Effects Of Micromolecular Hydrocarbons In Residual Gas On Gasoline HCCI Auto-ignition

Homogeneous charge compression ignition (HCCI) combustion has beenconsidered as one of the most promising methods to realize energy conservation andemissions resuction of internal combustion engine. However, the clear understandingof HCCI ignition mechanism is pressed for achieving optimized auto-ignition timingcontrol. In our previous experiment of the gasoline HCCI combustion based on therecompression of residual gas, it is found that some experimental results ofauto-ignition timing can not be explained well using the exsisting knowledge of theeffects of residual gas on auto-ignition. It has been proved that the auto-ignition couldbe influenced in chemical aspect by the low-concentration components in residual gas.Consequently, more concerns should be paid on this issue.In this research, a fast instantaneous in-cylinder sampling system is developed tostudy the compositions and concentrations of micromolecular hydrocarbons inresidual gas under the different operating conditions. And comparisons of evolutionsof the micromolecular hydrocarbons are made between internal and external exhaustgas. Furthermore, a zero-dimensional chemical kinetic model with detailed chemicalmechanism for gasoline surrogate is used to model the gasoline HCCI combustion.The effects of these micromolecular hydrocarbons on auto-ignition are analyzedthrough chemical kinetics simulation.It indicates that the micromolecular hydrocarbons with concentrations more than10ppm in residual gas are mainly CH4, C2H6, C2H4, C3H6, C2H2, CH2O and CH3CHO.Under the stoichiometric condition, CH4, C2H4and C2H2occupy about80percent ofthe total concentration of these7kinds of micromolecular hydrocarbons. It is foundthat C2H2and C2H4have the greatest contribution for promoting auto-ignition, whileCH4presents the potential of retrading auto-ignition. Under the condition of partiallean mixture, CH2O contributes more in promoting auto-igntinon. The combinedeffect of these micromolecular hydrocarbons on auto-ignition improves by0.14°CAapproximately compared to the sum of their individual effects under thestoichiometric condition. As the residual gas fraction increases, the chemical effectadvances auto-ignition more due to the increasement of micromolecular hydrocarbonsconcentration in fresh charge. Under constant load operating condition, the auto-ignition is advanced throughthe adjustment of effective compression ratio. It is found that the concentrations ofCH4、C2H4and C2H2increase by8.79times、9.88times and5.71times respectively asthe auto-ignition is advanced. It can be concluded that the advance of auto-ignitionand the improvement of micromolecular hydrocarbons concentrations interact witheach other greatly. Furthermore, the charge heating effect resulted by the elevatedeffective compression ratio is separated through chemical kinetics simulation. Thesimulation results show that the increase of micromolecular hydrocarbons enhancesthe chemical effect of residual gas on auto-ignition.In this research,7.73%external exhaust gas is introduced to expand the operatingrange of HCCI for analyzing the influence of external recirculation on the chemicaleffect of exhaust gas. The evolution of micromolecular hydrocarbons in externalexhaust gas is analyzed by the combination of in-cylinder, intake and exhaust gassampling. The concentrations of micromolecular hydrocarbons which are able toadvance auto-igition are reduced in external exhaust gas compared to that in internalexhaust gas due to the external recirculation. This difference in micromolecularhydrocarbons concentrations results in a0.04°CA retard in auto-ignition, which ismainly resulted by the phase from cylinder to exhaust pipe during external exhaustgas recirculation (EGR). The chemical effect of external recirculation process onauto-ignition is weaker because of the lower external exhaust gas recirculation rate.