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Experimental and Theoretical Investigation of Mixture Formation in CNG Direct Injection of Spark Ignition

Chitsaz, Iman | 2012

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 43913 (08)
  4. University: Sharif University of Technology
  5. Department: Mechanical Engineering
  6. Advisor(s): Saidi, Mohammad Hasan; Mozafari, Ali Asghar
  7. Abstract:
  8. Due to increasing of internal combustion engine applications in metropolitan, reduction of emission is one of the challenging issues for engine companies. Natural gas is a promising fuel for the strict legislation of emission in many countries. Vast variety of natural gas sources in Iran and also lower carbon to hydrogen content of natural gas motivates us to use this fuel for engine application. Using natural gas also helps to increase the variety of fuel basket in Iran. Present research has focused on mixture formation and injection characteristics of natural gas direct injection engine. Injection and mixture formation in direct injection engines is very important for good combustion and emissions can be reduced by a targeted injection and mixture formation. In the present study, a semi analytical model has been presented for the transient injection of compressible gas. Then, transient injection and jet characteristics of injector has been investigated by Schilieren method. Jet characteristics and mass flow rate of gaseous jet injection through gasoline direct injector was also investigated to understand the actual condition of commercial direct injectors. Finally, numerical investigation of mixture formation which is coupled with genetic algorithm was presented to optimize geometry of combustion chamber and injection timing. Base geometry was founded on the EF7 engine that is first national Iranian engine. Numerical simulation was performed by FLUENT and coupled with GAMBIT and MATLAB in MODEFRONTIER software. MODEFRONTIER is a medium that can link different software together and also has lots of optimization methods. Analytical results showed that isentropic was the most appropriate process for steady state injection, especially in the nearfield of injection but in unsteady condition; there was no exact agreement with theoretical tip penetration for different processes. Experimental Results showed that tip penetration of direct injector of gaseous in spark ignition engine was clearly different from those Diesel engines. Slope value of nondimentional data was obviously different from the range of previously reported values between 2.8 and 3.1. This trend indicated that tip penetration in CNG–Spark Ignition Direct Injection has a lower tip penetration than those of used for Diesel engine. Numerical optimization showed that the high-speed operating condition was not sensitive to the combustion chamber geometrical design as well as lowspeed condition. This was due to the limited time of gas diffusion at low speed in comparison to high speed. At 2000 rpm, the shallow type bowl geometry was more appropriate for the CNG-DI engine because this shape prepares the mixture better than the deep narrow bowl. Among the optimization variables, injection timing was very important and advanced injection for high speed and late injection for low speed would result in optimized fuel economy
  9. Keywords:
  10. Numerical Optimization ; Compressed Natural Gas (CNG) ; Schileren Optical Method ; Direct Injection Spark Ignition (DISI)Engine ; Underexpanded Jet

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