Mechanisms of HC Formation - Engine Combustion - Lecture Notes, Study notes of Sustainability Management

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Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture8/8_1.htm[6/15/2012 2:57:39 PM] Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 8:Mechanisms of HC Formation in SI Engines Mechanisms of HC Formation in SI Engines The Lecture Contains: Flame Quenching in SI Engines Quench Layer Thickness HC Emissions from Wall Quenching Crevice HC Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture8/8_2.htm[6/15/2012 2:57:39 PM] Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 8:Mechanisms of HC Formation in SI Engines Flame Quenching in SI Engines Photographic studies of flame region in a spark ignition engine immediately after arrival of flame close to the combustion chamber walls have shown existence of a thin non-radiating layer adhering to the combustion chamber. Flame propagates through unburned charge when the energy released on combustion is able to maintain the reaction zone temperatures at a high enough level to sustain the rapid combustion reactions. However, as the flame approaches combustion chamber walls, more and more heat is lost from the flame to the walls. Due to heat transfer from the flame to the walls, temperature of the reaction zone gets lowered that slows down combustion reactions reducing heat release rate. Finally, as the flame reaches in close proximity of the walls, the gas temperature ahead of flame falls below ignition point and the flame gets extinguished. This phenomenon is known as flame quenching. The flame propagating normal to the single wall will quench at some distance away. When the flame is propagating through a tube it may not propagate if the tube diameter is smaller than a critical value. Similarly, flame may not propagate between the two parallel plates if the distance between the plates is below a critical limit. The normal distance from the wall where flame gets quenched, or the gap between two parallel plates, or diameter of the tube in which flame is just unable to propagate under the given charge conditions, is called quench distance or quench layer thickness. The wall-quenching effects are primarily due to heat transfer and not due to diffusion of species. Quench Layer Thickness Let us consider that flame is propagating normal to a single wall. At the Instant of Flame Quenching For a laminar flame and also as the flame is very close to the walls, heat from the reaction zone is mainly transferred by conduction and, the convection effects may be neglected. Thus, (2.30) where k = Thermal conductivity of the unburned mixture, = Characteristic temperature difference for heat transfer, = Quench distance, Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture8/8_4.htm[6/15/2012 2:57:39 PM] Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 8:Mechanisms of HC Formation in SI Engines HC Emissions from Wall Quenching Single wall quench layer thickness typically varies from 0.05 to 0.1 mm. It decreases with increase of engine load as higher wall temperature results at higher engine loads, which reduces heat loss to the walls from the reaction zone, and consequently a smaller quench layer thickness is obtained. However, at top dead centre the surface to volume ratio of the combustion chamber is at its maximum and at this point the wall quench layer may comprise of 0.1 to 0.2 percent of the total charge inducted into the cylinder. Studies on combustion of pre-mixed fuel air mixtures in combustion bombs show that when all the crevices in the bomb are eliminated by filling with solid material, unburned HC concentrations were just about 10 ppmC only. Such low concentrations result as after flame quenching the hydrocarbons in the quench layer thickness on the single walls diffuse in the hot burned gas quite early and get oxidized. Typically, most hydrocarbons would get oxidized on diffusion in the high temperature burned gases within 2-3 milliseconds of the flame quench. These studies showed that the contribution of single wall quench layers to the total unburned HC emission is quite small. Crevice HC Crevices in the combustion chamber are narrow regions into which fuel-air mixture can flow but flame cannot propagate due to their high surface to volume ratio causing high heat transfer rates to walls. The largest crevice in the combustion chamber is between cylinder wall and piston top land, and second land. Other crevices present are along the gasket between cylinder head and block, around intake and exhaust valve seats, threads around spark plug and space around the central electrode of the spark plug. Piston – ring - cylinder crevice is shown schematically in Fig. 2.12. Table 2.3 gives typical volumes contained in different the crevice regions in the cylinder of a production engine. Total crevice volume is about 3 to 5 percent of the clearance volume and the piston and cylinder crevice constitutes around 70 to 80 percent of the total crevice volume. Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture8/8_4.htm[6/15/2012 2:57:39 PM] Figure 2.12 Typical dimensions of piston top land crevices. Objectives_template file:///C|/...nts%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture8/8_5.htm[6/15/2012 2:57:39 PM] Module 2:Genesis and Mechanism of Formation of Engine Emissions Lecture 8:Mechanisms of HC Formation in SI Engines contd... Table 2.3 Typical Volume Contained in Engine Crevices, cm3 (Engine Displacement Volume/ Cylinder = 352 cm3, CR = 9:1) Volume, cm3 Percent Clearance volume per cylinder 44 100 Volume above first ring (top land) 0.51 1.32 Volume behind first ring 0.32 0.86 Volume between Ist and 2nd rings (Second land) 0.40 0.88 Volume behind second ring 0.32 0.86 Total ring crevice volume 1.55 3.5 Spark plug thread crevice 0.20 0.45 Head gasket crevice 0.20 0..45 Total crevice volume 1.95 4.4 During compression and combustion, unburned charge is pushed into these crevices and at peak pressure, maximum gas would be stored in the crevices. The gas composition into the crevices depends on the location of spark plug. In the piston-cylinder crevices mostly unburned charge would be filled in unless the flame has reached piston top in some location nearest to the spark plug before the peak pressure occurs, which would result also in small amounts of burned gas being pushed into the crevice in this location. The other crevices close to spark plug would be filled with a larger fraction of the burned gas. During expansion, the stored gases in the crevices begin to flow back into the cylinder. Part of the unburned charge from crevices that expands back into the combustion chamber is oxidized on mixing with the hot burned gases. Amount of HC Stored in Crevices include: Contribution of crevice volume to HC emissions may be understood as follows. The crevice gas temperatures are nearly equal to the temperature of walls which are cooled. Hence, the density of the charge stored in the crevices is higher than in the cylinder. The maximum fraction of the unburned charge stored in crevices, Es occurs at peak pressure and is given by; (2.32) where m, V, T and P are mass volume, pressure and temperature. The subscripts cr and o refer to the conditions in the crevices and at the end of intake stroke in the cylinder, respectively. Pmax is the peak