Post-flame Oxidation - Engine Combustion - Lecture Notes, Study notes of Sustainability Management

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Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture10/10_1.htm[6/15/2012 2:58:19 PM] Module 2: Genesis and Mechanism of Formation of Engine Emissions Lecture 10:Post-flame Oxidation of HC and Transport to Exhaust Post-flame Oxidation of HC and Transport to Exhaust The Lecture Contains: Post-flame HC Oxidation HC Transport to Exhaust Summary of HC Emission Processes in SI Engines Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture10/10_2.htm[6/15/2012 2:58:19 PM] Module 2: Genesis and Mechanism of Formation of Engine Emissions Lecture 10:Post-flame Oxidation of HC and Transport to Exhaust Post-flame HC Oxidation During expansion stroke, the hydrocarbons from the quench layer, oil film and crevices diffuse back into the bulk combustion gases. These hydrocarbons that diffuse back into the burned gases oxidize inside the cylinder and in the exhaust depending upon the burned gas temperature and the availability of oxygen. HC levels in the cylinder prior to opening of exhaust valve are 1.5 to 2 times higher than the concentrations in the exhaust. Empirical correlations have been obtained from the experimental data to estimate HC oxidation in the cylinder and exhaust as given below: (2.35) where [ ] denotes concentration of reactants in moles per cm3, XHC and XO2 are the mole fractions of HC and O2, respectively, t is time in seconds, temperature T in K and the density (p/RT) is in moles per cm3. From the oxidation rate given by the above expression, oxidation time, tox for a given concentration [HC] can be estimated as below: (2.36) As the quench layers on the cylinder walls are thin, HC from these diffuse rapidly into burned gases and get oxidized. When the bulk gas temperatures are higher than 1300-1400 K, the HC oxidize rapidly. The unburned hydrocarbons from the crevices between piston and cylinder expand back into the bulk gases later in the expansion and exhaust strokes when temperatures have fallen below 1300 K. Just before exhaust valve opens, the gas temperatures are generally around 1250 K, but decrease below 1000K after exhaust blows down. Thus, a significantly large fraction of HC emerging from crevices and oil layers may not be oxidized. It is estimated that about two third of the hydrocarbons stored in the crevices and absorbed in oil film get oxidized inside the cylinder of the conventional gasoline engines under steady state, mid-load and mid-speed engine operation. At lower loads, extent of postflame HC oxidation would be lower. During postflame oxidation, partial combustion products and intermediate hydrocarbons due to decomposition of fuel molecules are also produced. These compounds are not present in the original fuel and constitute about 50% percent of the exhaust HC. Oxidation to an extent of up to 45% of HC which leave the cylinder is possible if high enough temperatures are maintained and the oxidation reactions are not quenched by sudden cooling. In the exhaust port and manifold if residence time is of the order of 50 ms or longer and temperatures greater than 1000 K are maintained, significant oxidation of HC is possible. The engine could be run at stoichiometric or richer mixtures and with retarded spark timing to obtain a high exhaust gas temperature, and calibrated amount of secondary air is injected at the exhaust port to oxidize and reduce HC emissions. This technique was employed on production engines prior to catalytic emission control. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture10/10_3.htm[6/15/2012 2:58:19 PM] Figure 2.14 Schematic of hydrocarbons exiting from engine cylinder Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture10/10_3.htm[6/15/2012 2:58:19 PM] Figure 2.15 Trends in variation of HC concentration and HC mass flow rate at the exhaust valve during the exhaust process of an SI engine. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture10/10_4.htm[6/15/2012 2:58:19 PM] Module 2: Genesis and Mechanism of Formation of Engine Emissions Lecture 10:Post-flame Oxidation of HC and Transport to Exhaust Summary of HC Emission Processes in SI Engines In a port fuel injection SI engine, typically: About 9 percent of the total fuel supplied escapes normal combustion. Of the this Approximately 3 percent escapes as fuel itself through absorption in oil and deposits routes, and as liquid fuel films deposited in the cylinder. About 5 percent as fuel-air mixture due to flame quenching on walls and crevices, and leakage through exhaust valves. Around 1% of fuel –air mixture escapes as crankcase blow by. Of the fuel contained in crevices and quench layers about 2/3rd is oxidized inside the cylinder. Of the fuel in oil layers, deposits and liquid fuel film only about 1/3rd is oxidized inside the cylinder. About 2.7 % of fuel exits the exhaust port and nearly 1 % is retained in the residual burned gases. Of the hydrocarbons exiting the exhaust port about 1/3rd is further oxidized in the exhaust system. Under warmed up operation, engine out HC emissions from a PFI are approximately 1.8 percent of the fuel supplied. The balance 7.2 percent fuel that does not leave the cylinder is either oxidized in the cylinder or at exhaust port and manifold or recycled in the residual gases and crankcase blow by gases. Approximate contribution of different sources to HC emissions for a port fuel injected SI engine is shown in Fig. 2.16