Effects Of Low Temperature Reforming(LTR) Products Of Fuels On Engine Combustion

In order to achieve efficient and clean combustion of ICE（Internal Combustion Engine）,HCCI（Homogeneous Charge Compression Ignition）,RCCI（Reactivity Controlled Compression Ignition）and other combustion modes have been proposed,and fuel reforming technology for hydrogen has also been widely used in ICE.The single reactivity of hydrogen limits the flexible control of the in-cylinder reactivity in ICE.The diversity of low temperature oxidation products of fuels makes it possible to change the in-cylinder reactivity flexibly.Low temperature oxidation can cause the flexible product combination according to the change of reforming conditions,and is not restricted by the selection and the reactivity optimization of catalysts.And the low reforming temperature is easier to achieve.Therefore,low temperature reforming（LTR）has great potential in realizing efficient and clean combustion of ICE at full load.In this work,a new model of low temperature reforming combustion for ICE is proposed,and the influence mechanism of low temperature reforming of fuel on engine combustion is explored.In this work,an LTR system for the engine was established,which could realize the flexible control of reforming conditions.In addition,an on-line quantitative GC detection system was established to detect reforming products.Optical diagnostic methods（high-speed imaging and PLIF）were used to analyze the development of combustion flame in cylinder.Meanwhile,CHEMKIN software coupled with detailed chemical reactions mechanism,was used to evaluate the activity of reforming products and analyze the reforming combustion.Firstly,the HCCI combustion mechanism with LTR was studied.Results show that at the lower reforming temperature（423 K）,PRF0（n-heptane）and PRF50 will not undergo low-temperature oxidation.The reforming products mainly include hydrogen,carbon monoxide,olefins,aldehydes,alkanes,alkynes,alcohols and ketones.Compared with PRF50,PRF0 can produce more reforming products.Low temperature reforming can delay the ignition of HCCI,slow down the combustion rate of HCCI,promote the complete combustion of more fuel,and further inhibit the soot formation during the combustion.The reactivity of each low temperature reforming product is different.Most of LTR products decrease the in-cylinder charge reactivity,which should ultimately lead to the delay of ignition timing in HCCI combustion.Compared with PRF0,the reforming products promoting the ignition of PRF50 increase significantly,while for PRF50 the influences of most reforming products on ignition timing are weakened.LTR can be used as an effective way to solve the problems of difficulty of combustion phase control and high combustion rate in HCCI combustion.By premixing the LTR products entering into the engine cylinder through the inlet and in-cylinder direct injection,the concept of single-fuel RCCI through LTR was proposed and studied in this paper.Results show that with increasing reforming temperature（523K→623K）,the reforming conversion of n-heptane increases.LTR can delay the ignition of single-fuel RCCI,and the higher the reforming temperature,the longer the ignition delay.When reforming does not occur,two stages of low temperature heat release（LTHR）appear in the single-fuel RCCI.The first stage of LTHR is caused by the no-reformed n-heptane entering into the cylinder through the intake port.When reforming occurs,the two stages of LTHR due to the low reactivity of reforming products.The PLIF signals produced by aldehydes and ketones from LTR products,appear before LTHR.And LTR slows down the evolution of formaldehyde during LTHR,makes the single-fuel RCCI combustion more smoothly and reduces soot formation.The ignition timing depends on both the thermal dilution effect and chemical effect of reforming products.The thermal dilution effect of each product will lead to the delay of ignition.Under the chemical influence of the reforming products,LTR products can delay ignition or promote ignition.For the products promoting the ignition under the comprehensive influence,their chemical influences are dominant.For the products which can delay the ignition under the comprehensive influence,whether the thermal dilution effect or the chemical influence is dominant,it should be judged according to the degree of delay.LTR can reduce the main reaction rate during LTHR,which is the main reason for the delayed ignition.Then the thermal dilution effect of reforming products dominates the factors affecting ignition.As an oxygen additive for diesel fuel,n-butanol has attracted wide attention because of its excellent engine performance.Therefore,the effect of LTR of n-heptane/n-butanol on engine combustion was studied.Results reveal that the mole fractions of CO and aldehydes in reforming products increase with the increasing volume fraction of butanol（30v%<B30>,50v%<B50>and 70v%<B70>）in the blended fuel,while the amounts of hydrocarbons,H₂,ketones and alcohols decrease.Regardless of in-cylinder early or late injection,LTR delays the ignitions of B30 and B50,but has less effect on B50.However,LTR promotes the ignition of B70.The change trend of reactivity caused by LTR is not affected by the concentration stratification of in-cylinder mixture.Compared with early injection,LTR has less influence on late injection.LTR can lead to flame development of B30 and B50 to tend to the flame propagation,and the flame development slows down.Although flame propagation dominates the flame development of late-injected B70 whether reforming occurs or not,LTR still makes the flame development of early-injected and late-injected B70have a trend of sequence autoignition,and accelerates the flame development.However,for late injections of B30 and B50,LTR leads to flameless zone in the combustion chamber,which may lead to an increase in unburned hydrocarbon emissions.Compared with early injection,late injection will cause more soot formation.However,LTR can reduce soot formation of B30 and B50,while soot formation of B70 will increase due to LTR.More than half of the reformed products can delay the ignitions of three blended fuels.3-C₇H₁₄,1,3-C₅H₈,C₃H₈ and C₂H₆ have strong ability to delay ignition,while C₂H₃CHO,C₂H₂ and CH₃CHO have stronger ability to promote ignition.Except for CO,1-C₅H₁₀,1-C₆H₁₂ and 1-C₇H₁₄,with the increase of equivalence ratio,the influences of reforming products on the ignition of three blended fuels are weakened.The influence trend of main reforming products on the ignition of the three fuels is consistent regardless of the mixture concentration.Aldehydes in reforming products are the key substances to change the in-cylinder charge reactivity.Effect of LTR product on ignition timing depend on both the type and the species concentration in the reforming products.The concentration,even the type of reforming product usually changes along with the change of reforming conditions（fuel type,reforming equivalent ratio,reforming temperature,residence time,etc.）.Therefore,it can be inferred that LTR is a method of controlling in-cylinder charge reactivity flexibly,and has the potential of realizing efficient and clean combustion of ICE at full load.