What is a Diesel Particulate Filter ?

G.Fueling Considerations   Sulfur in diesel fuel is the primary contributor to SO2 formation in exhaust (engine oil is a secondary contributor). High sulfur fuels also have a higher particulate emission rate due to the formation of sulfates from the SO2. Sulfates when combined with water vapor lead to the formation of sulfuric acid, a component found in acid rain. When exposed to catalytic devices used for controlling exhaust emissions, the catalyst acts to increase conversion of SO2 to sulfate. Another aspect to the formation of sulfate is the propensity of the sulfate to mask the catalyst and thus lower the catalytic activity.   For these reasons regulations have been implemented requiring a reduction in the amount of sulfur present in the fuel. Depending on the market, fuel could have a maximum sulfur content of 15 to 500 parts per million. For applications where a diesel emission control system is in use, often it is required that the engine can only use Ultra-Low Sulfur Diesel (ULSD), which contains less than 15ppm sulfur. The primary driver for this requirement is that the creation of sulfate can lead to the decrease in catalytic activity. Not only does this decrease the conversion efficiency of emissions, but can also cause lower soot oxidation rates leading to increased filter regeneration temperature requirements.   Diesel particulate filters require a minimum temperature at which to regenerate. Most passive systems will function well when ULSD fuel is used, but when higher concentrations of sulfur are present in the fuel, the sulfate which forms masks the catalyst material leading to a decreased conversion rate. As a result, the temperature must be increased further before soot regeneration can effectively take place. Studies have shown anywhere from 15°C to 65°C higher regeneration temperature requirements. Often this increased regeneration temperature leads to poor functioning filters since the exhaust is at the higher temperature less often. A poorly performing filter can also lead to a clogged filter if sufficient temperatures are not reached.   Another concern with high sulfur fuels is that the sulfate adsorbs to the surface of catalysts, then at high temperature will come off resulting in high particulate matter (PM) emissions from the exhaust – this can actually lead to higher PM emissions than would be observed in an engine without a filter. . SO2 can also react with the additives found in engine lube oil. This interaction can lead to the formation of sulfate salts such as calcium sulfate. The sulfate salt is considered a component of exhaust ash. This ash can build-up in the filter and act to cover the catalytic sites. Like the accumulation of sulfate, the ash will have a negative effect on the performance of the filter and lead to higher temperatures for regeneration to occur. By using strictly ULSD fuel, the very low sulfur content makes it less likely that problems will arise with the equipment   H. Selection and Use Criteria   It is very important to remember that DPF technology is an application specific technology. This means that in some applications there may be factors that preclude a DPF technology from being used. In making a decision whether to use DPF technology in a particular engine application, the following criteria must be considered:   . • The level of engine-out emission levels, including those from the engine lubricating oil. • The engine operating exhaust temperature profile. • Available space to equip the DPF. • The level of sulfur in the diesel fuel. • For certain catalyst-based DPFs, the proper NOx/PM ratio in the exhaust must be available.     1 | 2 | 3 | 4 | 5 | 6