Chapter 15 Fuel and Emission Control Systems 321 Copyright by Goodheart-Willcox Co., Inc. Toxic substances Exhaust manifold Exhaust pipe Thermo sensor Harmless products Muffler Catalytic converter Figure 15-24. A catalytic converter is a thermal reactor that burns any emissions entering the exhaust system. Ideally, harmless CO 2 and water come out of the exhaust pipe. (Toyota) The air pump is usually belt driven. In some systems, the air pump is driven by a small electric motor. This reduces emissions even more because the air pump speed does not change with engine speed. A rubber hose or metal line connects the output of the pump to a diverter valve. The diverter valve keeps air from entering the exhaust system during deceleration. This prevents back­ ring in the exhaust system. The diverter valve also limits the maximum sys- tem air pressure, when needed. It ­ eases excess pressure through a silencer or muffler. An air distribution manifold directs a stream of air toward each engine exhaust valve. Fittings on the air ­ distribution manifold screw into threaded holes in the exhaust manifold or head. An air check valve is usually located in the line between the diverter valve and the air distribution manifold. It keeps exhaust from entering the air injection system. Pulse Air System A pulse air system performs the same function as an air injection system. However, it uses the pressure pulses in the exhaust system to operate check valves. The check valves—also called aspirator valves, gulp valves, or reed valves—block airflow in one direction and allow airflow in the other direction. Pulse air systems are no longer used on modern vehicles. Catalytic Converter A catalytic converter is a thermal reactor for burning and chemically changing exhaust byproducts into harmless substances. It oxidizes (burns) the remaining HC and CO emissions that pass into the exhaust system. Extreme heat of approximately 1400°F (760°C) ignites these ­ emissions and changes them into harmless carbon dioxide (CO 2 ) and water (H 2 O), Figure 15-24. The catalytic ­ operation starts treating emissions when the catalysts are above about 300°F (150°C). A catalytic converter contains a catalyst, usually ­ platinum, palladium, rhodium, or a mixture of these ­ substances. A catalyst is any substance that speeds a chemi- cal reaction without itself being changed. The ­ talysts platinum and palladium treat the HC and CO emissions. Rhodium acts on the NO X emissions. Newer converters also contain a base metal called cerium to attract excess oxygen and release it into the exhaust. A catalytic converter using a catalyst in the form of a ceramic block is often termed a monolithic catalytic converter. When small ceramic beads are used, it is called a pellet catalytic converter. Pellet converters are no longer used. A dual-bed catalytic converter contains two ­ parate catalyst units enclosed in a single housing. A ­ xing cham- ber is provided between the two. Air is forced into the cham- ber to help burn the emissions. Most modern catalytic converters are three-way ­ converters. This type of converter can reduce HC, CO, and NO X emissions. A three-way converter is usually ­ ated with rhodium and platinum. Figure 15-25 shows a catalytic converter system that uses three three-way ­ nverters and three oxygen sensors. Diagnosing Problems Using an Exhaust Gas Analyzer An exhaust gas analyzer is a testing instrument that measures the chemical content of engine exhaust gases. Four- or five-gas analyzers are the most common type. The analyzer probe (sensor) is placed in the vehicle’s tailpipe. With the engine running, the exhaust analyzer indicates the amount of pollutants and other gases in the exhaust. The technician can use this information to determine the condition of the engine and other systems. An exhaust gas analyzer is an excellent diagnostic tool that will indicate: • Engine mechanical problems. • Vacuum leaks. • PCV troubles. • Fuel injection problems. • Ignition system problems. • Clogged air filter. • Faulty air injection system. • Evaporative control system problems. • Computer control system troubles. • Catalytic converter condition.