Currently, the development of higher specific output and higher efficiency S.I. engines requires better control and knowledge of knock mechanisms. As it is not easily possible to instrument an engine to determine the beginning of fuel auto-ignition, knock modeling by means of 3D CFD simulation, can be a powerful tool to understand and try to avoid this phenomenon [1, 2, 3]. The objectives of the work described in this paper are to develop and validate a simple model of auto-ignition.
This model, developed at IFP, is implemented in the 3D CFD code KMB [4, 5]. It is based on an AnB model [6, 7] which creates a ‘precursor’ species transported with the flow in the combustion chamber. When its concentration reaches a limiting value, the auto-ignition phenomenon occurs.
This model is used for a large range of engine conditions by varying the fuel injection method (DISI or PFI), spark advance, air/fuel ratio, spark location, internal flow, load, engine speed and cylinder head temperature (uniform or not). The computed delays for knock occurrence are compared with experimental data obtained on a single-cylinder engine, equipped with a pressure transducer. In most cases, the differences never exceeded one crank angle degree. The results suggest that, even if this model is based on a relatively simple chemical mechanism, it is able to give a good overall estimation and reproduce all the experimental trends. Moreover, using KMB, the location of the knock occurrence in the chamber could be found and used to propose combustion chamber shape and cooling circuit modifications.