Research On Mixture Formation And Combustion Characteristics Of The Conical Spray In A Gasoline Direct Injection Engine

The national economy and social environment are affected by the rapid development of automobile industry,simultaneously energy consumption and environmental pollution increase as the economic growth.To meet the more stringent emission regulations and social development of energy saving and environmental protection,it is imperative to optimize the existing engine technologies to further improve combustion efficiency and reduce emissions.With a higher compression ratio,the gasoline direct injection(GDI)engine can achieve better power and economy.There has great potential in improving GDI fuel economy and reducing emissions due to multiple mixture formation and combustion modes,which attracts a lot of academic research and industry attention.However,fuel is injected directly into the cylinder under higher injection pressure,which increases the wall wetting and reduces the mixing time.The quality of the mixture formation is significantly affected by the operating conditions and injection strategy.It is important to study GDI engine operating parameters and injection strategy which are helpful to improve mixture formation and combustion.In order to optimize the wall wetting and poor mixture distribution,this paper,combining the conical spray and GDI technology,studies the effect of operating parameters and injection strategy on the mixture formation and combustion characteristics.First of all,the conical spray characteristics variation with the development of time were obtained by optical measurement system,and the constant volume bomb simulation model was generated with FIRE to verify the spray shape and penetration.Then,the engine model and moving mesh were built by CATIA and FEP module of FIRE.Finally,to ensure the accuracy of the spray and combustion models in subsequent calculations,the in-cylinder pressure and heat release rate were verified.Under the engine speed of 2000r/min,the variation of in-cylinder flow field with crank angle show that the valve interception effect is strong when the intake valve opened.There is tumble generating under the intake valve and at the top of the combustion chamber.A large range of clockwise tumble is generated into the cylinder as the piston moves down toward.The field velocity is reduced before ignition,and as the piston moves upward the forming squish is beneficial to the mixture formation.The fuel injection quantity,injection timing,injection pressure and ignition timing were kept constant,and the effects of the intake temperature,the engine speed and the excess air ratio on the mixture formation and combustion characteristics were studied under the single injection strategy.The results indicated that:(1)As the intake temperature increases,the evaporation rate of fuel film on cylinder wall and piston becomes faster,and the uniformity of the mixed gas concentration increases.The peak pressure and heat release rate are gradually increased,and the combustion heat release is more concentrated.The maximum in-cylinder pressure is 6.7MPa.(2)With the increase of engine speed,the peak pressure and heat release rate decrease firstly then increase.The intensity and range of the in-cylinder tumble are increased,and the mixture can be mixed uniformly.The evaporation time of fuel film is limited when the engine speed is too high.When the engine speed is 3000r/min,the maximum in-cylinder pressure is reduced by 2.3MPa compared to that of 1500r/min.(3)With the excess air ratio increase from 0.8 to 1.05,the peak pressure increases at first and then gets worse,and the maximum in-cylinder pressure is 6MPa.When the excess air ratio is low,the spray penetration increases because of more cyclic fuel supply,and the amount of fuel that hits the wall and piston increases.The mixture quality is poor which is not conducive to the ignition and flame propagation.The engine speed,total fuel injection quantity and overall mixture equivalent ratio were kept constant,and the effects of the injection times,the second injection timing and the third injection timing on the mixture formation and combustion characteristics were studied under the multiple injection strategy.The results indicated that:(1)Under the strategy of double injections,the second injection timing has a significant effect on the in-cylinder pressure and heat release rate in cylinder.The mixture quality is improved with the delay of the second injection timing,and an ignitable mixture is concentrated around the spark plug.When the second injection timing is 170°CA,the peak in-cylinder pressure reaches 6.4MPa,and the corresponding crank angle is 375°CA(2)By comparing the changes of engine parameters under different injection times,the fuel film mass decrease and the fuel evaporation increases under the three injections,which results in shorter combustion duration and higher thermal efficiency.Compared with single and double injection,the peak in-cylinder pressure of three injections is increased by 2MPa and 1.1MPa respectively.(3)Under the strategy of three injections,the in-cylinder pressure and heat release rate first increase then decrease with the delay of third injection timing.The maximum in-cylinder pressure is 7.8MPa at the third injection timing is 250°CA,and the corresponding crankshaft angle is 12oCA after top dead center.When the third injection timing is 270oCA,the over-rich and excessively lean region of mixture is increased due to the short formation time,which causes combustion deterioration.In addition,increasing the third fuel injection ratio and delaying the third injection timing appropriately are facilitate to improve the formation and distribution of mixture,which promote the fuel burning sufficiently.