Study On The Effect Of Gasoline Reforming Products On Combustion And Emissions Of Gasoline Compression Ignition(GCI) At Low-load Conditions

Nowadays,under the multiple background of severe challenge to energy supply,CO₂emission and air environmental pollution facing international and domestic pressure,it is of great practical significance to seek for high-efficiency and clean combustion concepts for internal combustion engines.Considering the ultra-low NOx and soot emissions along with relatively high thermal efficiency,gasoline compression ignition（GCI）is turned out to be a research focus in engine combustion field.Previous research has shown that,GCI combustion concept is considered as a promising advanced combustion concept,which has the possibility to realize high-efficiency and clean combustion in a wider operating range,as well as a broad commercial prospect.However,due to the poor auto-ignition quality of gasoline,how to extend the lowest load limit becomes an urgent issue to be solved.Therefore,the investigation on the effect of gasoline reforming products on combustion and emission characteristics of GCI combustion concept at low-load conditions was performed systematically in this paper by applying engine bench test and numerical simulation.Firstly,a gasoline low-temperature reforming test bench was designed and built to examine the effect of reforming conditions（reforming temperature,reforming oxygen concentration and reforming time）on the composition and proportion of gasoline reforming products experimentally,and the chemical kinetic analysis on the reforming process was simulated by CHEMKIN software.The results show that,under the conditions of reforming temperature below 600 K,reforming oxygen concentration range from 15%to 21%,and reforming time between 3.23 s and 5.65 s,an improved gasoline conversion is arrived by increased reforming temperature,reforming oxygen concentration and reforming time.CO is the most abundant combustible gas in reforming products,followed by H₂ and CH₄.In comparison of other two reforming conditions,reforming temperature is proved to be of key importance to the production of CO,H₂ and CH₄.Furthermore,the highest dependence on reforming temperature exists in the CH₄production.In addition,the dehydrogenation reaction between fuel molecules and OH plays a prominent role in the production of CO and H₂,while the one between i C₈H₁₈ and O₂ is proved to exert a primary part in the production of CH₄.In order to explore the influence mechanism of major components in the reforming gas on combustion process,three-dimensional CFD software CONVERGE and CHEMKIN software for thermodynamic analysis were coupled.In addition,numerical investigation was carried out to study the effect of H₂/CO proportion in the intake air on combustion and emission characteristics when internal EGR was employed.Result indicates that the identified proportion of H₂/CO enables obviously improved combustion stability at low-load conditions（combustion efficiency more than 95%）.Significant reduction in combustion efficiency and even misfire may be observed when blending a small amount of H₂/CO.As the mole fraction of H₂/CO exceeds 6%,the combustion efficiency rises sharply first and then decreases slowly with the increased H₂/CO mole fraction.Besides,slightly higher combustion efficiency will be achieved if a greater proportion of internal EGR is adopted.Compared with H₂,the peak of combustion efficiency can only be reached by blending CO with a higher mole fraction in the intake air,but the combustion efficiency peak is still lower than that of H₂.As for emissions,soot emission is always maintained at an extremely low level when H₂/CO is blended.Besides,significantly reduced HC emission is obtained by blending H₂,and it is recommended to select a relatively low internal EGR rate for comprehensively decreased emissions.For blending CO,there is an evident trade-off relationship between soot emission and emissions of NO_X,CO and HC.