Développement d'un moteur 4-soupapes fonctionnant en mélange dilué. Une nouvelle approche basée sur l'optimisation de l'aérodynamique interne Application of Flow Field Optimization to Lean Burn Engine Development. A New Approach Based on Internal Flow Field Optimization

Abstract

L'objectif du projet GSM Moteur de Synthèse à Allumage Commandéest de concevoir un moteur fonctionnant en mélange pauvre à la fois dépollué et économe. La première phase analyse finement sur moteur monocylindre les interactions entre l'aérodynamique et la combustion qui déterminent l'aptitude au fonctionnement en mélange pauvre ou dilué. La procédure employée présente la particularité d'associer des outils complémentaires tels que code de calcul tridimensionnel, diagnostics optiques (anémométrie laser et ombroscopie) et mesures classiques sur banc monocylindre. La modélisation tridimensionnelle est utilisée comme moyen efficace de sélection et de prédiction de l'aérodynamique interne. Les paramètres les plus influents sur la stabilité de l'initiation de la combustion sont la direction et l'intensité de la vitesse au point d'allumage et le niveau de turbulence. Le meilleur compromis favorable au fonctionnement en mélange pauvre est constitué par une culasse possédant une chambre de combustion en toit avec une seule soupape d'admission. Son aérodynamique interne est caractérisée par la combinaison d'un mouvement de rotation d'axe vertical (swirl) avec un tourbillon d'axe horizontal balayant l'arête du toit (tumble). Son niveau de turbulence est ajusté de manière à accroître la vitesse de combustion en limitant les instabilités cycle-à-cycle. La deuxième phase est consacrée à la transposition de cette solution sur un moteur multicylindre. Les principales difficultés rencontrées sont liées aux disparités de comportement entre cylindres accentuées par le fonctionnement en mélange pauvre. Seul un contrôle individuel des paramètres de combustion de chaque cylindre (avance, injection, richesse) associé à un écartement d'électrodes de bougie accru permet de re-trouver des résultats proches de ceux acquis sur monocylindre. Dans ces conditions, les limites pauvres se situent à des richesses comprises entre 0,60 et 0,70 suivant la charge et le régime. La diminution des émissions d'oxydes d'azote qui en résulte atteint en moyenne sur des points représentatifs du cycle ECE15 74 % par rapport au fonctionnement à la stoechiométrie. Enfin, des essais avec recyclage des gaz d'échappement ont démontré le potentiel de la solution optimisée d'un point de vue aérodynamique à tout mode de fonctionnement en mélange dilué. The constraints that must be accounted for inbuilding a new engine are increasing. It is necessary to contend with user demands for performance, while improving efficiency, limiting pollutant emissions and reducing knock sensitivity. Given the number of parameters to be taken into account, the response to these demands would require large investments in time and resources if conventional development methods (which usually follow an empirical approach) were applied. The purpose of this research supported by Groupement Scientifique Moteur (GSM) was to design a lean-burn sparkignition engine with the object of reducing pollutant emissions and fuel consumption. We try to develop a promising new approach that follows a scientific methodology based in particular on the use of predictive computer codes. The objective of this project is to design an engine that runs on a lean or possibly dilute mixture that will meet European pollution regulations. This lean-burn solution, combined with oxidation catalysis, is considered as an alternative to threeway catalysis, which imposes operation at stoichiometry. The original configuration is a 4-valve engine. One of the advantages this engine offers is great flexibility in changing the inlet conditions. This provides a way of optimizing internal fluid motion, which turns out to be a determining factor in the ability to operate with a lean or dilute mixture. The operation of an engine with a lean or dilute mixture results in substantially reducing nitrogen oxide (NOx) emissions, virtually eliminating carbon dioxide (CO) emissions, and in lower specific fuel consumption. On the other hand, unburnt hydrocarbon (HC) emissions generally increase, which implies the use of an oxidation catalyst if the antipollution standards become too severe. The first phase was to analyze the interactions between fluid dynamics and combustion, which determine the capability of this engine to run with a lean or dilute mixture. The methodology relies on complementary means :(a) Three-dimensional computer code (KIVA). (b) Optical diagnostics (Laser Doppler Velocimetry). (c) Single-cylinder engine equipped with conventional measurement systems. Three dimensional modeling is used to predict and to optimize fluid motion in the cylinder for different intake configurations. The most important parameters influencing the stability of initial combustion are found to be the direction and magnitude of the mean velocity at the spark location, and the turbulence level. We should note that this flow field optimization is also applicable for operation with any dilute mixture (diluted by exhaust gases for example). The question of the minimization of the cyclic variability remains. The most favorable configuration for lean-burn operation was a pent-roof combustion chamber with a single operating intake valve. Fluid motion in this engine is characterized by the combination of a swirling and a tumbling motion and can be described as an inclined tumble. This motion leads to a flow at the spark plug location directed along the edge of the cylinder head. Moreover, the turbulence level is optimal for a high burning rate and low cycleto-cycle instability. The second phase was to apply this solution to a multicylinder system. The main difficulties came from the variability between cylinders, which was amplified during lean-burn operation. Each cylinder must be independently controlled (spark timing, sequential injection, fuel-air ratio, etc. ). Moreover, an increased spark gap is needed in order to reproduce the performance (i. e. efficiency) obtained with the single-cylinder. For these conditions, the minimum fuelair equivalence ratio is between 0. 60 and 0. 70, depending on the engine load and speed. Nitrogen oxide emissions are then reduced from about 74% on the average (compared with stoichiometric emissions) at several selected running conditions representative of the ECE 15 cycle. Finally, the optimized solution was proven to be capable of accommodating any dilute mixture. This was demonstrated via tests using exhaust gases for dilution.