Browse Publications Technical Papers 2009-01-0495

Influence of the Valve-lift Strategy in a CAI™ Engine using Exhaust Gas Re-Breathing - Part 2: Optical Diagnostics and 3D CFD Results 2009-01-0495

Among the existing concepts that help to improve the efficiency of spark ignition engines at part load, Controlled Auto-Ignition™ (CAI™) is an effective way to lower both fuel consumption and pollutant emissions without major modifications of the engine design. The CAI™ concept is based on the auto-ignition of a fuel mixture highly diluted with burnt gases in order to achieve high indicated efficiency and low pollutant emissions through low temperature combustion.
Large amounts of burnt gases can be trapped in the cylinder by re-breathing them through the exhaust ports during the intake stroke. For that, a 2-step exhaust valve-lift profile is used. The interaction between the intake and exhaust flows during the intake stroke was identified as a key parameter to control the subsequent combustion in a CAI™ port fuel injected (PFI) engine. Not only does this interaction directly act on the mean in-cylinder conditions (temperature, dilution), but it can also induce some mixture stratification.
As identified in part 1, the phasing and the duration of the second exhaust valve event have a very significant influence on combustion. Nevertheless, the combustion variations that were observed cannot only be explained by the mean in-cylinder conditions. A difference in mixture stratification was proposed as an explanation. To confirm this hypothesis, optical diagnostics and 3D CFD calculations were used. Using acetone-LIF (Laser-Induced Fluoresecence), optical diagnostics provided information about the stratification between fresh and burnt gases. Those results were related with the auto-ignition locations which were observed using direct visualisation of the combustion. 3D CFD results explained the origin of such mixture stratification and balanced the species stratification with the temperature stratification. The combination of both tools allowed the CFD results to be validated and provided complementary information on the combustion process.


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