Browse Publications Technical Papers 2011-01-0850

An Investigation of Cascading Autoignition and Octane Number using a Multi-zone Model of the CFR Engine 2011-01-0850

This paper describes a quasi-dimensional multi-zone model of the CFR engine. The engine cylinder was divided into multiple zones containing the unburned air-fuel mixture, which experienced different temperature-pressure histories during the compression stroke and flame propagation phases of the engine cycle. This allowed for the simulation of a temperature gradient within the cylinder, which is postulated to be the cause of the Cascading Autoignition characteristic of the CFR engine.
A Wiebe function description of the flame front propagation was used to describe the normal combustion process; mass and energy were transferred proportionally from the unburned zones to a single burned zone. A Functional Global Autoignition Model (FGAM) was used to describe the autoignition chemistry in each of the unburned zones and an equilibrium approach was used to determine the composition of the burned zone.
This multi-zone model successfully reproduced the non-instantaneous pressure rise seen in knocking CFR pressure traces. A parametric modeling study was then conducted to investigate the influence of inlet pressure, inlet temperature, residual exhaust gas fraction, burn duration, compression ratio and in-cylinder temperature distribution on the cascading autoignition.
Having been calibrated on knocking pressure traces of iso-octane running under RON 100 test conditions, the model was applied on nine Primary Reference Fuels (PRFs) and a Toluene Standardization Fuel (TSF) under their respective Research Octane Number (RON) test conditions. For each of the fuels, the calibration constants of the FGAM were optimized to fit a comprehensive set of Constant Volume Autoignition simulations, generated by the CHEMKIN™ Chemical Kinetics Software, based on a well-validated Detailed Kinetic Mechanism.
The combination of the computationally inexpensive FGAM that accurately reproduced cool flame heat release with a multi-zone engine model was shown to simulate the post-knock pressure development of a knocking pressure trace in the CFR engine, which the method of using a detailed kinetic model in a simple 2-zone engine model does not. An accurate description of this knock pressure development would enable a more representative simulation of knock intensity as measured in the Octane Rating tests.

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