Numerical And Experimental Investigations On Combustion Process Of Coal-derived Naphtha For Homogeneous Charge Compression Ignition (HCCI) Engines

With the increasing energy crisis and pollution,alternative fuels for vehicles and new combustion technologies for internal combustion engines have received worldwide attention.As a low-cost clean fuel,coal-derived naphtha has the characteristics of stable source,mature preparation technology,high low calorific value,good evaporative property,moderate octane number and almost no aromatic hydrocarbon.It is an ideal alternative fuel for HCCI(Homogeneous Charge Compression Ignition)engines.The combustion of coal-derived naphtha in HCCI engines is a very complex process,which is mainly controlled by the chemical reaction kinetics and the process directly determines the performance and emissions of HCCI engines.At present,we are still faced with the problems of lack of mechanism for coal-derived naphtha,insufficient understanding of low-temperature reaction characteristics of coal-derived naphtha and lack of kinetics basic experiments for coal-derived naphtha.The major works and results of this thesis are as follows:Firstly,the HCCI experiments of coal-derived naphtha injected by intake pipe were carried out.Not only the operating range of coal-derived naphtha on the HCCI engine was measured,but also the effects of the ratio of fuel to air(?),the intake temperature(T_in)and engine speed(n)on the combustion characteristics,emission characteristics and performance of the HCCI engine were studied.Experimental results show that the combustion of coal-derived naphtha in the HCCI cylinder is very sensitive to T_in.Keep n=1200r/min and?=0.35 constant,when T_in increases from 333K to 373K,the peak values of four parameters(the cylinder-pressure P,the cylinder-temperature T,the cylinder-pressure increase rate d P/d?and heat release rate HRR)increase accordingly and the timing of these peaks are also advanced,the combustion timing(CA10)and combustion midpoint(CA50)advance,the combustion duration(CD)gradually shortens.With the increase of T_in,the combustion fluctuation(COV_Pmax)initially decreases and then increases.As T_in increases,the indicated mean effective pressure(IMEP)and the indicated thermal efficiency(?_i)initially increase and then reduce,HC and CO reduce,but NOx increases slightly.Keep n=1200r/min and T_in=353K constant,as?increases from 0.25 to 0.40,the peak values of the parameters(P,d P/d?,HRR and T)increase,and these peaks occur early,CA10and CA50 slightly move forward,CD shortens.With the increase of?,COV_Pmax firstly decreases and then increases,CO and HC emissions are slightly reduced,NOx emissions increases.when?increases from 0.25 to 0.40,IMEP increases,but?_i firstly increases and then decreases.It must be pointed out that if the mixture is too thick,the IMEP will not increase but decrease.The engine speed(n)has a limited effect on the combustion of coal-derived naphtha.Keep?=0.35 and T_in=353K constant.When n increases from 1000r/min to 1400r/min,P_max,HRR_max,d P/d?_maxand T_max all increase and their corresponding crank angles slightly move forward.When n increases from 1000r/min to 1200r/min,CA10 and CA50 advance and CD shortens.However,when n is further increased to 1400r/min,the advancement of CA10 and CA50 is not obvious.when n increases,COV_Pmax firstly decreases and then increases,and HC and CO emissions initially decrease and then increase,the production of NOx is always extremely low.The operating range of the coal-derived naphtha injected by intake pipe on the HCCI engine is relatively narrow.When T_in is high,the stable operation range of the HCCI engine tends to shift to low load.However,the higher T_in easily causes the heat release to be too rapid and concentrated,which can easily cause the crude operation of the HCCI engine.On the contrary,lower T_in is more suitable for heavy load conditions,but misfire is likely to occur under low load conditions.When the mixture is relatively thin,it will greatly limit the speed of HCCI engines.Secondly,the optimal alternative fuel was selected and a reduced mechanism of coal-derived naphtha was developed.Based on physics and chemical characteristic property of coal-derived naphtha,the coal-derived naphtha alternative fuels were selected,and then on the HCCI engine test bench,the important parameters(P,HRR,T and combustion phase parameters)in the HCCI combustion chamber of coal-derived naphtha and different alternative fuels were measured and compared to determine the optimal alternative fuel.And then,in order to get a small-size mechanism of coal-derived naphtha,the directed relation graph with error propagation(DRGEP)method was used to remove insignificant components efficiently,followed using sensitivity analysis(SA)and reaction path analysis(RPA)to further reduce the sub-mechanism.In addition,by comparing the characteristic parameters of the reduced sub-mechanism with the experimental data,the unpredicted parameters were found out,and the sub-model can be further modified.Then,the modified sub-models were combined,the pre-exponential factors of the reactions with high sensitivity to H was optimized.Finally,a reduced mechanism consisting of 403 reactions and 88 components was constructed.A good agreement can be obtained by comparing the experimental data and reduced mechanism predictions on ignition delay time(?),the laminar flame speed(S_L)and the mole fraction of the important species.In addition,the mechanism can accurately predict S_L under the lean mixture(?<1.0)condition.The reduced mechanism is compact and reliable,which can accurately reproduce the chemical reaction kinetics process of coal-derived naphtha and is suitable for simulating the combustion process of coal-derived naphtha in HCCI engines.Finally,numerical simulation study on the combustion process of coal-derived naphtha in a HCCI engine.the AVL-FIRE software was used to construct a multi-dimensional dynamic model of the HCCI combustion chamber,and the reduced mechanism established in this thesis was coupled with it to simulate the combustion process of coal-derived naphtha in the HCCI cylinder.The numerical simulation results show that the predicted parameters(P,HRR and T)are basically consistent with the experimental data,which verifies the accuracy of the reduced mechanism and the dynamic model of the HCCI combustion chamber.High temperature area in the HCCI cylinder is mainly distributed near the piston boss,while the temperature is lower near the connection between piston top and cylinder wall.The pressure near the boss of the piston crown is high,while the pressure around the circular grooves is relatively low.This indicates that the geometry of the combustion chamber has a significant effect on the combustion of coal-derived naphtha in the HCCI cylinder.In addition,compared with the combustion of naphtha in the cylinder of diesel engine,the distribution differences of transient pressure field,temperature field and the concentration field of components(methyl cyclohexane,OH and CO)in the HCCI cylinder are significantly small.This not only indicates that the construction of the HCCI engine test bench is successful,but also confirms that the HCCI combustion mode is dominated by two main factors:fuel chemistry and strong transient turbulence.