282 Diesel Technology Copyright Goodheart-Willcox Co., Inc. The exhaust system collects combustion gases from the diesel engine and carries them away. Removing spent combustion gases from the cylinders with as little resis- tance as possible is as important for effective combustion as filling the cylinders with fresh air. In addition to removing combustion gases, the exhaust system is designed to remove engine heat, reduce NOx emissions, reduce soot emissions, muffle exhaust noise, quench sparks, and in cases, furnish energy to a turbocharger. Exhaust System The exhaust system of a mobile diesel engine generally consists of the exhaust manifold, exhaust piping, EGR valve, diesel oxidation catalyst (DOC), diesel particulate filter (DPF) muffler (some engine manufactures use the DPF as the muffler), and outlet pipe (the latter is called the stack in heavy duty vehicles or the tailpipe on cars and light trucks). Gases must be pushed around bends, elbows, adapters and connectors in the piping system, and of course, through the muffler, Figure 13-1. Some systems contain a resonator. On most engines, a turbocharger is installed in the exhaust system and driven by exhaust, Figure 13-2. A stationary diesel exhaust system usually contains an exhaust manifold, exhaust piping, EGR valve, DOC, DPF, silencer and/or muffler and outlet pipe, Figure 13-3. The system is designed to discharge the exhaust in a manner that is not objectionable or dangerous. The system must also conduct and discharge exhaust without creating a fire hazard at the installation site or surrounding structures. The exhaust system must be in compliance with any applicable building codes and enable engine to meet EPA emission standards while efficiently removing exhaust gases without creating excessive back pressure. Back Pressure Back pressure is the resistance to the free flow of gas. This resistance can be caused by friction between the gases and the system piping, undersized exhaust piping, a sudden constriction in the piping, and pipe elbows and bends. The higher the back pressure, the harder it is for the hot exhaust gases to get out of the engine. Excessive back pressure leads to high valve temperatures, water jacket overheating, and premature engine wear. Another result of excessive back pressure is loss of power. The hot combus- tion gases trapped in the cylinder dilutes the fresh, incoming air. Less oxygen is available to support fuel com- bustion, and engine power output drops. The harder the pistons have to work against the buildup of the combustion gases, the less power for turning the crankshaft. A 2 psi increase in back pressure can cause an engine to lose approximately 4 horsepower. A low restriction exhaust system minimizes the amount of energy needed to pump combustion gases from the cylinders, leaving more power for operating the engine. This results in better fuel economy at any given speed. Exhaust stack Muffler Exhaust manifold Exhaust pipe Figure 13-1. The diesel engine exhaust system shown in this illustration is used for many mobile applications. Emission control devices are not shown in this illustration. Compressor side of turbocharger Turbine side of turbocharger Air inlet to turbocharger Exhaust outlet from turbocharger Exhaust manifold Engine cylinder Air line Air cooler Figure 13-2. Turbochargers are commonly used on diesel engines. (Caterpillar Tractor Company) Outlet pipe Supports Diesel engine Exhaust manifold Exhaust pipe Muffler/ silencer Figure 13-3. Typical stationary diesel engine system. Designs will vary with each installation and application. (Waukesha Engine Division)