Book Description

The phenomenon called the ''engine knock'' is an abnonnal combustion mode inspark ignition (SI) engines. it might lead to very high peak pressure in the cylinderand serious damages in engines. Knock limits the compression ratio of the ~ngine. The higher compression ratiomeans the higher fuel conversion efficiency of the engine. it also means highercylinder pressure and thereby higher gas temperature which can cause knock becauseof shorter ignition delay time. Increasing compression ratio is the simplest strategyfor increasing the efficiency of combustion, so a more detailed understanding of theprocesses goveming knock is important.it is generally accepted that knock is initiated by autoignition in the unbumed gasmixture as a result of compression due to the f1ame front propagation and the piston movement. Auto ignition can be defined as spontaneous ignition of some part of thecharge in the cylinder. The autoignition is may cause an extremely rapid chemicalenergy release. it causes a high local pressure and propagation of pressure waveswith high amplitude across the combustion chamber. The rapid rise in pressure andthe vibration of the resultant pressure wave across the combustion chamber cause erosion of the piston, piston rings and head gaskets. Known measures to avoid theoccurrence of engine knock cause either environmental problems, for example theusage of MTBE or reduce the engine thennal efficiency , for example lowcompression ratio, high swirl or early ignition timing. Because of this, the occurrenceof knock was subject of continuous public and industrial research.A detailed investigation of the combustion processes in intemal combustion engines is necessary for the improvement of engine technology .Chemical kinetic model ofthe combustion process implemented into the computational f1uid dynamic sapplications for the prediction of gas f1ow in the combustion chamber provides anefficient tool in tenns of time and cost for the investigation and improvement of the combustion process.The software tools for the modeling of combustion processes in combustion devicesrequire the reduction of the kinetic model to a limited number of species. Since the engine geometry is very complex, the performnnance of commercial software productsfor combustion device optimization decreases considerably if the number of species exceeds about 10. Consequently, a variety of methods in chemical kinetic modelingare needed to construct a reaction mechanism for a complex fuel such as PRF and toreduce it to a low number of capable species without a loss of information that mightbe important for the accuracy of the calculations. One method having the following steps is The generation of a ''detailed reaction mechanism'',The construction of the ''skeletal mechanism'',The final reduction of the reaction mechanism using Quasi Steady State Approximations (QSSA).This study concentrates on the construction of the problem oriented reduced mechanism. A method for automatic reduction of detailed kinetic to reduced mechanisms for complex fuels is proposed. The method is based on the simultaneoususe of sensitivity, reaction-f1ow and lifetime analyses. The sensitivity analysis detects species that the overall combustion process is sensitive on. Small in accuracies, in calculating these species, result in large errors in the characteristic behavior of the chernical scheme. Species, not relevant for the occurrence of autoignition in the end-gas, are defined as redundant. The automatic detection of there dundant species is done by means of an analysis of the reaction f1ows from and towards the most sensitive species, the fuel, the oxidizer and the final products. Theyare identified and eliminated for different pre-set levels of minimum reaction flow and sensitivity to generate a skeletal mechanism. The resulting skeletal mechanism is investigated with lifetime analysis to get the final reduced mechanism. A measure ofspecies lifetimes is taken from the diagonal elements of the Jacobian matrix of the chernical source terms. The species with the lifetime shorter than and mass-fractionIess than specified limits are assumed to be in steady state and selected for removalfrom the skeletal mechanism. The reduced mechanism is valid for the parameter range of initial and boundary values that the analysis has been performed for.The proposed reduction method is exemplified on a detailed reaction mechanism foriso-octane/n-heptane rnixtures. The gas-phase chernistry is analyzed in the end gas of an SI engine, using a two-zone model with conditions chosen relevant for engine knock. Comparing results obtained from the skeletal and the reduced mechanism swith results from the detailed mechanism shows the accuracy of the resulting mechanisms. it is shown that the error in the mechanisms increase with increasingpre-set Ievels of reduction. This is visualized by the help of the predicted crank angle degree at which auto ignition in the end gas of the engine occurs.The reduced mechanism is used for investigation of the modeling of the auto ignitionin the SI engines. The effects of engine operator parameters such as compression ratio, spark advance, fuel equivalence ratio and engine speed on autoignition onsettime have been studied.This work shows that it is possible to achieve a simplified reaction mechanism withgood agreement to the original mechanism by the reduction method. Fundamental knowledge about the detailed mechanism is not necessary to apply the method. Theprocedure used for reduction is fully automatic and provides a fast technique togenerate the problem oriented reduced mechanisms.