Is the scr constantly active?

Discussion in 'Trucks [ Eighteen Wheelers ]' started by circle h, Sep 20, 2021.

  1. circle h

    circle h Light Load Member

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    I have searched the internet and this forum and can't totally understand how the scr actually works. My first question is is def being injected into the scr constantly when the engine is running? Second, what is in the scr? Is it built like a muffler or a dpf or something else? Third, Does the scr get hot when the dpf is not regenerating? Last, can the scr be cleaned like a dpf or it that not necessary? I went to the Cummins store and asked all these questions. No one seems to comprehend how this stuff really works. I walked out a little more confused than I started. Any help understanding this would be greatly appreciated. Obviously this stuff is here to stay and I am going to have to live with it. I hope to understand it enough to learn how to avoid down time. Thanks for reading.
     
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  3. Goodysnap

    Goodysnap Road Train Member

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    DEF is not being Dosed or injected all the time.

    SCR stands for Selective Catalyst Reduction - Do some Google searches and you can find plenty of reading to do. Basically DEF is injected and atomized ahead of the catalyst to covert NOX- (oxides of Nitrogen) to less harmless- Carbon Dioxide-(CO2) and Water. The exhaust temperatures or SCR temp must be above 500 degrees F for sufficient NOX conversion to occur.

    A catalyst does not require normal cleaning or routine service as it is not a filter but a flow thru catalyst- Similar to an automotive catalytic converter. The only reason cleaning wound be needed is if it were contaminated with heavy amounts of oil or coolant discharge from the engine that caused the unit to become restricted or plugged.

    Maintenance to the system would include- keeping the fill cap in place and fill the system with good clean DEF- Change and DEF filters as per Manufacturer recommendation. Think of the SCR-DEF a completely separate system from the DPF. They serve two completely different purposes to the treatment of the Diesel Engine exhaust.
     
  4. circle h

    circle h Light Load Member

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    Thanks for the reply. I have tried to read all I can about how the scr works and all the information is vague at best. I now understand the concept and it seems to make a lot of sense. It is unfortunate that egr has not been eliminated since the dpf and scr would appear to achieve the same results without running soot back through the engine.
     
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  5. Arctic_fox

    Arctic_fox Heavy Load Member

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    Mostly correct though the catalyst or sootcan rather does require an occasional clean based on age. It does eventually fill with unburned soot and other contamination over time even when operateing normally. Typically idleing a lot causes this much faster as the engine cant get to the proper burn off temp and not all the soot is correctly converted. In a normal system it needs to be cleaned and/or replaced every 500k or so to keep everything operateing at peak.
     
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  6. circle h

    circle h Light Load Member

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    So what triggers the def to be injected and how often and for how long is it injected? I understand the dpf a little better. It clogs up a bit and triggers unburnt fuel to be injected to create the reaction that turn soot to ash.
     
  7. Goodysnap

    Goodysnap Road Train Member

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    The SCR typically should never see any soot unless the DPF were cracked or damaged and soot was able to pass thru. Of course I speak generally of Cummins product emissions. There is no recommended maintenance cleaning of an SCR. I see many original with no issues closing in on and past the million miles.

    Now DOC will have soot build that is cleaned or washed during a DPF maint. Oil and coolant buildup- otherwise known as face plugging is common with Oil and coolant consuming engines.
     
  8. Goodysnap

    Goodysnap Road Train Member

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    There has been some reduction of EGR% over the years and DEF has been increased to compensate. I have heard of some trials of strictly DEF and No EGR oversea's but have seen nothing close in North America. EPA restrictions pretty much made it impossible to reach requirements without EGR.
     
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  9. Goodysnap

    Goodysnap Road Train Member

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    The Engine and aftertreatment ECU monitor NOX levels out of the engine and out of the tailpipe via NOX sensors measured in PPM. The goal is to have as close to 0 PPM as possible coming out of the tailpipe. DEF is dosed once the exhaust temps reach a certain threshold and adjusts the dosing rate as needed to achieve adequate levels. The NOX output varies greatly based on engine operating conditions. As long as conditions are met DEF will continue to Dose until the engine is shutdown or temps fall back out of operating range.
     
  10. Arctic_fox

    Arctic_fox Heavy Load Member

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    To a point yes. It may be diffrent with the newer ones but i know all three pete dealers ive been in have told me that as a rule the soot can should be cleaned at about 500k as they do tend to fill up. Im sure if the system is maintained correctly it likely can go a million plus but every one of the epa 13 engines i have used over the years typically start to show problems at 500ishK. Then again a lot of companys seem to just let the EGR and def maintaince slide until the truck shows problems then kick it down the road for a new one.

    Mine for example only had 600k and the can was full to near bursting and had to be cleaned.
     
    circle h Thanks this.
  11. Goodysnap

    Goodysnap Road Train Member

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    I think you may be confused on the components.

    The SCR does not trap or collect soot. Neither does the DOC.

    It is the DPF that contains the soot and particulates from the engine which is converted into ash. Once the ash load reached a certain level, then a cleaning or replacement is needed.
     
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