| High-speed direct-injection(DI)diesel engine chamber structure has an important effect on organizing the in-cylinder fuel-air mixing and combustion,designing the appropriate chamber structure to realize the reasonable airflow and spray motions could reduce the fuel adhesion on the wall effectively and promote the fuel-air mixing and combustion,which is the effective way to improve the diesel engine performance.The continuously increased fuel injection pressure made the chamber structure of the high-speed DI diesel engine developed in the direction of higher diameter-depth ratio and lower surface-volume ratio,however,there were still the problems of the insufficient top clearance utilization and the low fuel spray spreading rate.To solve these problems,the concept of Double-Layer Diverging Combustion chamber(DLDC chamber)was proposed,it was aimed to guide the fuel mixed with the air fast and homogeneously,realize the rapid top clearance utilization and ensure the utilization of the chamber center region by making the fuel double-layer spreading in the upper and lower layers after the fuel-wall impingement,and achieve the purpose of improving the diesel engine performance.The former researches indicated that this chamber had the potential to improve the fuel-air mixing and decrease the fuel consumption.For further exploring the energy-saving and emission-reduction potentials of the DLDC chamber,it was necessary to research the influence of the chamber structure parameters on the diesel engine in-cylinder fuel spreading and combustion processes,emissions formation processes,space utilization and performance under different fuel injection conditions,clarify its chamber-spray-charge motion matching characteristics,and provide the structure design basis of the DLDC chamber.In this thesis,there were four different DLDC chambers designed which reflected the six main structure parameters based on a single-cylinder 135-type diesel engine,the numerical simulation,the high-speed photography visualization measurement and the engine bench experiment were applied for more in-depth researches on the DLDC chamber.The numerical simulation results of the fuel spray spreading in the DLDC chamber indicated that:the fuel impinging platform height and the throat diameter would affect the fuel-wall impinging state and the fuel spreading velocity after the fuel-wall impingement,the cancellation of the fuel stripping surface was helpful for the rapid fuel spreading.The upper layers in an approximate shallow dish type with smaller volume and in an approximate shallow ? type with larger volume promoted the fuel spray spreading and the top clearance utilization respectively under different fuel-wall impingement states.The lower layer with larger volume and a smaller bottom protrusion was more helpful for the rapid fuel spreading.Type ? and type ? DLDC chambers,which contrasted the structure features obviously and made the fuel spray spreading well,were selected for the following researches.The high-speed photography results of the fuel spray spreading in the two-dimension chamber models showed that:compared with the traditional structure chambers,the DLDC chamber made the fuel-wall impingment happened earlier,promoted the fuel spray spreading and the top clearance utilization,the fuel spray spreading of different layers were affected by the fuel injection conditions differently,and appeared different chamber-spray-charge motion matching characteristics.Under this research experimental conditions,it was also found that:there was a specific injection pressure providing the largest fuel spray distribution region for the DLDC chamber;advancing the injection timing was not good for the lower layer utilization,the utilization of the upper layer and the top clearance were affected by the injected fuel mass and the top clearance volume together,adjusting the injection timing showed different influence on the fuel spray distribution region of the DLDC chamber in different injection timging ranges;decreasing the spray cone angle and the nozzle hole diameter showed more obvious influence on the fuel spray spreading in the lower layer with smaller volume and less injected fuel mass,and the fuel spray spreading in different upper layers were affected more obviously in different stages.All the regions of type ? DLDC chamber were utilized well,its fuel spray spreading rate was higher during the later stage.Type ? DLDC chamber made the fuel spray flow along with the upper layer bottom and stay around the side wall of this layer,its fuel spray spreading rate was higher during the earlier stage.The in-cylinder numerical simulation results of the DLDC chamber appeared that:this chamber provided weaker squish and reverse squish;its NOx formation was mainly caused by the chamber guiding the fuel double-layer combustion directly and less affected by the reverse squish than the traditional structure chambers;if it was easy to form more homogeneous mixture,the soot was just formed at the end edge of the fuel spray spreading in the lower layer,if the fuel stayed around some local regions easily,the formed rich mixture led to the obvious soot formation.The numerical simulation and diesel engine bench experiment results indicated that:adjusting the injection conditions showed different influence on the fuel-air mixing and combustion in different layers,which made the influence of the DLDC chamber on the diesel engine in-cylinder processes and performance different from the traditional structure chambers.There were different injection pressures making the DLDC chamberobtain its lowest fuel consumption and lowest soot emission respectively.Advancing the injection timing was more helpful to decrease the DLDC chamber fuel consumption.When the nozzle hole number was decreased or the spray angle and the nozzle hole diameter were decreased,the DLDC chamber provided lower fuel consumption and soot emission than the ?chamber and lower NOx and soot emissions than the re-entrant chamber.Type I DLDC chamber had a higher maximum in-cylinder pressure with higher heat release rates during the premixed combustion phase,and could provide lower fuel consumption and soot emission but higher NOx emissions.Type ? DLDC chamber had a lower maximum in-cylinder pressure with lower heat release rates during the premixed combustion phase,and could provide lower NOx emissions but higher fuel consumption and soot emission.The influence of the DLDC chamber on the diesel engine performance was clarified by the multiple operation modes diesel engine bench experiment.Therein the most significant results were as follows:with Nozzle A(8 holes,150° spray angle,?0.18 mm nozzle hole diameter)and the same NOx emissions,type I DLDC chamber could provide lower fuel consumption and soot emission than the traditional structure chambers.Herein its weighted mean fuel consumption was 5.5%lower than the ? chamber and 1.9%lower than the re-entrant chamber,its weighted mean soot emission was 52.1%lower than the ? chamber and 35.4%lower than the re-entrant chamber.With Nozzle B(7 holes,150? spray angle,?0.18 mm nozzle hole diameter)and the same fuel consumption,type ? DLDC chamber could provide lower NOx and soot emissions than the traditional structure chambers.Herein its weighted mean NOx emissions were 17%lower than the ? chamber and 4.6%lower than the re-entrant chamber,its weighted mean soot emission was 45%lower than the ? chamber and 28%lower than the re-entrant chamber.In summary,the DLDC chamber promoted the fuel spray spreading and improved the top clearance utilization effectively.The fuel-air mixing and combustion in the upper and lower layers were affected by the fuel injection conditions differently,which indicated that they had different chamber-spray-charge motion matching characteristics,both of them determined the fuel-air mixing and combustion processes in the DLDC chamber and the influence on the diesel engine performance together.The DLDC chamber design principle was also obtained in this thesis:when there were a higher impinging platform and a larger throat diameter,the DLDC chamber should employ a lower layer with larger volume and a smaller bottom protrusion and an upper layer in an approximate shallow dish type;when there were a lower fuel impinging platform and a smaller throat diameter,the DLDC chamber should employ an upper layer in an approximate shallow ? type with larger volume,and there would be a higher protrusion in the lower layer bottom,however,the piston cavity bottom region could be still utilized effectively. |
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