Diesel Exhaust Fluid (DEF)<\/strong><\/h3>\n\n\n\nDiesel exhaust fluid is sometimes also known as \u201cAdBlue\u201d. It is a non-toxic, non-flammable liquid composed of one-third urea and two-thirds deionized water.<\/p>\n\n\n\n
Ordinarily, your diesel exhaust fluid will be housed in a container under the hood of your vehicle.<\/p>\n\n\n\n
\u201cUrea Definition\u201d: Urea is a chemical compound consisting of carbon atoms bonded to nitrogen, oxygen, and hydrogen atoms. It is organic and naturally produced in the urine of mammals.<\/em><\/p>\n\n\n\nWhen the urea is introduced to diesel exhaust along with a catalyst, it forms a \u201credox\u201d reaction with nitrogen oxide, completing the transformation of NOx into nitrogen gas and water.<\/p>\n\n\n\n
Particulate Filters<\/strong><\/h3>\n\n\n\nParticulate filters are used in diesel aftertreatment systems to capture particulate matter such as soot and ash before they are released into the atmosphere.<\/p>\n\n\n\n
Particulate filters often have the ability to filter exhaust down to a range of 2 to 10 microns. For reference, one micron is equal to 1 millionth of a meter, or 0.001 millimeters (mm).<\/p>\n\n\n\n
Diesel Exhaust Aftertreatment Systems<\/strong><\/h2>\n\n\n\nNow that we have an understanding of the science behind common aftertreatment methods, let’s have a look at the diesel aftertreatment systems installed on common vehicles.<\/p>\n\n\n\nDiesel Oxidizing Catalyst (DOC)<\/strong><\/h3>\n\n\n\nAfter exhaust first leaves the engine, its first stop is usually through the diesel oxidizing catalyst.<\/p>\n\n\n\n
The purpose of the DOC is to facilitate a chemical reaction that converts carbon monoxide and hydrocarbons into harmless carbon dioxide and water vapor before it is released into the atmosphere.<\/p>\n\n\n\n
The DOC is able to perform this conversion by oxidizing the CO and HC using heat and a catalyst. The heat is sourced from the hot exhaust itself. The catalyst is made from precious metals such as platinum (Pt) and palladium (Pd).<\/p>\n\n\n\n
Disadvantages to the Diesel Oxidizing Catalyst System<\/strong><\/h4>\n\n\n\nBecause the DOC system relies on heat from the exhaust system itself, it becomes much less effective in colder climates and at lower operating temperatures.<\/p>\n\n\n\n
Additionally, because the DOC system is often located upstream of the diesel particulate filter, it can accumulate soot over time, reducing its effectiveness.<\/p>\n\n\n\n
Lastly, the catalyst within the DOC system tends to degrade over time due to contamination and thermal cycling. This degradation leads to an increase in emissions released into the atmosphere.<\/p>\n\n\n\n
Selective Catalytic Reduction and Diesel Exhaust Fluid<\/strong><\/h3>\n\n\n\nThe Selective Catalytic Reduction systems (SCR) and Diesel Exhaust Fluid work together to reduce or eliminate nitrogen oxide from being released into the atmosphere.<\/p>\n\n\n\n
The reason for the name \u201cselective\u201d in \u201cselective catalytic reduction\u201d is because the system is designed to only target nitrogen oxide, while leaving other components of the exhaust unaffected.<\/p>\n\n\n\n
Diesel exhaust fluid is injected directly into the exhaust pipe upstream of the nitrogen oxide catalyst.<\/p>\n\n\n\n
The diesel exhaust fluid, consisting of urea and water, creates the needed \u201credox\u201d reaction with the diesel exhaust when in the presence of the catalyst, which reduces the amount of nitrogen oxide released into the atmosphere.<\/p>\n\n\n\n
This SCR system can achieve up to 90% efficiency in converting nitrogen oxide into nitrogen gas and water when operated under ideal conditions.<\/p>\n\n\n\n
SCR and DEF Disadvantages<\/strong><\/h4>\n\n\n\nSCR systems usually experience a reduction in efficiency at cold temperatures. Therefore, they are often not considered effective until the exhaust gas is at operating temperature.<\/p>\n\n\n\n
An SCR system consumes a lot of space in a vehicle and is expensive to manufacture and maintain.<\/p>\n\n\n\n
The urea in diesel exhaust fluid can naturally decompose after some time.<\/p>\n\n\n\n
