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Basics of HCCUI/CAI Process
Control of emissions of nitrogen oxides and soot from the IC engines has proven to be most difficult. The origin of NOx and soot during combustion conditions are shown on a Fuel- Air Equivalence Ratio – Temperature ( –T) plot on Fig 7.5. This plot is based on calculations done for n- hydrocarbon and air combustion. In CI engine, the combustion conditions in terms of equivalence ratio and combustion temperature vary across the combustion chamber and also vary with respect to time. Regions of SI and CI engine combustion are identified on this plot and discussed as below:
Conventional SI engine combustion takes place in a premixed homogeneous charge with around 1 and is shown in the right hand bottom corner. A point is shown for stoichiometric homogeneous SI engine combustion. The combustion temperatures here are in the range of 2400 to 3000 K. NO formation is a function of oxygen content and temperature of the burned gases. NO concentration is shown qualitatively by the density of dotted points and is maximum at higher temperatures for slightly leaner than stoichiometric mixtures. At a given temperature, NO reduces for the further lean mixtures and for the rich mixtures with > 1.8 NO is negligible even at the high combustion temperatures.
The region of conventional diesel combustion is shown by a cluster of circular points on this plot. These points refer to local mixture conditions and not the overall fuel-air ratio. The conditions show formation of high concentrations of either soot or NOx locally in CI engine combustion. During combustion of spray, in the core region > 3 and combustion temperatures are in the range 1800 to 2400K. This is the region of soot formation. Concentration of soot increases as increases at a given temperature of combustion. Here any hardly NO formation is expected. At the spray boundary, fuel burns as lean premixed charge or in near stoichiometric conditions as diffusion flame, which is shown by the cluster of points on the left side of SI combustion region. These and T conditions belong to NO formation in CI engine. Here, no soot but high NOx is formed.
The region of desired HCCI/CAI operation is shown in the lower left corner on this plot which would result in low NOx as well low soot formation. Here mixture is much leaner than =1and the combustion temperatures are lower than 1900 C. This is the combustion region which is being created in the combustion chamber for implementation of HCCI concept.
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