Developing a methodology based on
for predicting cyclic variability in
spark ignition engines
A 3 years project from April 2007 to March 2010
Today's 3D simulation methods for piston engine flows allow a qualitative selection of the most promising designs in terms of fuel consumption and pollutant emissions at a reduced cost because they limit the necessary number of expensive test bench evaluations. Despite their widespread usage, they are limited to the prediction of stable operating points. Now, the usage of new technologies like the advanced engine combustion modes CAI or HCCI, or of flexible direct fuel injection, can often induce the appearance of important cyclic variations, the source and amplitude of which are not accessible to today's standard methods.
SGEmac proposes to develop a simulation methodology for flow and combustion in piston engines that are radically different from standard methods and based on Larg-Eddy Simulation (LES). The latter is naturally afdapted to the simulation of high frequency unsteadiness, thus allowing to extend the field of application of simulation to the prediction of cyclic variability, and thus contribute to reduce their negative impact on fuel consumpition and pollutant emissions, right in the design phase.
To reach this objective, a reference experiment based on a spark ignition single cylinder engine operated under port-fuel and indirect injection homogeneous combustion is acquired. Combining pressure sensors and advanced optical diagnostics allows to characterise the engine operation from cycle to cycle. Operating points with small cyclic variability will serve for an extensive validation of the developed LES models, whilst points with high levels of variability will serve to validate LES methodologies developed by the project partners.
Scientific and technical challenges
LES for piston engines is a highly innovative approach, at the leading edge of piston engine combustion research. On one hand, it has the potential to predict engine combustion in a detailed and realistic way, taking a step towards the emergence of a "numerical engine test bench". On the other hand, it allows envisaging the prediction of unsteady phenomena not accessible to today's methods. .
SGEmac proposes to demonstrate its real potential, and to define a methodology for allowing to exploit it in an industrial context, starting from the recognised expertise of the project partners in LES. In this sense, SGEmac clearly aims taking a worldwide leading position on LES for piston engines, and its industrial application.
In addition to the outcomes in terms of LES methodology, the acquisition of experimental data combining cycle resolved global techniques with advanced optical diagnostics will allow to achieve a bettre understanding of the reasons for the appearance of cyclic variability in engines. .
Expected economic impact
In the medium term, SGEmac shall contribute to extend the range of application of 3D CFD to cyclic variabilities in spark ignition engines , which are often limiting the real benefits from advanced cmbustion modes, and not accessible to today's methods. Th ensuing better understanding of the reasons for their occurence allows to envisage to minimise their ilmpact in early design phases, thus limiting the necessity for a posteriori complex control strategies.
In the longer term, the extension of LES to all types of piston engines, combined with the increase of computational power, allow to envisage the emergence of the "virtual engine" based on LES, allowing quantitative predictions, without a priori experimental knowledge, of all characteristics of a piston engine combustion chamber under stable and unsteady operation.
In the medium to long term, LES shopuld allow to push even farer the limits of the project partners' future engines in terms of energy efficiency and pollution. This shall strengthen their competitiveness in a worldwide context characterised by a constant tightening of emission regulations and an increasing competition.