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Exhaust aftertreatment system – a cleaning process for exhausts

Maybe you have heard about a Diesel Oxidation Catalyst, a Diesel Particulate Filter, Selective Catalyst Reduction and an Ammonia Slip Catalyst? But do you know how they depend on each other to function? Or, why ammonia is an important component in an engine system in order to meet Stage V emission legislations? In part two of our article series about engine technology, I will walk you through the different components of an exhaust aftertreatment system and describe how they work.  

Stage V emission legislations require an updated engine technology

In order to live up to Stage V emission legislations, the exhaust gases being produced in the engine must go through a cleaning process before they can be released into the environment. The cleaning process takes place in the Exhaust Aftertreatment System (EATS). The difference with the exhaust aftertreatment system used for Stage V compared to previous emission legislations is that, in addition to the Selective Catalyst Reduction (SCR) catalyst and an Ammonia Slip Catalyst (ASC), a Diesel Oxidation Catalyst (DOC) and a Diesel Particulate Filter (DPF) are added. The harmful exhaust emissions from the engine are present through the whole exhaust aftertreatment system. However, the quantities decrease more and more as the exhaust gases flow through the system.

Five different components cooperating for cleaner exhaust fumes

The exhaust aftertreatment system consists of five different components: A Diesel Oxidation Catalyst (DOC), a Diesel Particulate Filter (DPF), Urea (AdBlue) injection, a Selective Catalyst Reduction (SCR) catalyst and an Ammonia Slip Catalyst (ASC).

Diesel Oxidation Catalyst oxidizes nitrogen oxide to nitrogen dioxide

The exhaust coming from the engine first passes through the DOC. It is mainly used for oxidizing nitrogen oxide (NO) to nitrogen dioxide (NO2), substances that together create NOx. However, the DOC also has another important function, which is related to regeneration of the DPF. Regeneration of the filter needs to be done in order to empty the filter from soot. There are three different types of regeneration; stand still regeneration, moving regeneration and passive (continuous) regeneration.

Stand still regeneration is performed while the vehicle is standing still. After the combustion, a small amount of fuel is injected into the cylinder. The hydrocarbons from the fuel are oxidized (burned) in the DOC. The burning of hydrocarbons results in increased temperature of the exhaust gases which, in turn, increases the temperature in the DPF, enabling regeneration of the filter. The temperature needs to be optimized so the soot is burned as quickly as possible to decrease the amount of time the machine needs to stand still.

A passive regeneration, on the other hand, is performed continuously during operation. If the DOC is able to produce a sufficient amount of NO2, the burning rate of the soot will be high enough to perform passive regeneration without having to stop the vehicle. However, a vehicle that drives on low loads in minus 20°C will probably have to perform stand still regenerations, since the engine never will generate sufficiently high temperatures. It all depends on how high temperature the engine generates, and that is affected by type of machine, where it is operating as well as the machine’s load and speed. Moving regeneration, on the other hand, utilizes other heat management systems than injecting fuel in the DOC. This also enables regeneration at low loads and in a cooler climate.

Particles are removed in the Diesel Particulate Filter

The DPF comes after the DOC in the exhaust aftertreatment system. The DPF removes most of the particles (more than 99 %) from the exhaust gases. The exhaust gases pass through the filter, which collects particles from the gases. The particles eventually turn into a soot cake. The increasing amount of soot will create a high pressure in the DPF, which means the soot needs to be removed. This can be done either by stand still regeneration or passive/moving regeneration, as mentioned above. The disadvantage with a DPF is that it creates back pressure. That means the engine needs to work harder, which leads to a slightly higher fuel consumption, unless the engine is optimized for this. However, a DPF is a must in order to live up to Stage V emission legislations.      

