288 Diesel Technology Copyright Goodheart-Willcox Co., Inc. Its function is to remove acoustic energy from the gases without unnecessarily obstructing their flow to the outlet pipe. In industrial installation, the muffler is often called a silencer. Types of Mufflers Mufflers are grouped into two general categories: dry mufflers and wet mufflers. There are two main silencing strategies used in dry mufflers: dispersive and reaction. Dispersive Mufflers Dispersive mufflers, or straight-through mufflers, Figure 13-15A, are designed to force advancing sound waves through small openings of high restriction, reducing their intensity. These mufflers consist of a per- forated inner pipe that is enclosed by an outer pipe roughly three times larger in diameter. The space between the pipes is sometimes filled with a heat-resis- tant, sound-absorbing material. Reactive Mufflers Reactive mufflers, or reverse-flow mufflers, Figure 13-15B, use various assemblies, such as tubes, chambers, and baffles to cause a portion of the advancing sound waves to be reflected. This decreases the intensity of the waves as they travel back-and-forth through the muffler. Most modern muffler designs actually use some com- bination of dispersion and reaction in their acoustic pack- aging. Nevertheless, even a well-designed muffler will produce some back pressure in the system. However, a small amount of back pressure can be used intentionally to allow a slower passage of exhaust gases through the par- ticulate oxidizer catalytic converter (described later in this chapter), which results in a more complete conversion of harmful gases. An exhaust system that contains many bends can use up as much as half the engine’s total allowable back pressure before the exhaust gases even reach the muffler. Such a complex system is hardly ideal from an efficiency standpoint. Nevertheless, exhaust system engineers can prevent excessive back pressure in the system as a whole by using the proper muffler. Marine Mufflers Since raw water is readily available in marine applica- tions, most modern marine engines use a wet muffler. A cutaway view of a wet muffler is shown in Figure 13-16. Wet mufflers are usually constructed of cast iron or sheet iron and contain a system of internal baffles that break up the exhaust gas pulsation. This produces a silencing effect without back pressure. The water used in the wet muffler also aids in reducing noise. The water cools the exhaust gases, causing them to contract. This decrease in volume reduces the velocity of the exhaust gases and, therefore, reduces the exhaust noise. The water itself also absorbs some of the sound. Some marine engines are equipped with dry mufflers. In both wet and dry marine mufflers, circulating water reduces the temperature of the exhaust gases. The differ- ence between the two is that the exhaust gases in a dry muffler do not contact the cooling water and the water in a dry muffler does not flow through the muffler compart- ment. Instead, it flows through a jacket around the muffler. In wet mufflers, the gases are expanded into the muffler and directly contact the water. In passing through the baf- fles and the water, the gases are cooled, condensed, and effectively silenced. Water Removal Water accumulation in the exhaust system may result in rusted pipes and fittings, early engine component failure, and excessive noise caused by internal muffler corrosion. A B Figure 13-15. Two types of dry muffler designs. A—Straight- through muffler. B—Reverse-flow muffler. (Deere & Company, Inc.) Water injection Baffle plate Exhaust out Exhaust in Figure 13-16. Cutaway view of a wet muffler. (Caterpillar Tractor Inc.)