Keith Browning Posted July 10, 2011 Share Posted July 10, 2011 After reading the PCED and OBD Theory and Operation publications I am still left with the question as to the DEF system "works." Not the nuts and bolts but rather the strategy or calibration: when and how much DEF is used. Is it constantly injected at a fine rate and increased under load or injected at times where high NoX levels are present? One would think it's not constant and definitely load dependent when higher combustion temperatures are present. How does SCR work with EGR? Anyone with some knowledge on this or knows where to find it please post! Quote Link to comment Share on other sites More sharing options...
Jim Warman Posted July 10, 2011 Share Posted July 10, 2011 During the course of maintenance oil changes, we top off the Add Blue tank. Trucks that are driven lightly rarely take any more than 3 or so liters of DEF. Being that the SCR dosing system is a feedback system (akin to the relationship between fuel injectors and O2 sensors in a gas engine), I would consider that there is a base strategy of look up tables in ROM and that this is where the adaptive strategy begins learning from. Problems with the SCR system can drop the engine into a derate mode... one can imagine that this will be two fold... The intended reaction would be to ensure that tailpipe emissions limits aren't exceeded. The unintended yet beneficial reaction would be to provide impetus for the owner to repair the system to regain lost power. I have yet to discover if the SCR system has closed loop or open loop operating conditions but I currently must assume that the NOx sensor has a temperature range. Looking at the wiring diagrams, we see that it has a heating element like an O2 sensor... making me assume that my assumption might be assumably correct. What we need to consider is that the manufacturers haven't added SCR systems to their trucks because they want to... but because this is the easiest way they could meet the new and tougher emissions requirements. A brief sidebar on adaptive strategy. For several years, gas engines have had a toggle called MP_LRN.... misfire profile learn. For the uninitiated, the PCM was given the opportunity to learn the profile of the crankshaft tone ring in order to more accurately map any manufacturing deviations in the tone ring giving a more accurate measure of variations in crank speed that would indicate a misfire. On older engines, you would have to perform a series of closed throttle, no brake coast downs on the highway watching for the MP_LRN PID to toggle to YES. On new engines, all that is required is the IDS and a couple of brief WOTs - adaptive strategy at it's simplest. Quote Link to comment Share on other sites More sharing options...
rlchv70 Posted July 11, 2011 Share Posted July 11, 2011 Yes, the amount of urea injection will vary based on the load and quantity of NOx present. The SCR catalyst doesn't work well when cold. Therefore, it should not be injecting urea until a certain temperature threshold is present. The SCR catalyst is not 100% efficient, its closer to 60% efficient. That is why EGR is still needed. Also, since the SCR doesn't work when cold, the engine will rely on EGR to control NOx under those conditions. Quote Link to comment Share on other sites More sharing options...
Fredsvt Posted July 12, 2011 Share Posted July 12, 2011 I found this a while ago on motorcraft's site, regarding DEF usage, not sure if it's helpful or not. Quote Link to comment Share on other sites More sharing options...
Keith Browning Posted July 12, 2011 Author Share Posted July 12, 2011 It's in the Resources page here under "Inside DTS" Quote Link to comment Share on other sites More sharing options...
Cetane Posted July 12, 2011 Share Posted July 12, 2011 I know you are looking for something specific but this is all I found in the public domain. It may answer some questions, it will probably just raise more. http://www.bosch-engineering.de/pool/de/pdf/Calibration_Tier2_Bin5_Demo_Vehicle.pdf Quote Link to comment Share on other sites More sharing options...
Fredsvt Posted July 13, 2011 Share Posted July 13, 2011 That's probably where I got it, and I'm just having some senior moments. I found this last night: http://www.arb.ca.gov/cap/manuals/cntrldev/sncr_etc/309scr.htm It seems more geared to industrial applications, it is detailed though. Quote Link to comment Share on other sites More sharing options...
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