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Formulation of a Knock Model for Ethanol and Iso-Octane under Specific Consideration of the Thermal Boundary Layer within the End-Gas
Abstract:
Knock is often the main limiting factor for brake efficiency in spark ignition engines and is mostly attributed to auto-ignition of the unburned mixture in front of the flame. In order to study knock in a systematic way, spark angle sweeps with ethanol and iso-octane have been carried out on single cylinder spark ignition engine with variable intake temperatures at wide open throttle and stoichiometric premixed fuel/air mixtures.
Much earlier and stronger knock can be observed for iso-octane compared to ethanol at otherwise same engine operating conditions due to the cooling effect and higher octane number of ethanol, leading to different cycle-to-cycle variation behavior.
Detailed chemical kinetic mechanisms are used to compute ignition delay times at conditions relevant to the measurements and are compared to empirical correlations available in literature. The different correlations are used in a knock model approach and are tested against the measurement data. The importance of using accurate ignition delay time expressions in predicting the correct timing for the onset of knock is illustrated for both ethanol and iso-octane.
The probability of the occurrence of knock is significantly reduced towards the end of the cycle. A new model approach for the thermal boundary layer close to the cylinder walls is included in the knock integral to take into account its effect on the knock probability thus improving significantly the accuracy of the knock prediction. The formulation of the knock model can be derived from the geometry of the combustion chamber and includes its specific shape.
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