Urea (AdBlue™) turns into ammonia, carbon dioxide and water

After collecting the particles from the gases in the DPF and incinerating the soot in the filter, there is still nitrogen oxide (NO) and nitrogen dioxide (NO2) left in the gases. In order to reduce the NOx levels even further, Urea (Adblue) is injected in the exhaust stream. Urea is originally an organic salt that is dissolved in water, and the mixture is often called AdBlue. Urea contains two amino groups and when disintegrating in heat, they turn into ammonia, carbon dioxide and water. The challenge with the urea injection is to mix the ammonia evenly throughout the exhaust flow, going into the next step in the exhaust aftertreatment system, which is called the SCR. An even flow of ammonia into the SCR enables a maximum use of the system, which leads to a more effective reduction of NOx.

A disadvantage with urea is that if too much fluid is injected, or if the fluid hits a cold surface, it can create crystals. A too large crystal can create a pressure drop, which means the fuel consumption increases. The formation of crystals can be avoided by ensuring the piping has no rough edges (like bad welding, steps etc.) where the liquid can get stuck and crystals can start growing. It is also important to optimize the urea injection by ensuring the injector is in the right place and that the right amount of urea is injected at the right angle.  

After treatment system including DPF, SCR and UREA tank

Aftertreatment system including DPF, SCR and UREA tank

Selective Catalyst Reduction reduces NOx levels

After urea has been injected in the exhaust aftertreatment system, the exhaust stream enters the SCR where most of the NOx (normally more than 98%) is removed. Together with active substances in the SCR, the ammonia coming from the urea injection turns nitrogen oxides (NOx) to nitrogen gas (N2) and water (H2O).

Leftover ammonia is oxidized in the Ammonia Slip Catalyst

The ASC is the final component in the exhaust aftertreatment system. In order for as much NOx as possible to be removed in the SCR, the amount of urea being injected in the system is often slightly overdosed. However, the leftover ammonia must not be released into the environment. Therefore, ammonia that was not used in the SCR process is removed in the ASC. The ASC oxidizes the ammonia into water (H2O) and nitrogen gas (N2).    

An Exhaust Aftertreatment System that focuses on high uptime

The aftertreatment system is a complex system with many different functions and components that need to work together. Volvo Penta has designed an exhaust aftertreatment system that focuses on maintaining high uptime and low total cost of ownership. It is optimized in order to enable passive regeneration and ease of installation. To facilitate the installation of the system, Volvo Penta offers two different exhaust aftertreatment systems; a one-box system for the Stage V D13 engine and a two-box system for the Stage V D5, D8 and D11 engines. In the two-box system, the exhaust aftertreatment system is divided into two boxes that are connected through a pipe, which makes the installation more flexible. Thus, Volvo Penta’s exhaust aftertreatment system is adjusted to be as effective as possible, contributing to an engine with both high uptime and low emissions.

A prerequisite for the exhaust aftertreatment system to function is that it is provided with the right amount of heat. The engine’s heat management system will be the topic in our third article in our article series about engine technology. Stay tuned!

I hope you enjoyed reading my article about exhaust aftertreatment system. Feel free to contact me with questions, visit our website for more information or interact with us on social media. And don’t forget to read the first part in our article series “A combination of systems creates a modern engine technology” on our Professional Power Blog.

This article has 2 comments

  1. Miroslawa Milh Reply

    Urea contains two amino groups and when disintegrating in heat, they turn into ammonia, carbon dioxide and water. It is not right. In first step we have NH3 and HNCO . Hnco hydrolysed with H2O to NH3 but HNCOlike polymerized and formed cristals H3N3C3O3.

    • Hannes Kannisto Reply

      Hi Miroslawa,
      Thank you so much for your comment!
      Yes, you are absolutely correct! The detailed disintegration of urea (not mentioning the formation of crystals) is much more complex than stated in the text. However, for the general reader this might not be as important as for us who works with this every day. The text is very general to give an introduction to the subject. This means that some details have intentionally been simplified or left out to make the text easier to read and understand.

      Best regards //Hannes

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