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.