| Studying the new combustion technology and improving the thermal efficiency of natural gas engines have been the research focus of the corresponding industry all along.High pressure direct injection has the advantages of high thermal efficiency and low emissions,and thus being regarded as one of the most advanced techniques of natural gas engines.In this case,high pressure direct injection natural gas engines are more and more concerned nowadays.High pressure direct injection natural gas engines adopt natural gas as the main fuel,diesel as the pilot fuel,the combustion process of which involoves the mixing quality and chemical kinectic mechanisms of both fuels.Therefore,the control of the injection parameters and the selection of the optimization methods for high pressure direct injection natural gas engines are different from conventional diesel and natural gas engines.Meanwhile,both experimental and numerical studies related to this type of engines are very limited.Subsequently,it is of theoretical value to investigate the effects of injection strategy on combustion and emissions,the source and control methods of combustion noise and the development of chemical kinetics for high pressure direct injection natural gas engines.The aim of this thesis is to theoretically analyze the effects of injection parameters on the combustion,emissions,fuel economy and combustion noise of high pressure direct injection natural gas engines.The kinetic mechanism for the oxidation and emission formation of the surrogate fuels was constructed and the 3D numerical simulation model was developed,with which the optimization methods for the combustion and emissions of the corresoponding engine were explored.The main research work and results are as follows:1.The test bench for high pressure direct injection natural gas engine was built and experimental investigations were conducted using the control variate method.By processing and analyzing the experiment data,the effects of different injection parameters were investigated.The experiment results indicated that,for high pressure direct injection natural gas engine,using more advanced natural gas injection timing,raising injection pressure,adopting smaller interval between pilot diesel and natural gas injections as well as shortening the diesel pulse width are benefical for the advancement of combustion phasing and the improvement of fuel economy.NOx emissions can be reduced by delaying natural gas injection timing,decreasing injection pressure,enlarging the injection interval between pilot diesel and natural gas injections along with increasing diesel pulse width.HC emissions can be reduced by adopting relatively delayed natural gas injection timing with relatively high injection pressure as well as increasing diesel pulse width.CO and soot can be reduced by using relactively delayed natural gas injection timing with relatively low injection pressure,shortening the interval between pilot diesel and natural gas injections as well as decreasing diesel pulse width.2.The in-cylinder pressure signals were processed and analyzed with the aid of pressure decomposition,Fourier transformation and Wigner-Ville distribution to obtain the first and second deviations of the combustion pressure,pressure level curves of the combustion pressure and the combustion resonance pressure as well as the time-frequency power spectrum for the combustion resonance pressure.The processed signals were then used to predict and assess the engine noise.The results indicated that,the combustion noise of high pressure direct injection natural gas engine mainly comes from the combustion process of natural gas;the contribution of the first deviation of the combustion pressure to combustion noise is more profound than the energy of combustion resonance signals;the effective method for combustion noise control including delaying natural gas injection timing,decreasing injection pressure,increasing the interval between pilot diesel and natural gas injections in a proper range as well as extending diesel pulse width.3.After in-depth investigation of the formation and growth process of PAH species for different surrogate fuels,a new n-heptane/toluene/methane/PAH mechanism was developed using the hierarchical construction method based on the results of DRGEP and reaction path analysis.The mechanism was then coupled with the 12-step NOx mechanism,forming an integrated mechanism with 84 species and 336 steps,which is adaptable for the efficient simulation of the combustion process and emission formation of high pressure direct injection natural gas engines.The mechanism was proved to have the capapbility of predicting the ignition delay and laminar flame speed of the surrogate fuels as well as the concertration of key species,especially the concertration of soot precursors in the flames of surrogate fuels with high acccuracy.4.The 3D numerical simulation model for high pressure direct injection natural gas engine was built by combining the n-heptane/toluene/methane/PAH mechanism with RANS turbulence model,modified KH-RT spray breakup model and phenomenological soot model.The 3D numerical simulation model can predict cylinder pressure,NOx,CO and soot emissions precisely,implying that the model is reliable for the numerical investigation of the combustion and emissions of the test engine.5.Numerical investigations were performed on the basis of the 3D numerical simulation model built in this thesis.The effects of hydrogen addition,natural gas main-post injection strategy and EGR on the combustion and emissions of high pressure direct injection natural gas engine were studied by analyzing the results of the 3D numerical simulation model with the assistance of the sensitivities analysis of the key reactions in OD homogenous reactor and the production rate analysis of the related reactions in 1D stabilized laminar flame burner.The results showed that the variations of the concertrations for OH and H free active radicals can be used to indicate the progress of the combustion process,the occurrence of the increase of HO2 free active radical forebodes the ignition of pilot diesel and the values of the concertration of HO2 free active radical can be used to represent the rate of heat release during the same combustion process;lower CO and soot emissions as well as higher indicated thermal efficiency can be obtained when adopting natural gas/hydrogen blended fuels;however,higher NOx emissions will be accompanied.The implementation of natural gas main-post injection strategy can to some extent improve the indicated thermal efficiency of high pressure direct injection natural gas engine when interval between natural gas main and post injections is appropriate;when interval between natural gas main and post injections is excessively large,deteriorated indicated thermal efficicency can be observed.Additionally,lower CO and soot emissions can be achieved when combining intervals between natural gas main and post injections larger than 14?A with natural gas post injection proportions higher than 10%whereas NOx emissions will increase gradually with the increase of natural gas post injection proportion.In view of adding EGR,the most obvious effect is the reduction of NOx emissions.It was also found that adding EGR lower than 20%will not cause deteriorated indicated thermal efficiency,however,increases in CO and soot emissions are inevitable.When combines EGR with natural gas main-post injection strategy,compromise between indicated thermal efficiency and emissions of NOx,CO and soot can be achieved. |
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