Is the scr constantly active?

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.

 

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.

 

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.

 

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.

 

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.

 

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.

 

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.