Chapter 13 Exhaust Systems 289 Copyright Goodheart-Willcox Co., Inc. Vertically installed mufflers used on mobile installations are often equipped with drain holes, which prevent moisture from entering the engine. A hinged rain cap is often attached to the top of each stack to prevent water from rain, snow, or washing from entering the system. The cap opens only when the engine is running and closes when the engine is shut off. A simple alternative to the rain cap is a curved outlet tip, which is standard on most new trucks. Mufflers Used on Turbocharged Engines The internal design of most mufflers varies depending on engine and exhaust system design. On turbocharged engines, the turbine blades are located between the engine and the muffler. As the blades are turned by the exhaust gas, they tend to chop up the sound waves. For this reason, mufflers for turbocharged diesels are simpler and less restrictive than those for naturally aspirated die- sels. Mufflers designed for dual applications may differ in baffling and flow capability from mufflers designed for a specific application. Muffler as a Spark Arrester In addition to acting as silencers, most mufflers also function as spark arresters—an important safety factor when operating mobile and stationary diesel equipment in fire-prone areas. In some dry mufflers, a device is incorpo- rated to trap burning carbon particles and soot, Figure 13-17. In wet mufflers, the water serves as a spark arrester. As with any exhaust system part, spark arresters must be matched to the displacement and horsepower of the engine. The relatively low exhaust gas temperatures found in newer engines allow more moisture to remain in the exhaust system. This moisture and the acids that result from fuel sulfur may contribute to internal corrosion. To combat this internal corrosion, many of today’s mufflers have internal components made of stainless steel—even when the shell is a more common material, such as aluminized steel. Polished stainless steel shells are available and give the best overall life. Chromed steel shells are not as desir- able from the standpoint of appearance and longevity— one drawback being their tendency to turn blue when subjected to excessive heat. Resonator On some mobile vehicles, there is an additional muf- fler, known as a resonator or silencer, to further reduce the exhaust decibel level. When used in the exhaust system, it may be located before or after the muffler. The resonator looks like a muffler and is constructed like a straight- through muffler. It is connected to the muffler by an inter- mediate exhaust pipe. Industrial Silencers On naturally aspirated engines, a silencer will be required to reduce exhaust noise and a spark arrester is recommended. By placing the silencer or muffler as close to the engine as possible, interferences resulting from pres- sure waves are reduced, Figure 13-18. While large silencers are usually covered with insulation, Figure 13-19, they must be fitted with some means of draining condensation and cleaning trapped soot from the bottom of the unit. Exhaust out Exhaust in Carbon trap Cleanout pipe Figure 13-17. Exhaust flow through a spark arrester. Silencer located close to engine Support for piping and silencers Aluminum heat shields Flexible exhaust connections Long radius bends Figure 13-18. Location of silencers in an industrial application. (Waukesha Engine Division)