Visualization Investigation On The Fuel Air Mixing Process And Combustion Characteristics Of Flash Boiling Spray In Sidi Engine Under Cold Start Condition

Nowadays,gasoline direct injection technology has become the mainstream pathway for passenger vehicle engine due to its fast transient response and improved fuel economy.However,as the energy and environmental issues comes about,the emission regulations all over the world become more and more stringent.As the most common propulsion system for passenger vehicle,the performance and fuel consumption of gasoline engine have received more and more attention.For the most concerned challenges that gasoline engines are confronting including fuel economy,particulate matter emission and cycle-to-cycle variations,the widely adopted high pressure inj ection technique has its limitations.Flash boiling spray,which features finer atomization effect at a relatively lower injection pressure,become another viable technique pathway besides high-pressure inj ection.However,up to now,the characteristics superheated spray throughout the combustion process including atomization,evaporation;mixture formation,combustion and emission formation have not been fully studied in a systematic manor.This is also the main obstacle for the mass utilization of flash boiling spray in production engine.Different from quiescent chamber,flow field under realistic engine play important engine role on engine performance.So the effect of different level of swirl flow and tumble flow on engine working process was also studied.In this research,various laser diagnostics was adopted to visualize three main working processes of direct injection engine.First of all,the global structure and footprint of spray injection was recorded to study the spray characteristics in real engine condition.Next,the fuel air mixture distribution near the spark plug plane at the end of compression stroke was evaluated.Finally,the flame propagation and emission of firing condition was analyzed to clarify the benefits of flash boiling spray on combustion process.In order to make full use of the benefits of the enhanced atomization and prompt evaporation of flash boiling spray,cold start was selected as the operating condition.The fuel temperature was adjusted along with the regulated MAP temperature to achieve different level of spray superheat degree.Different intake flow organization was used to examine the influence of intake flow on combustion characteristics by changing the intake port design.Firstly,the spray structure was recorded through Mie-scattering technique to measure and analyze the influence of spray superheat degree and intake airflow field on spray inj ection process.The results shows that flow field affect the surface of the spray strongly with smaller droplet surrounding the spray indicating improved atomization and evaporation;high tumble flow results in notable offset of the global spray structure along clockwise direction;under different superheat degree,typical liquid,transitional flash boiling and flare flash boiling spray structure could be observed verifying that the effectiveness of changing the spray superheat degree by adjusting the fuel temperature.Different from the results obtained in the constant chamber,the spray appearance in real engine condition was distorted and showed larger cycle-to-cycle variation.Higher magnitude to intake flow leads to high level of cycle-to-cycle variation while improving the spray superheat degree suppresses the stability of the spray.However,the orientation and appearance of spray is completely different from the original design.Therefore,the characteristics of flash boiling spray should be carefully studied and the occurrence of flash boiling should be considered at as early as the concept designing stage of combustion system.Secondly,the fuel concentration distribution around the spark plug plane was measured using the laser induced fluorescence method.The characteristics of fuel distribution under typical cold start condition was examined and the effect of spray and flow field on fuel distribution was summarized.Results shows that the atomization and evaporation was poor under could start condition with large amount of liquid fuel cluster still existing at the end of compression stroke.With the increase of superheat degree,the evaporation of fuel was improved greatly.However,under unorganized condition,due to lack of dominant flow pattern,the distribution of the fuel was random and showed strong cycle-to-cycle variation.With high tumble flow,the homogeneity of the fuel was improved especially near the spark plug.Similarly,the repeatability of the fuel distribution was improved strongly with strong swirl flow.Dominant fuel distribution pattern was observed with rich mixture located at the left side of the spark plug.This distribution showed good relationship with flame initiation results in firing test.Proper orthogonal decomposition analysis quantified the magnitude of the distribution pattern and explained the poor repeatability under unorganized flow field condition.Finally,high speed imaging of flame natural luminosity along with simultaneous in-cylinder pressure measurement was conducted to study the flash boiling combustion characteristics in detail.First,high tumble port improves the combustion efficiency but results in the occurrence of partial burn due to excessive turbulence level near the spark plug.Therefore,the effect of flash boiling on cold start combustion was conducted under high intake swirl flow since the first injection cycle throughout the transient and stable stage.Results shows that it took almost 30 cycles for liquid fuel condition to achieve stable operating due to the poor fuel air mixture formation.In contrast,the engine reached stable operating with the introduce of flash boiling spray without partial burn cycle observed.Under stable stage,faster flame propagation speed was found leading to advanced combustion phasing,improved output performance and lower cycle-to-cycle variation all of which are crucial to cold start condition.According to the emission measurement,good relationship between yellow flame intensity and particulate number emission.While the PN,THC and CO emission was reduced considerately under flare flash boiling condition,the NOx emission was deteriorated.The combined analysis of flame image and in-cylinder pressure shows that the flame propagation rate of early flame directly decide the final output capability.Critical flame area was defined such that the CAD reaching the critical area and IMEP shows strong linear relationship.In a summary,this paper utilized various visualization technique to study the characteristics of flash boiling spray combustion throughout the spray structure,fuel distribution,flame propagation and emission process under different intake airflow organization.Based on the prior conclusions drawn on flash boiling spray characteristics concerning atomization,evaporation and interaction with surrounding air in constant chamber and the results obtained in this study,the thorough influence of flash boiling spray on combustion process was unveiled.