Study On Combustion And Emission Characteristics Of An HCCI Engine Fueled With N-Pentanol/n-Heptane

HCCI(Homogeneous Charge Compression Ignition)has been widely recognized as a potential combustion mode.Although the HCCI combustion mode has many advantages,the most critical challenges are that the combustion phase of HCCI cannot be accurately controlled and the operation range is too narrow.Many studies have shown that this is mainly due to the combustion process of HCCI is controlled by chemical kinetics.So it is necessary to study the fuel characteristics and combustion mode to improve the combustion of HCCI.To solve the above difficulties,the design is comprehensively optimized by combining Pipe Injection(PI)with Direct Injection(DI),and considering the direct injection pressure,injection timing,physical and chemical properties of the fuels and component ratio of the fuels and so on.Thus,the combustion performance of HCCI is improved and its operation range is expanded.The research of this paper not only enriches the research of compound HCCI,but also has a significant guiding and application value for HCCI engines to achieve efficient and clean combustion.First of all,the effects of intake temperature Tin,fuel equivalence ratio Φt,and engine speed n on the combustion characteristics,emission characteristics and engine performance of n-pentanol PI HCCI are studied.The results are as follows: with the increase of the intake temperature,peak in-cylinder pressure(p),peak Heat Release Rate(HRR)and peak in-cylinder temperature(T)all increase obviously and occur in advance,combustion timing(CA10)occur earlier and Coefficient Of cyclic Variation(COV)is reduced significantly.HC and CO emissions are reduced,and NOx emission is kept at a very-low level.IMEP(Indicated Mean Effective Pressure)and ηi(indicated thermal efficiency)are improved significantly.With the increase of n-pentanol equivalence ratio,peak in-cylinder pressure,peak heat release rate and peak in-cylinder temperature all increase obviously and occur in advance,HC and CO emissions are reduced,and Combustion Duration(CD)is shortened obviously.IMEP and ηi are also increased significantly.With the increase of engine speed,peak in-cylinder pressure and peak heat release rate first increase then decrease slightly.HC and CO emissions show the trend of first decrease and then increase.In addition,NOx emission is kept at a very-low level.By establishing the direct injection system,in-depth research is conducted on compound HCCI combustion mode of n-pentanol PI/n-heptane DI.The effects of many factors(including direct injection pressure Pin,Pre-injection energy Ratio Rp,direct injection timing φin,total equivalence ratio Φt,premixed equivalence ratio Φp,direct injection equivalence ratio Φd,and intake temperature Tin)on the engine combustion characteristics,emission characteristics and performance are systemically studied.The results show that the addition of the direct injection system(compound HCCI)has a significant effect on the combustion improvement of n-pentanol PI HCCI,which can enhance the control of HCCI combustion phase to a certain extent and expand the operation range.Finally,the operation stability of the engine within a certain range is improved.Under certain test conditions,when Pin=6MPa,the injector and combustion chamber achieve the best matching degree and the engine achieves to the best comprehensive performance.With the increase of Rp from 0.45 to 0.65,peak in-cylinder pressure,peak heat release rate and in-cylinder temperature all increase.CA10 and CA50 occur earlier.Cyclic variation tends to decrease,and IMEP and ηi are improved significantly.However,the combustion process will become worse,when Rp continues to increase to 0.77.When Φt=0.45 and Rp=0.65,the optimal injection timing φin is 340°CA.With the increase of total equivalence ratio Φt,peak in-cylinder pressure,peak heat release rate and peak in-cylinder temperature all increase.The combustion phasing is retarded.The combustion duration become longer.ηi first increases and then decreases.In addition,HC and CO emissions are reduced,and NOx emission tends to rise.Under certain test conditions,the parameters of Φp and Φd have the relative optimal values to achieve the better comprehensive performance of the engine.With the increase of intake temperature,COVPmax is significantly reduced.IMEP and ηi are significantly improved.HC and CO emissions are significantly decreased.Then,the compound HCCI combustion mode of n-heptane PI/n-pentanol DI is further explored.The effects of the total equivalence ratio Φt,Pre-injection energy Ratio Rp,and direct injection timing φin on the engine combustion characteristics,emission characteristics and performance are investigated.It is concluded that the combustion mode mainly include three stages of combustion heat release,namely,low temperature reaction heat release of n-heptane,high temperature reaction heat release of n-heptane and combustion exothermic reaction of n-pentanol.With the comparison between the n-heptane PI/n-pentanol DI compound mode and n-pentanol PI HCCI mode,it is evident that n-heptane may improve combustion of n-butanol,reduce the cyclic variations,and make the engine operate more stable.The peak in-cylinder pressure of the compound mode is higher than that of common HCCI,and the corresponding positions of the crank angle is also advanced.The peak in-cylinder temperature of compound mode is higher than that of common HCCI.The timing of CA10 and CA50 are advanced obviously,but the combustion duration CD is increased.Also,the compound mode may improve the emissions to some extent.The HC emission is significantly reduced.CO emission is slightly reduced.NOx emissions are kept at a very-low level in both modes.Especially,the compound mode may obviously improve IMEP and ηi of the engine under the condition of small equivalent ratio(Фt=0.40).With the comparison between n-pentanol PI/n-heptane DI compound HCCI mode(referred as Mode 1)and n-heptane PI/n-pentanol DI compound HCCI mode(referred as Mode 2),it is evident that,when the equivalence ratio is similar,ηi of Mode 1 is higher than that of Mode 2;the cyclic variation of Mode 1 is lower than that of Mode 2;the HC and NOx emissions of Mode 1 are generally higher than these of Mode 2;the CO emission of Mode 1 is generally lower than that of Mode 2.But Mode 1 requires a higher intake temperature to ensure stable operation.