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Improvement of Combustion Stability under Cold Ambient Condition by Mixture Control
Abstract:
For diesel engine, lower compression ratio has been demanded to improve fuel consumption, exhaust emission and maximum power recently. However, low compression ratio engine might have combustion instability issues under cold temperature condition, especially just after engine started.
As a first step of this study, cold temperature combustion was investigated by in-cylinder pressure analysis and it found out that higher heat release around top dead center, which was mainly contributed by pilot injection, was the key factor to improve engine speed fluctuation.
For further understanding of combustion in cold condition, particularly mixture formation near a glow plug, 3D CFD simulation was applied. Specifically for this purpose, TI (Time-scale Interaction) combustion model has been developed for simulating combustion phenomena. This model was based on a reasonable combustion mode, taking into account the characteristic time scale of chemical reactions and turbulence eddy break-up. In addition, parameters of the ignition model and computational grids near glow plug were improved to apply under cold start conditions.
As a result, the result of this study reveal that controlling an equivalence ratio and temperature surrounding glow plug appropriately by multiple pilot injections was contributing to the improvement of combustion stability under cold start condition.