Numerical Study On The Chemical Reaction Kinetics Of N-heptane For HCCI Combustion Process

Homogeneous Charge Compression Ignition (HCCI) combustion has beenattracting growing attention in recent years due to its potential for simultaneousreduction of exhaust gas emissions and fuel consumption in diesel engines. However,it is very difficult to control the initiation and rate of combustion over the requiredspeed and load range of HCCI engine.First, HCCI combustion process is researched on a single-cylinder direct injectiondiesel engine with heptane/air. The results show that the HCCI combustion of DMEpresents two-stage combustion process;Both the In-cylinder pressure and the averagetemperature decrease with the increase of excess air ratio and the ignition timing ofthe second stage puts off simultaneously.The auto-ignition and combustion mechanisms of n-heptane have beeninvestigated by using Chemkin-â…¢ software. The results indicate that n-heptane showstwo-stage auto-ignition, heat release with low temperature reaction(LTR) and hightemperature reaction(HTR), and heat release with HTR is consist of blue-flamereaction and hot-flame reaction. The second O2 addition is the most importantelementary reaction for the low temperature branching, and the successivedecomposition of its production is the main source of OH radical at LTR. Blue-flamereaction is controlled by H₂O₂ decomposition, which is the main source of OH atHTR. In this stage, OH abstracts H-atom from CH₂O to generate HCO, and then HCOoxidation to CO. Hot-flame reaction is dominated by the oxidization of CO to CO₂.OH radical is the most important radical in the oxidation of CO and H-abstractionfrom n-heptane. Both different initial temperature and equivalence ratio can generatethree different combustion regions: complete combustion;low temperature reactionand blue-flame reaction;only low temperature reaction. The reason without hot-flamereaction is that the absence of reaction 8 H+O2=O+OH result in the lack of OH radicalto oxidize CO;The key of only low temperature reaction is the absence of H2O2decomposition and H₂O₂ decomposes rapidly only when the cylinder temperatureincreases above 1000K,so there is not enough OH radical to oxidize CH2O .In addition, on the basic of the elementary reaction path generated starting fromthe study above, a reduced model of n-heptane has been developed by sensitivityanalysis and reaction path analysis of intermidate species. The new reducedmechanism consists of 35 species and 41 elementary reactions and it has beenvalidated under engine condition. The results show that it gives predicitions similar tothe detailed model in the ignition timing,in-cylinder temperature and pressure.Furthermore, the reduced mechanism can be used to simulate boundary condition ofpartial combustion in good agreement with the detailed mechanism.