surface of the injector,and most deposits precursors were produced close to the wall and in the combustion area.7.The conclusions of injector deposits inhibition measures were as follows: 1)Increase fuel temperature,reduce after-injection and fuel deposition.2.Increase the temperature of the injector tip and apply dual-injection technology to eliminate carbon deposition.3.Other measures including: ptimize the step-hole,avoid the impact of the spray plume to the step-hole,reduce the SAC volumes,the nozzles arrangement on the protrusion should be kept away from the seat base,optimize the structure of the needle valve component,restrain the rebound of the needle valve,optimize the sealing structure of the ball valve,and improve the processing precision. |
