Chapter 10 Emission Control and Exhaust System Fundamentals 201 Copyright by Goodheart-Willcox Co., Inc. Some engines, especially high-performance V-type engines, have two separate exhaust systems, one for each cylinder bank. This is known as a dual exhaust system and it increases the power of the engine by reducing the amount of backpressure. Vehicles with dual exhaust also have two catalytic converters. A dual exhaust system is noisier than a single exhaust on the same engine because the cancel- ing effect of multiple cylinders is reduced. To regain some of this canceling effect, equalize pressure between the two banks, or provide an access point for an oxygen sensor, dual exhaust systems are sometimes equipped with an equalizer pipe. The exhaust system has a harder job than its simple construction would indicate. If the exhaust gases cannot flow freely, the pistons must push against excess backpres- sure in the exhaust system to remove the exhaust gases from the cylinder. This excess backpressure can reduce engine power or cause overheating. In extreme cases, it can keep the engine from starting. Exhaust gases contain water and corrosive byproducts of combustion. When these enter the exhaust system, which is cooler than the combustion chamber, they condense and settle at the low points. These pools of corrosive liquid can quickly rust out the exhaust system. Pipes and mufflers are made of rolled steel and other quality metals that resist engine power. With the valves open, the muffler still con- trols sound to some extent. Exhaust Pipes and Tailpipes Exhaust pipes are used to conduct the exhaust gases between the other exhaust system components. The pipes are steel tubes bent to conform to the underside of the vehicle. Pipes that connect major components are called exhaust pipes. The final pipe in the system, which exits to the atmosphere, is called the tailpipe. Many exhaust and tailpipes are made of two tubes, one inside of the other. This is known as double-wall construc- tion. The double-wall construction deadens sound waves. Some exhaust pipes on luxury automobiles have a high- temperature plastic film sandwiched between the two metal walls for additional sound reduction. Exhaust System Factors Vehicles with fewer engine cylinders tend to be noisier than engines with a greater number of cylinders. This is because the sound waves caused by the exhaust pulsations from each cylinder cancel each other out. The more cylin- ders, the greater the canceling effect. At Low Speeds At High Speeds Bypass flow at high speeds Chamber A Small-diameter pipe Large-diameter pipe Valve closed state Valve opened state Variable valve Variable valve Figure 10-34. The muffler shown here uses two valves to control internal exhaust flow based on engine speed. The valves are closed at low speeds and are opened by exhaust pressure at high speeds. (University of Toyota/Toyota Motor Sales USA)