Chapter 13 Exhaust Systems 285 Copyright Goodheart-Willcox Co., Inc. Caution: Never reuse an exhaust manifold gasket. Exhaust Gas Recirculation Nitrogen oxides (NOx) are produced during the com- bustion process when the engine is burning lean, and most modern diesel engines run lean. Most engine manufac- turers have added exhaust gas recirculation (EGR) systems to their engines in order to meet EPA emissions standards for NOx. EGR is the most effective means of reducing NOx. EGR systems take a portion of the exhaust gas and reroute it back into the air intake system. The exhaust gas is made up of mostly carbon dioxide and water vapor. This recirculated gas takes up space in the combustion chamber, decreasing the amount of usable intake air and reducing the lean burn. Most EGR systems are also pre-cooled. The exhaust gas is run through an EGR cooler that uses engine coolant to cool the gas before it is reintroduced to the air intake system. There are disadvantages to using an EGR system. Using exhaust gas in the combustion process reduces engine power and fuel economy. However, today’s elec- tronically managed engines help minimize these losses. The reintroduced exhaust gas also contains carbon parti- cles produced during the combustion process that can pass the piston rings, causing wear on the cylinder walls and creating acids in the engine oil. Components of an EGR System A butterfly valve controls the amount of exhaust gas flow in the EGR system. Piping routes the gas from the but- terfly valve to the EGR cooler. From the EGR cooler the gas is fed to the EGR mixing valve, which is used to combine the exhaust gas with charged, cooled intake air. The mixing valve is controlled by the ECM which receives signals from various engine sensors. These sensor signals include baro- metric pressure, ambient air temperature, coolant and oil temperature, and mass air flow. Diesel Particulate Filter To meet current emission standards, engine manufac- turers use a diesel particulate filter (DPF) to reduce soot or particulate matter (PM). Particulate matter is any solids in the exhaust gas. Particulate matter is produced by incom- plete combustion of the fuel. Lower combustion tempera- tures are the major cause of incomplete combustion. A DPF is similar to a muffler it has a steel outer housing and a honeycomb interior, typically composed of cordierite (ceramic material) or silicon carbide (silicon and carbon). Exhaust gas passes through the honeycomb where it is forced through the ceramic walls, while soot is trapped on the walls. A DPF usually removes 85% to 100% of exhaust particulates. Over time, however, the filter plugs and must be cleaned. The diesel particulates will burn off the DPF at temperatures of 1200°F (650°C) or higher. The process of raising the temperature high enough to burn off soot is known as regeneration. There are two regeneration processes: passive and active. A passive regeneration system incorporates a cata- lyzed DPF that has an interior coated with a metal catalyst. Palladium and platinum are the most common catalysts. These catalysts enable regeneration at approximately 574°F (300°C). This lower exhaust temperature can be achieved under load during normal vehicle operation. Active regeneration uses heat, rather than a catalyst, to clean the DPF. The most common method of generating this heat is to inject fuel (dosing) into the diesel oxidation cata- lyst to raise the temperature above 1200°F (650°C), which is high enough to burn off the soot. The amount and timing of fuel injection is controlled by the engine ECM or a separate control module. Most highway trucks use a catalyzed DPF that is capable of passive and active regeneration, while some vocational applications use a non-catalyzed DPF that requires active regeneration. Selective Catalytic Reduction Selective catalytic reduction (SCR) is another exhaust after treatment device, Figure 13-11. An SCR is designed to remove nitrogen oxides or NOx from exhaust gasses. NOx is produced when nitrogen and oxygen combine at extremely high combustion temperatures. An SCR is used to separate the nitrogen from the oxygen, both harmless gasses, before the exhaust gas is discharged. Urea, or crystallized nitrogen in a water solution, is used as a catalyst in this operation. Urea is also known as DEF (diesel exhaust fluid), and comes in a 32% urea-to-water solution. It is stored in a separate tank. The DEF is injected into the input side of the SCR. The amount of fluid injected is normally controlled by the engine ECM and will depend on the operating conditions. If too much DEF is used, there will be ammonia in the exhaust, if too little is used, there will be NOx in the exhaust. DEF injection is usually done with a dosing con- trol unit and pump. Because the DEF is water based, it can freeze. To prevent freezing when the engine is shut down in cold weather, the dosing pump will reverse and pump the DEF out of all lines back into the DEF tank. The EPA requires all systems to have a level indicator for the DEF, a warning for driver when the DEF level is low, and an automatic engine shutdown when the DEF has been depleted. It also requires the control system to be able to verify the DEF in the tank is at the proper 32% solution and not watered down. Diesel/Water Emulsions The diesel/water emulsion system shown in Figure 13-12 is capable of reducing emissions of NOx par- ticulates, hydrocarbons, and carbon monoxide up to 90% from stationary and mobile diesel engine applications.