Nature of CAI Combustion and Air/Fuel Ratio Stratification Effects

Abstract : The purpose of this study was to gain a better understanding of the fundamental aspects of the CAI combustion process in order to assess the possibilities of controlling CAI combustion through air/fuel mixture stratification. The experimental work was conducted on a single-cylinder gasoline engine equipped with optical accesses. Two engine configurations (NVO-Negative Valve Overlap-and EGRB-Exhaust Gas Re-Breathing) were tested. Investigation of the combustion process using optical diagnostics showed that CAI combustion is characterized by auto-ignition reactions triggering simultaneous combustion in large volumes of fresh gases. The high global heat release rate results from combustion reactions with moderate local chemical reaction rate occurring over a large volume of the burning gases. From this analysis, it was concluded that controlling the heat release rate in CAI operation implies limiting the volume of the reaction zone by charge stratification, or decreasing the reaction rate by increasing the dilution. Consequently, a significant part of the work was focused on the investigation of the correlation between the fuel/air mixture quality and the CAI combustion process. It was found that low level of fuel stratification obtained with asymmetric port fuel injection does not significantly affect the combustion phasing even though it directly influences the location of the auto-ignition zones. Indeed, single cycle observations of the fuel distribution and the combustion process showed that the location of the first auto-ignition zones is strongly correlated with the position of the fuel rich areas. Finally, direct fuel injection was found to provide a potential mean of controlling auto-ignition timing and to some extend the heat release rate. Combining multiple injections during the negative overlap period, the intake stroke and in the late compression stroke, the auto-ignition delay may be accurately adjusted to optimize the heat release phasing. It is believed that the control capability offered by multiple direct fuel injections can be used to extend the CAI operating range and further improve the fuel efficiency benefits.
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B. Thirouard, J. Cherel. Nature of CAI Combustion and Air/Fuel Ratio Stratification Effects. Oil & Gas Science and Technology - Revue d'IFP Energies nouvelles, Institut Français du Pétrole, 2006, 61 (1), pp.95-119. ⟨10.2516/ogst:2006007x⟩. ⟨hal-02005793⟩

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