14,16 Diesel Dialogue 7/17/13 9:24 AM Page 16
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How the System Works The DEF fluid is a mixture of 32.5% high-purity synthetic urea and 67.5% deionized water. The solution is added to a tank on the vehicle that is located generally near the fuel tank for convenience. The DEF tank has a blue lid where the fuel tank has a green lid. The DEF fluid is transferred from the tank to an injector via a pump. The injector for the DEF fluid is usually placed downstream (after turbo) in the exhaust system. The injector is operated electronically by a controller that will open it to allow a low dose of DEF into the exhaust stream. The controller for the injection of DEF is programmed to inject the right amount of fluid based on inputs from the engine controller. As engine demands change, the amount of DEF will change depending on engine load, rpm, speed, temperature, etc. When the DEF is injected into the exhaust stream, it becomes a catalyst for the NOx. The NOx level is reduced between 70%
16 July 2013 | EngineBuilder
and 90% based on application. Here’s the beauty of SCR: by using urea, more NOx gas can be reduced than other treatments, which has always been a major issue when trying to reduce diesel emissions. This is why the EGR valve system was placed on diesel engines. The EGR was used to revert exhaust gas back into the intake in order to lower the oxygen content of the the incoming charge of air into the engine. When the oxygen level is reduced, the combustion temperature is also lowered. In a diesel, when the combustion temperature is reduced, you begin to form soot instead. Then soot becomes an issue in the intake manifold along with the rest of the exhaust system. When the soot particulate had to be dealt with, manufacturers began incorporating an expensive solution known as a DPF (Diesel Particulate Filter). This is known as a regeneration process where the soot is collected in a filter in the exhaust system and then later burned off by injecting
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fuel to clean the filter. But naturally, this creates higher fuel consumption. By incorporating the use of SCR, NOx levels can be maintained while producing more power. Then the manufacturers rely less on other emission devices such as the EGR and DPF. So, more power can be made with less pollution. This is one of the reason today’s diesel engines can make almost twice the power on the same platform. Injection timing can be tailored to make power instead of being altered to produce less NOx and deal with cooler combustion temperatures faced with EGR. Reflecting back, EGR systems have caused their share of problems. Look at the Ford 6.0L diesel and the problems the EGR coolers impose on those engines. So the use of DEF has proven to be a positive, low-cost solution to lower emissions while offering more power and reliability. ■