Chapter 13 Exhaust Systems 295 Copyright Goodheart-Willcox Co., Inc. Noise Levels In the past years, federal, state, and local governments have adopted noise control laws that affect on-highway vehicles. Enforcement of these laws takes place through certification of newly manufactured trucks, through vehicle registration controls, or by actual roadside noise moni- toring stations where citations may be issued. A decibel is a unit used in measurement of sound intensity. Federal regulations require total decibel read- ings not to exceed 80 decibels (dB) at 50′. Engineers try to reduce the engine exhaust’s contribution to less than 75 dB. The turbocharger cuts about 6 dB and the muffler 10–20 dB. The measure of a decibel’s effects are pro- gressive. A 3 dB drop equals about one-half the sound power, but is only the threshold of what the average person can perceive as “quieter.” It takes a 10 dB reduc- tion for that person to perceive that something is “twice as quiet.” There are five major sources that contribute to the noise level of a highway truck: ❑ Excessive fan speed is one of the biggest noise sources in this area. This can be controlled by the use of a temperature controlled or thermostatic fan. Other noise concerns are bent fan blades and broken or missing shrouds. ❑ Both the type of air intake system and the location of the inlet affect the truck noise level. Air inlets that open to the side are generally noisier than those that do not. Intake air silencers, described in Chapter 12, are available. ❑ The engine and drive train are a major source of noise. Operating speed, type of engine, and drive train all affect the total noise level. Shielding the exposed areas with acoustic barriers will reduce noise levels. Generally, any part of the engine or drive train that can be seen while standing away from the truck will contribute toward the total vehicle noise level. ❑ Tires are often a major noise source at high speeds. The condition of the tires and the tread pattern affect the noise level. At low speeds, this is a less noticeable noise source. ❑ Worn or inadequate exhaust systems are the greatest and most frequent contributor to high noise levels. A visual inspection and repair of leaking connections or replacement of failed com- ponents will noticeably reduce noise levels. If the noise level is still too high, the addition of resona- tors, or a change in muffler is recommended. Reducing Exhaust Noise To control engine exhaust noise without a corre- sponding decrease in performance or fuel economy is important. Here are several specific ways to reduce exhaust noise levels on mobile vehicles. Worn or leaking flexible tubing is a common noise source. Replace the part if necessary. The service life of flexible tubing can be improved if it is installed in a relaxed position. Bending, stretching, and compression all reduce life because they limit the tubing’s ability to “flex.” The entire exhaust system should be well supported. This will reduce the noise generated by the exhaust pipes and the muffler shell. Care must be taken to isolate engine vibra- tion from the exhaust system and to provide for expansion when the system is hot. The addition of a universal resonator, silenced “wye” or splitter muffler in the system will generally reduce the exhaust noise level by 3 to 6 dB. Both items have a min- imal effect on engine back pressure. Change from a single to a dual system. For maximum benefit, when changing from a single to a dual system, special “dual only” mufflers are recommended. Lower system back pressure is an addi- tional benefit of dual systems. On horizontal systems, the tail spouts should be pointed toward the center of the roadway. This helps reduce the noise reflected off the road surface toward curbside observers. On vertical systems, straight stacks will yield lower sound levels than curved stacks. Straight stacks direct the noise upward, while curved stacks direct more noise toward the observer. Turbochargers Turbochargers are used on almost all diesel engines in production today. They utilize exhaust energy, which is nor- mally wasted, to drive a turbine-powered centrifugal air com- pressor that converts air velocity into air pressure to increase the flow of air to the engine cylinders. The air drawn into a naturally-aspirated engine cylinder is at less than atmospheric pressure. A turbocharger packs the air into the cylinder at more than atmospheric pressure. A turbocharger improves combustion, resulting in decreased exhaust emissions, smoke, and noise. Increased power output and the ability of the tur- bocharger to maintain nearly constant power at high altitudes are also benefits of a turbocharged engine. Fuel economy is another very significant reason for using a turbocharger with a diesel engine. Today, con- serving fuel has become nearly as important as controlling air pollution. The extra air provided by the turbocharger allows increased horsepower output without increasing fuel consumption. Lack of air is one factor limiting the engine horsepower of naturally aspirated engines. As engine speed increases, the length of time the intake valves are open decreases, giving the air less time to fill the cylin- ders and lowering volumetric efficiency. Types of Turbochargers There are two types of turbochargers commonly used in today’s diesel engines constant geometry (CGT) and variable geometry (VGT) turbochargers. A constant geom- etry turbocharger utilizes all exhaust gas that enters the