STOKES CUSTOM HOSE

Tier 1

     In 1994 the EPA started releasing federal regulations on diesel exhaust emissions. These regulations are Tier 1 standards. Tier 1 placed exhaust gas standards for all on and off road engines producing greater than 50hp. These regulations were to be phased in slowly from 1996 to 2000.

Tier 2 and Tier 3

      From 2000 to 2008, Tier 2 and 3 standards were implemented and were increasingly more stringent. All Tier 1 through 3 requirements could be achieved by more advanced engine design, with little to no use of an exhaust aftertreatment system.

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Tier 4/4B

      In May of 2004, the EPA signed the final ruling introducing Tier 4 emissions standards. Tier 4 was to be phased in from 2008 to 2015 which brings us to the current Tier 4B standards of today. Tier 4 standards require that the levels of Particulate Matter (PM) and Nitrogen Oxide (NOx) to be reduced by approximately 90% more than previous standards. In order to achieve this goal would require emission levels near 0% for both Particulate Matter (PM) and Nitrogen Oxides (NOx). In order to reach the near zero levels, manufacturers developed devices to create chemical reactions to change the harmful gasses to gasses that are more natural to the air. 

     The diesel exhaust aftertreatment system consists of four major components. The Exhaust Gas Recirculation system (EGR), the Diesel Oxidation Catalyst (DOC), the Diesel Particulate Filter (DPF) and the Selective Catalytic Reduction (SCR). These four components work together to reduce the levels of Hydrocarbons, Carbon Dioxide (CO₂), Nitrogen Oxides (NOx), and Particulate Matter (PM).

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EGR

     The Exhaust Gas Recirculation system (EGR) consists of to major components. The valve and the cooler. When combustion temperatures rise above 2,370°f NOx emissions are created. The EGR systems circulates exhaust gasses back into the intake to reduce the oxygen level in the combustion chamber. The lower oxygen level produces a less efficient burn of the fuel which reduces combustion temperatures. The lower combustion temperatures reduce the formation of NOx gasses.

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DOC

     The Diesel Oxidation Catalyst (DOC) is a ceramic flow through substrate coated with a catalyst wash-coat. The catalyst on the DOC cell wall creates an oxidizing environment that promotes oxidation of several of the exhaust gasses. The DOC oxidizes Carbon Monoxide (CO) to cause a chemical reaction that converts it to Carbon Dioxide (CO₂). Gas phase hydrocarbons are broken down to create Carbon Dioxide (CO₂) and water vaper (H₂O). The organic fraction of diesel particulates (SOF) are broken down to form of Carbon Dioxide (nCO₂) and a form of water vapor (mH₂O). The Sulfur Dioxide (SO₂) is turned into Sulfur Trioxide (SO₃) which has the negative effect of causing the formation of sulfate particles.

     Simply put, the oxidation catalyst creates an environment where harmful gasses in the exhaust can bond with the free oxygen molecules that are present. The additional oxygen molecules effectively change harmful gasses into much safer gasses.

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DPF

     After the exhaust gasses have passed through the DOC and have been changed over to safer gasses, there are still solid diesel particles present in the exhaust. This is why the Diesel Particulate Filter (DPF) is necessary. The DPF is a wall-pass flow style of filter that is constructed of a ceramic similar to the DOC. The wall-flow filter has straight passages that terminate near the exiting end and the passage wall is porous. This design allows gasses to pass through the wall to the adjacent passages, while the larger solid material is trapped in the original passage. DPF filters will remove 85% or more of the total particulate matter.

     As the DPF catches the solid particles, it will eventually become full, requiring a regeneration to take place. The Engine Control Module (ECM) monitors the level of restriction via multiple back-pressure differential sensors to determine when a regeneration cycle is needed. The regeneration process simply heats the DPF to a temperature high enough to cause combustion of the trapped particulate matter. This process turns the particulate deposits into ash, and effectively compresses the matter into the filter, allowing more particulate matter to accumulate. This ash will continue to be trapped in the DPF and regeneration will not be able to remove it. Eventually the accumulated ash will require the DPF to be removed and cleaned or replaced.

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SCR

     Tier 4 or Tier 4b final requires the Nitrogen Oxide (NOx) levels and the particulate matter levels to be near 0% when exiting the exhaust

The DOC and DPF are not able to achieve this alone, so a third stage of aftertreatment was developed to meet these high standards. The third stage is called a Selective Catalytic Reduction (SCR). The SCR removes the Nitrogen Oxides (NOx) selectively by using a liquid-reductant agent within the catalyst to accelerate the start of the oxidation process. The liquid-reduction agent commonly used is called Diesel Exhaust Fluid or DEF. The active ingredient in DEF is automotive grade urea which is easily broken down to ammonia. The ammonia is what allows the accelerated oxidation reaction to occur within the SCR. The SCR selectively oxidizes the Nitrogen Oxides (NOx) to create Nitrogen (N), Water (H₂O), and small amounts of Carbon Dioxide (CO₂). The SCR is similar to the DOC's flow through substrate. The SCR alone can reduce Nitrogen Oxides (NOx) outputs by 90%.

     All on road diesel engines produced after January 2010 must meet Tier 4b standards an require a SCR. Tier 4b off-road requirements of a SCR vary by horsepower and application.

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