DPF/DEF/SCR - It's here, so let's deal with it.

For the past year (or more) I have seen and participated in the "battle" between brands: which is better, who's right, what sucks, who's wrong... I have since discovered that it was an exercise in wasted breath and energy. Why are we focusing on each other's opinion instead of focusing on gathering information and discovering what works and what doesn't. Every driver has specific habits, and using those habits and gathering information on breakdowns could help answer some of the baffling questions about why the DPF systems constantly fail, why the DEF dosers fail and get clogged, and other issues that is costing us operating time and $$$$$. Well, I am no expert in how the DPF and DEF systems work on my truck, let alone all engines, so I spent some down time studying the basics on Wikipedia. I will be diving into white papers and other webpages (I will post them here) and coming up with some ideas on why these things are so unreliable. If the geeks that developed these things have faith that they are a solid product, and we have plenty of trucks that have pushed past the 600k mark with these system on them, I would conclude that three things are causing failures. 1. Drivers habits. 2. improper maintenance and 3. bad manufacturing.

I did find something interesting in the Wiki article. I have been suspicious that shutting down the engine during a passive regeneration could be bad for the system. This article kinda supports this.

Diesel particulate matter burns when temperatures above 600 degrees Celsius are attained. This temperature can be reduced to somewhere in the range of 350 to 450 degrees Celsius by use of a fuel borne catalyst. The actual temperature of soot burn-out will depend on the chemistry employed. The start of combustion causes a further increase in temperature. In some cases, in the absence of a fuel borne catalyst, the combustion of the particulate matter can raise temperatures above the structural integrity threshold of the filter material, which can cause catastrophic failure of the substrate. Various strategies have been developed to limit this possibility. Note that unlike a spark-ignited engine, which typically has less than 0.5% oxygen in the exhaust gas stream before the emission control device(s), diesel engines have a very high ratio of oxygen available. While the amount of available oxygen makes fast regeneration of a filter possible, it also contributes to runaway regeneration problems.

Since having my doser clog up at 94,000 miles, I have been allowing 5 minutes for warm up and cool down. I watch the coolant temp on the truck and can tell a passive regeneration cycle is in progress when the temp rises to just above 200 degrees F. If this is the case, I let it run a little longer. My idle usage is about 3 gallons per week, or 7 gallons per week if I am teaming. I don't shut the engine off after pulling forward to fuel the reefer, as I think that starting the truck 4 or 5 times in a 10 minute timeframe is not good for the engine in any way. Same for shippers/receivers, I shut it off when I initially go to the guardshack, but then leave it running until I get it to the parking space, get the trailers swapped, or get it into the door.

Please remember that this thread is focused on finding best practices and identifying/eliminating bad practices for running trucks with the DPF/DEF/SCR systems in them. Be specific about your engine, but don't be hatin on the other guys.....

 

Now lets look at the SCR (DEF System). It also requires elevated exhaust temperatures. Failure to elevate the exhaust temp will cause "slip" where the DEF injected does not burn or do its job. This causes the filter media (usually made of precious metals) to break down and lose it's effectiveness. This is another reason I think that a proper warm up and cool down period is CRITICAL to keeping these trucks from failing. Here's an excerpt from the Wikipedia article on SCR:

LimitationsSCR systems are sensitive to contamination and plugging resulting from normal operation or abnormal events. Many SCRs are given a finite life due to known amounts of contaminants in the untreated gas. The large majority of catalyst on the market is of porous construction. A clay planting pot is a good example of what SCR catalyst feels like. This porosity is what gives the catalyst the high surface area essential for reduction of NOx. However, the pores are easily plugged by a variety of compounds present in combustion/flue gas. Some examples of plugging contaminates are: fine particulate, ammonia sulfur compounds, ammonium bisulfate (ABS) and silicon compounds. Many of these contaminants can be removed while the unit is on line, for example by sootblowers. The unit can also be cleaned during a turnaround or by raising the exhaust temperature. Of more concern to SCR performance is poisons, which will destroy the chemistry of the catalyst and render the SCR ineffective at NOx reduction or cause unwanted oxidation of ammonia (forming more NOx). Some of these poisons include: halogens, alkaline metals, arsenic, phosphorus, antimony, chrome, copper.
Most SCRs require tuning to properly perform. Part of tuning involves ensuring a proper distribution of ammonia in the gas stream and uniform gas velocity through the catalyst. Without tuning, SCRs can exhibit inefficient NOx reduction along with excessive ammonia slip due to not utilizing the catalyst surface area effectively. Another facet of tuning involves determining the proper ammonia flow for all process conditions. Ammonia flow is in general controlled based on NOx measurements taken from the gas stream or preexisting performance curves from an engine manufacturer (in the case of gas turbines and reciprocating engines). Typically, all future operating conditions must be known beforehand to properly design and tune an SCR system.
Ammonia slip is an industry term for ammonia passing through the SCR un-reacted. This occurs when ammonia is: over-injected into gas stream; temperatures are too low for ammonia to react; or catalyst has degraded (see above).
Temperature is one of the largest limitations of SCR. Gas turbines, cars, and diesel engines all have a period during a start-up where exhaust temperatures are too cool for NOx reduction to occur.

 

Repeat.... see below.

 

DPF Wikipedia article

http://en.wikipedia.org/wiki/Diesel_particulate_filter

DEF Article

http://en.wikipedia.org/wiki/Diesel_exhaust_fluid

SCR Wikipedia Article

http://en.wikipedia.org/wiki/Selective_Catalytic_Reduction

 

You bring up a good point with the EGR system, a totally useless thing anymore, and a costly one too. Just a small EGR cooler leak allowing coolant into the exhaust stream can destroy the DPF filter media and if bad enough, can make it to the more expensive SCR filter media. The coolant drips on the super hot media (kinda like ceramic) and puts holes through it. This causes the filter media to stop working properly, soot builds up in the holes created by the coolant, then superhot diesel fuel gets injected into the system, causing those holes to almost double in temperature, melting the media even more. Guess what??? The filter media in this case is not under warranty, and costs drivers between $2000-$4500 a pop. Good reason to pay close attention to your coolant level and fuel economy (which can indicated a EGR issue)