Chapter 13 Exhaust Systems 283 Copyright Goodheart-Willcox Co., Inc. Back pressure is measured in inches of mercury, or water, with a manometer. The back pressure limits set by the engine manufacturer are given in the service manual and will vary with engine use. On medium and heavy duty mobile engines, for example, the acceptable maximum might be 3″ of mercury (or 40.8″ of water). Figure 13-4 shows a manometer connected to check the exhaust restriction near the turbo outlet. Exhaust System Components While the design of exhaust system components varies from one manufacturer to another, the primary objectives of all systems are the same. Exhaust systems are designed to provide optimum noise silencing, exhaust gas after treatment, minimum back pressure, and long life. They are also designed to be as light as possible without sacrificing durability. The exhaust system on a heavy duty truck, for example, has to carry out its tasks in a harsh environment. It must survive engine and vehicle twist, internal and external temperature extremes, as well as shock and bounce. Components must be mounted away from expo- sure to wheel splash and other external hazards and be accessible for repair. Mountings also should allow for maximum trailer swing clearance behind a tractor cab, a wide range of body types for straight trucks, and maximum ground clearance for both trucks and tractors. Note: It is important to remember that the intake air system and the exhaust system are very closely related and must work together. The intake system brings fresh air into the engine and the exhaust system gets rid of the burnt gases, Figure 13-5. Therefore, there is a cer- tain overlap of data between this chapter and Chapter 12. Exhaust Manifolds The exhaust manifold collects engine exhaust gases from the cylinder ports and carries them to an exhaust pipe. The exhaust manifold is essentially a large pipe that has exhaust ports, or openings from each cylinder of the engine feeding into it. The exhaust ports and passages in the exhaust manifold are large enough to allow the escaping gases to flow freely and expand. This permits better scavenging of the engine cylinders. If burned gases are left in the cylinders following the exhaust stroke, the amount of air mixture that can be taken in on the next intake stroke will be limited. This reduces engine power and increases fuel consumption. While most exhaust manifolds are made of cast iron or nodular iron, some are made of steel plate with welded joints and branch elbows that are made of steel castings. Other manifolds are made of cast aluminum. Most cast iron manifolds are made of two or more pieces with slip joints to allow for heat expansion, Figure 13-6. On many marine engines, the exposed surfaces of the exhaust mani- fold and related parts may be insulated with layers of spun glass held in place by woven insulation covers, Figure 13-7. Manometer Turbocharger outlet Figure 13-4. A manometer is used to check diesel exhaust back pressure. (Cummins Engine Co.) Fitting Positive crankcase ventilator valve Valve cover Exhaust manifolds Inlet manifolds Air inlet pipe Mounting for air cleaner Figure 13-5. The air intake system and the exhaust system are closely related. (Caterpillar Tractor Company) Three-piece exhaust manifold Figure 13-6. This three-piece manifold contains slip rings that allow movement during heat expansion. (Detroit Diesel Corp.)