Three Way Catalytic Converters - 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/lecture22/22_1.htm[6/15/2012 3:04:50 PM] Module 5:Emission Control for SI Engines Lecture 22:Oxidation and Three Way Catalytic Converters The Lecture Contains: OXIDATION AND 3-WAY CATALYTIC CONVERTERS Oxidation Catalytic Converters Reduction and 3-Way Catalytic Converters 3 -Way Catalytic Converters Oxygen Sensor Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture22/22_2.htm[6/15/2012 3:04:50 PM] Module 5:Emission Control for SI Engines Lecture 22:Oxidation and Three Way Catalytic Converters OXIDATION AND 3-WAY CATALYTIC CONVERTERS Since 1975, in the production gasoline vehicles two main types of catalytic converters have been used: Oxidation catalytic converters 3-Way catalytic converters. The catalytic converters for the first time were used to reduce only HC and CO emissions from the US gasoline passenger cars in 1975. As these converters reduced HC and CO by oxidation, they were called as ‘oxidation' catalytic converters. NOx emission standards were met by use of EGR at that time. The engines were operated on rich mixtures and with application of EGR engine out NOx emissions were reduced. Secondary air was injected in the exhaust system upstream of the converter to provide sufficient oxygen for oxidation of CO and HC on the catalyst. Later, when the NOx standards were made stringent from 1981, reduction catalysts were also developed. An exhaust gas oxygen sensor developed in the early 1980s facilitated engine operation at near stoichiometric mixtures that made it possible to simultaneously oxidize CO and HC to CO2 and H2O and reduce NOx to N2 . As all the three pollutants were converted simultaneously in the same reactor these were termed as 3-way catalytic converters. T he three-way catalytic converter is presently a standard fitment on most gasoline passenger cars. During mid-1990s, gasoline direct injection (GDI) engines with charge stratification were introduced in the market by two Japanese car manufacturers. During part load city operation, these GDI engines work as stratified-charge engines with overall very lean air/fuel mixtures. For NOx control in these engines, lean de-NOxcatalytic converters have been developed. The de-NOx x catalysts and other advanced catalyst systems such as for cold start HC control are discussed later. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture22/22_5.htm[6/15/2012 3:04:51 PM] Module 5:Emission Control for SI Engines Lecture 22:Oxidation and Three Way Catalytic Converters contd... 3 -Way Catalytic Converters The essential condition to simultaneously oxidize CO and HC and reduce NOx on the same catalyst bed is to operate engine at very close to stoichiometric air-fuel ratio. Under stoichiometric engine operation, enough reducing gases CO and HC are present in the exhaust to reduce NOx to N2 and at the same time enough oxygen to oxidize CO and HC. Thus, all the three pollutants are removed simultaneously in a 3-way catalyst. . Dependence of conversion efficiency for the three pollutants on fuel-air equivalence ratio in a 3-way catalyst is shown in Fig. 5.14 . High conversion rates of over 80% of all the three pollutants are obtained in a small window of about 0.12 A/F unit width (0.997 < f < 1.005) around the stoichiometric air-fuel ratio. Figure 5.14 Conversion efficiency of a 3-way catalytic converter as a function of air-fuel ratio. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture22/22_6.htm[6/15/2012 3:04:51 PM] Module5:Emission Control for SI Engines Lecture 22:Oxidation and Three Way Catalytic Converters contd... A closed loop feedback controlled fuel management system is used for precise control of air-fuel ratio. A simple closed-loop feed back engine fuel management system is shown schematically in Fig. 5.15. An oxygen sensor installed in the exhaust system detects presence of free oxygen in the exhaust gas which determines whether the fuel-air mixture is leaner or richer than stoichiometric. The signal from the oxygen sensor is fed to a microprocessor controlled fuel management system to adjust fuel injection rate so that the engine operates in a narrow window around the stoichiometric set point. Fuel-air ratio oscillates around the set point at a frequency of 0.5 to 1 Hz as the fuel flow is varied. Signals of air mass flow rate, engine load, speed, spark timing temperatures and several other parameters are also fed to the engine electronic control unit for management of engine operation. Carburetors were found to be incompatible with such control systems although initially electronically controlled carburetors were developed and used. Now, multipoint port fuel injection system is a standard feature of engines using 3-way catalytic converters. Figure 5.15 A simple closed-loop feedback control system forair/fuel ratio control Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture22/22_7.htm[6/15/2012 3:04:51 PM] Module 5:Emission Control for SI Engines Lecture 22:Oxidation and Three Way Catalytic Converters contd.... Modulation of F/A Window Oscillations in fuel flow rate around the set point widen considerably the F/A window, thereby adversely affecting the conversion efficiency. To maintain high conversion efficiency for all the three pollutants, the adverse effect of these oscillations in F/A ratio is countered by use of an oxygen storage/release system in the catalyst wash coat. Components like cerium oxide (CeO2 ) and zirconium oxide (ZrO2 ) are added to the washcoat. CeO2 acts as oxygen storage system and widens the air-fuel ratio window when high conversion rates of all the three pollutants are possible. CeO2 undergoes the following chemical changes as the mixture transits from rich to lean and back due to variations in fuel-flow rate. Rich Operation: (5.4) Lean operation: (5.5) During fuel rich operation, CeO2releases oxygen for oxidation of CO and HC and in the process gets itself reduced to Ce2O3When the engine operation becomes lean as a result of control of fuel flow by engine fuel management system, Ce2O3 gets oxidized back to CeO2by reacting with excess O2 or NO. These reduction-oxidation (Redox) reactions in cerium oxide continue and effectively even in a wider F/A (nearly 0.06 F/A ratio) widow high conversion efficiency of all the three pollutants is obtained. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture22/22_9.htm[6/15/2012 3:04:51 PM] Figure 5.17 Oxygen sensor output voltage as a function of fuel-air equivalence ratio Objectives_template file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture22/22_10.htm[6/15/2012 3:04:52 PM] Module 5:Emission Control for SI Engines Lecture23:Advanced Catalysts for HC Control contd... The temperature of sensor electrolyte affects its conductivity and hence the output voltage. The optimum operation temperature for the oxygen sensor is between 300 and 600º C. During engine warm- up the unheated EGO sensor is not operative. Electrically heated exhaust gas oxygen (HEGO) sensors are therefore used. For meeting ULEV emission requirements, the performance of 3-way catalyst is to be further improved and oscillations in air-fuel ratio are to be minimized. For this, a new sensor which measures actual oxygen content in the exhaust and the engine control unit responds gradually to the changes in air-fuel ratio is used. This sensor is known as universal exhaust gas oxygen (UGEO). With the use UEGO, a better control of air-fuel ratio is obtained and 3-Way catalyst operates in a very narrow window of high conversion efficiency (Fig 5.18). Figure 5.18 Comparison of fuel- air ratio window of operation for UEGO and HEGO