Research On Combustion System And Spray Characteristics Of Direct-Injection Sub-Chamber Gasoline Engines

Compared with PFI gasoline engine,GDI engine has become its substitute for higher fuel economy,faster engine response,easier cold-start,precise air/fuel ratio control,larger EGR rate,and lower emissions.However,it is difficult to realize the ignition reliability and combustion stability in GDI engines.Therefore,the concept of Direct-Injection Sub-Chamber (DISC) gasoline combustion system is advanced.A sub-chamber is set in the cylinder head, and the sub-chamber is connected with the main combustion chamber by a channel.The spark plug is located in the sub-chamber,and the injector is mounted inside or outside the sub-chamber.Under all working conditions,the mixture inside the sub-chamber is in the range of easy to be ignited and the ignition reliability is realized.In the three executive schemes,the injection characteristics of the mechanical 3-injection nozzle,inlet airflow and squish,and spray tip penetration under different back pressure and squish are utilized to ensure the mixture concentration inside the sub-chamber.Due to the much larger energy of the flame compared with the spark plug discharging,the combustion stability is enhanced and the lean-bum range can be expanded.This combustion system can improve the fuel efficiency and decrease the difficulty of the control system.A DISC prototype engine is rebuilt from a DI diesel engine by modifying the fuel delivery system and combustion system,and adding electronic control system.After that,the preliminary feasibility and the emission experiments under idle working conditions are conducted.The following conclusions can be concluded from the experiments:all parts of the engine and the electronic control ignition system work well,and they can fulfill the demands of feasibility experiments of the DISC gasoline engine.The engine can run at the air/fuel ratio of 48 with the super low idle speed of 400r/min smoothly.The emissions of the optimized engine are lower than the National 3 emission regulations.In order to keep the mixture inside the sub-chamber at the range within which the mixture is easy to be ignited,a mechanical 3-injection nozzle is designed.Numerical simulation about its performance is conducted and the characteristics of the injection from each kind of nozzle hole are analyzed.The computational results show that the injection quantity from the constant nozzle hole is almost independent with the fuel injection quantity per cycle per cylinder,and the performance of both the constant nozzle hole and the major nozzle holes can fulfill the demands of this DISC gasoline engine. When the spray from the 3-injection nozzle is injected into the sub-chamber,it is a complex process including near-impingement and multi-impingement.The mixture formation process inside the sub-chamber is studied with the Laser Absorption-Scattering(LAS) technique and CFD code experimentally and numerically.After that,the spray/wall interaction models available are estimated using experimental results.Experimental results show that:the spray and the mixture formation process are influenced by the impinging angle. A higher injection pressure can increase the vaporization rate,while there is no big difference on the absolute value of the vapor phase mass around the EOI.Under low ambient pressure and temperature,the vaporization rate is very low,and it will cause spark-plug wetting.So,it is not suitable to inject fuel into the sub-chamber too early.When decreasing the sub-chamber depth,the time interval from the start of injection to a homogeneous mixture is formed inside the sub-chamber,but that will endanger the possibility of spark-plug wetting.Calculation results varies very much when adopting different spray/wall interaction models,and none of these models can simulate all the aspects of the impinging sprays perfectly by now.According to the demands of the combustion system on atomization and injection advance angle,the hole-axis-angle(HAA) among the spray jets in the DISC gasoline engine cannot be so large as typical multi-hole nozzles in diesel engines.Two-hole nozzles with small HAA were manufactured in order to study the interactions between the neighbor jets. The characteristics of the two-hole sprays are measured using LAS technique,and comparisons are made with the single-hole spray which is considered to be the baseline.At the same time,numerical simulations about the single-hole and two-hole sprays were made using a 3-D CFD code AVL FIRE.Experimental and numerical results show that:when the HAA is smaller than 25°,the vaporization of the spray will be decreased because of the collisions and coalescences of the droplets in the neighbor jets.The two-hole spray has the tendency to have a longer penetration due to the behavior of smaller droplets forming larger droplets,while the lower relative pressure region inside the spray formed by the high axial spray velocity and lack of air entrained into this area makes its penetration to decrease.So the spray tip penetration results from the co-effect of these two factors.The low relative pressure region inside the spray also causes the two jets to incline to the center of the spray,and the offset is determined by the HAA of the spray.In the DISC gasoline engine,a relative smaller HAA is favorable for the jets pointing the far end of the piston,so that it will result in a longer penetration,which can enhance the air utilization.A relative larger HAA is favorable for the jets pointing the near end of the piston,in order to decrease or avoid impingement.