Numerical And Experimental Investigation On Diesel Surrogate Low Temperature Combustion Mechanism

In this study, combustion mechanism of Low temperature combustion （LTC）mode was investigated using fully coupled multi-dimensional CFD （KIVA3v-r2） andreduced chemical kinetics model combined with experiments. The combustionprocesses, main emissions and effects of injection strategy at different intake oxygenconcentrations had been discussed.An experimental work of the diesel fuel and several diesel fuel surrogates hadbeen investigated in a wide range of intake oxygen concentration ranging from21%toapproximately10%, covering both conventional diesel combustion and lowtemperature combustion conditions. The ignition delay and emission characteristics ofthe diesel fuel and its surrogates had been compared. The results show that: toluenehad larger effects on ignition delay at lower intake oxygen concentrations. A largereffect of various surrogates on the combustion and emissions can be observed at loweroxygen concentrations. TRF20/1-hexene （95/5vv） matched well with diesel at bothconventional combustion and low temperature combustion processes.Based on the analysis of mechanism with fuel surrogate TRF20/1-hexene（95/5vv）, a new reduced kinetic model of TRF/1-hexene included NO_x and PAHsformation was presented. In low-temperature reaction stage, the second oxygen additionis the most important elementary reaction for n-heptane oxidation and the decomposition ofits production is the main source of OH radical. However, this very reactive radical isreplaced by the lots of unreactive benzyl radical （C₆H₅CH₂·） through toluene reaction, whichretarded the auto-ignition timing. The cracking reaction of1-hexene provided theformation path of ethylene and acetylene. The high temperature reaction stage wasmainly the process of CO formation. The new mechanism, which includes67speciesand135reactions, matched well with experiments in shock tubes and constant volumespary chamber. Then the model was applied to predict diesel LTC process at11%oxygen concentration. Effects of intake oxygen concentration on combustion processof diesel engine were also investigated, covering PAHs formation mechanism and sootemission. The results show that: NO and NO₂declined both with the decrease ofoxygen concentration, however, the magnitude of NO decrease was much larger. Thatis, the ratio of NO in NO_x declined with the decrease of oxygen concentration. Sootemissions increased first then declined. The reason was that the amount of sootoxidation decreased first, while, the essential reason of low soot emissions at very lowoxygen concentration was that soot formation declined obviously. Pyrene （A4） had the same trend with soot at different oxygen concentrations, so soot emissions can bepredicted correctly when A₄as the soot precursor.Effects of injection strategy on combustion and emissions were investigatedusing coupled model. The results indicate: with the increase of injection pressure orwith the early injection timing at LTC，the mixing of fuel and air was improved whichled to the decrese of soot emission and the increase of NO_x emission. There is anoptimum quantity and timing of post injection in different post-injection strategies.Keeping total injection quantity constant, soot declined then increased with more postinjection quantity or later post injection timing. The hydroxyl radical concentrationincreased much in combustion zone through optimized post-injection, whichaccelerated soot oxidation rate. That is, the acceleration of soot oxidation rate bypost-injection fuel was the key reason to reduce soot emission.