Diesel exhaust fluid is a consumable and requires regular refilling. On some vehicles, running out of DEF may put the vehicle into a derated mode, reducing the power and performance of the vehicle until the DEF is refilled.<\/p>\n\n\n\n
Diesel Particulate Filters<\/strong><\/h3>\n\n\n\nThe purpose of diesel particulate filters is to trap and eliminate particulate matter from diesel exhaust before it enters the atmosphere.<\/p>\n\n\n\n
Particulate filters are normally placed upstream of the SCR and DEF injection to protect these components.<\/p>\n\n\n\n
Diesel particulate filters are commonly made from ceramic fibers or cordierite.<\/p>\n\n\n\n
DPF Regeneration<\/strong><\/h4>\n\n\n\nOver time, diesel particulate filters accumulate particulate matter which restricts the flow of exhaust gas through them. The restricted flow of exhaust gases leads to back pressure, which hinders the performance of the vehicle.<\/p>\n\n\n\n
In order to combat this, DPFs will have a \u201cregeneration\u201d method.<\/p>\n\n\n\n
Regeneration is achieved by heating the exhaust or filter to a temperature sufficient to oxidize and burn off accumulated soot in the filter.<\/p>\n\n\n\n
Ordinarily, this is achieved through \u201cpassive regeneration\u201d where the exhaust gases are hot enough to naturally burn off soot during highway or high-load operation.<\/p>\n\n\n\n
When a vehicle doesn\u2019t achieve sufficient temperatures for passive regeneration, an \u201cactive regeneration\u201d method can be used. This involves heating the filter through electrical methods or injecting raw fuel into the exhaust pipe to achieve sufficient heating to oxidize and burn off soot.<\/p>\n\n\n\n
DPF Disadvantages<\/strong><\/h4>\n\n\n\nThe particulate matter that the DPF is capturing contains soot and ash.<\/p>\n\n\n\n
As we have seen, the soot in a filter can be burnt off through \u201cregeneration\u201d methods. However, ash contains non-combustible materials such as metal oxides and sulfates and therefore cannot be burnt off.<\/p>\n\n\n\n
Because of this, DPFs must be periodically cleaned and replaced, resulting in higher maintenance costs.<\/p>\n\n\n\n
Additionally, DPFs increase engine backpressure. Any restriction to gas flow in an engine reduces its performance.<\/p>\n\n\n\n
Exhaust Gas Recirculation (EGR)<\/strong><\/h3>\n\n\n\nExhaust gas recirculation is a vehicle emissions control system used in both diesel engines and petroleum engines.<\/p>\n\n\n\n
The purpose of exhaust gas recirculation is to reduce the formation of nitrogen oxides.<\/p>\n\n\n\n
It\u2019s important to note that while the SCR system helps to remove already formed NOx gases, the EGR system helps to prevent them from forming in the first place.<\/p>\n\n\n\n
The EGR system achieves this by recirculating a portion of the engine’s exhaust gases back to the combustion chamber.<\/p>\n\n\n\n
The addition of exhaust gas into the combustion chamber reduces the amount of oxygen available in the combustion process. In effect, the reduced oxygen lowers the combustion temperature.<\/p>\n\n\n\n
This reduced temperature is important because nitrogen oxide is formed in the combustion process when Nitrogen and Oxygen molecules react at high temperatures. In fact, NOx production increases exponentially as temperature increases, making temperature control a key factor in reducing emissions.<\/p>\n\n\n\n
EGR Disadvantages<\/strong><\/h4>\n\n\n\n\nBy reducing the amount of oxygen available for combustion, the EGR system reduces the power and performance available to a vehicle.<\/li>\n<\/ul>\n\n\n\n\nBy introducing exhaust gas into the combustion chamber, the EGR system may cause soot deposits on glow plugs, intake valves, cylinder heads, turbochargers etc.<\/li>\n<\/ul>\n\n\n\nDiesel Engine Aftertreatment Maintenance – Avoiding Downtime and Extra Expenses<\/strong><\/h2>\n\n\n\nDiesel aftertreatment systems do require maintenance and regular servicing. Luckily, some of the regular aftertreatment maintenance tasks can be completed by the \u201cdo-it-yourself\u201d mechanic and are generally low cost.<\/p>\n\n\n\n
Regular \u201cDIY\u201d aftertreatment maintenance items include:<\/p>\n\n\n\n
\nCleaning and replacing DPF filters<\/li>\n<\/ul>\n\n\n\n\nDEF refilling<\/li>\n<\/ul>\n\n\n\n
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