198 Auto Engine Performance and Driveability Copyright by Goodheart-Willcox Co., Inc. Exhaust Manifolds The exhaust manifold conducts the exhaust gases from the cylinder head to the exhaust pipes, Figure 10-28. It may be made of cast iron or, for maximum durability and noise reduction, double-walled steel. One or more oxygen sensors are installed in the exhaust manifold or in the exhaust pipes just after the manifold. When used, the turbocharger is installed on the exhaust manifold or in a special exhaust pipe immediately downstream of the exhaust manifold. The turbine wheel is exposed to the exhaust gases, which spin the wheel. On engines equipped with an air pump, the exhaust manifolds are equipped with air pump–inlet tubes that have a threaded connection for maximum sealing. There is usu- ally one inlet tube for each engine cylinder. Catalytic Converter The catalytic converter is a device that reduces the amount of harmful exhaust gases. Unless it becomes clogged, it has no effect on engine operation. Most vehicles use a three-way catalytic converter. The catalytic converter contains the elements platinum (Pt), palladium (Pd), and rhodium (Rh). Each of these elements is catalysts, which is any material that causes a chemical reaction in something else. In addition, the converter contains cerium (Ce) to store and release oxygen. A three-way catalytic converter consists of two sepa- rate sections: • Front section for reducing NO X . • Rear section for oxidizing HC and CO. See Figure 10-29. This type of catalytic converter is always used with an air pump. Output from the air pump is forced into the center inlet between the front and rear sections. The front section contains platinum and rhodium. These catalysts react with the NO X in the exhaust break- ing it into nitrogen and oxygen with some residual carbon dioxide and water. This section of the converter is called the reducing converter since it reduces the NO X to the compo- nent elements of nitrogen and oxygen. The rear section of the converter is called the oxidiz- ing converter. It contains platinum and palladium. These catalysts cause a chemical reaction that converts HC and CO into carbon dioxide and water. In the rear section, the exhaust gas is also combined with air (oxygen) delivered by the air pump. This oxygen allows the catalysts to finish converting the remaining CO and HC into carbon dioxide and water. The air-injection system delivers air into the converter only when the engine is at operating temperature. The element cerium (Ce) has the ability to attract and release oxygen. The excess oxygen in the exhaust stream sta- bilizes the catalytic elements and enhances the converter’s ability to convert exhaust gases into carbon dioxide and water. Since catalysts deteriorate over time and with use, this extends the converter’s effective life. The more oxygen the catalytic elements receive, the better and longer they will be effective. Many newer exhaust systems have a separate reduc- tion converter immediately after, or as part of, the exhaust manifold. These converters are designed to make full use of the heat in the exhaust before any is lost to the surround- ing air. NO X reduction occurs in this converter, and the O 2 produced travels to a second larger converter to reduce HC and CO. On these systems, an air pump is not used. V-type engine exhausts usually have a reduction converter at each exhaust manifold. Since the catalytic converter operates by chemical reac- tion and is also receiving hot gases from the engine, it can easily reach temperatures up to 1600°F (875°C). Therefore, the parts of the converter are made of heat-resistant met- als, such as stainless steel. Metal heat shields are also placed around the converter to protect the rest of the vehicle and anything under the vehicle from the intense heat. Diesel Engine Catalytic Converters A diesel engine produces less CO and HC than a gaso- line engine. However, a diesel engine produces excessive amounts of two pollutants: soot and NO X . Soot can be cap- tured by a particulate trap in the converter and then drawn Figure 10-28. A double-wall, steel manifold is shown here. Other exhaust manifolds are made of cast iron. Inner liner Figure 10-29. A typical three-way catalytic converter. Air is pumped into the center section by the air pump. This increases the catalyzing action as well as catalyst life. Exhaust gases in Output from the air pump Front section (reduces NOX) Rear section (oxidizes CO and HC) Exhaust gases out