From the point of view of the customer purchasing a car the
ecological as well as the price aspect is in the main focus today
and in the years that come. This will increase due to global
warming, the accelerated depletion of raw materials and significant
price increases. Downsizing of spark ignition engines is an
opportunity to lessen these shortcomings by decreasing the
displacement volume of the engine and for a constant power
increasing the load. In the case of extreme downsizing, especially
in the case of low engine speed, auto ignition occurs in the
air/fuel mixture. As a consequence cylinder pressure tends to
exhibit high amplitudes and frequencies, which can lead to engine
damage.
This paper presents a model which allows linking 3D-CFD with a
detailed chemical reaction system. Therefore a three-dimensional
numerical model in OpenFOAM is formulated that includes all
physical characteristics of a direct-injected, highly charged spark
ignition engine. The conservation equation for mass, momentum and
energy form the mathematical basis for the modeling approach.
Additional equations for spray modeling and for
turbulence-chemistry interaction are required whereat the latter
bases on a partially stirred-reactor approach. This model was
developed and implemented in OpenFOAM at the Chalmers University of
Technology in Göteborg, Sweden.
A detailed chemical reaction system that describes the
combustion of iso-octane/n-heptane and air is used. This mechanism
could be reduced by means of sensitivity and 0D-reactor studies.
Thus it is adopted to the cold-flame range at which auto ignition
occurs. The reduction of the chemical mechanism has great
importance especially for the coupling with the CFD-code.
Several hypotheses for causes of autoignition already exist. The
aim of this paper is to verify such causes with CFD methods,
particular the sensitivity with respect to engine parameters. An
operating state at low engine speed and high load is chosen as a
reference point. Measured pressure history can be applied to
validate the complete numerical model. Finally it is used for a
systematic parameter study to gain insight into the character of
autoignitions.