Urea SCR service

This new technology keeps improving diesel emissions.

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A new day has dawned for the diesel engine. Once known as a dirty technology, diesels are breaking free from their old reputation. Diesel emission regulations have continued to tighten over the past 20 years, and diesels now are held to the same standard as gasoline engines. Cleaner fuels have played a role in the cleanup effort, but the diesel engine itself has been transformed and has come a long way from where it was even just five years ago. These refinements in design have changed the customer's driving experience but also impacted the way we go about things in our service bays.

Diesel engines traditionally have been "bad actors" in regard to two regulated emissions: particulate matter (PM) and oxides of nitrogen (NOx). Diesel PM is made up mostly of the black smoke that once was considered to be a status symbol, an indication that the engine was making power. NOx wasn't visible, but made your eyes and nostrils sting as it formed nitric acid in the presence of moisture. Diesels produced large amounts of both NOx and PM, which only reinforced their image as dirty workhorses. Looking back, it makes you wonder how we put up with it all those years, especially in light of where the technology is now.

Government agencies have been cracking down on PM and NOx emissions for many years, raising the bar incrementally and nudging diesel engine manufacturers toward cleaner technologies. Many changes have taken place, including limiting sulfur in diesel fuel to 500 ppm in 1993 and then 15 ppm in 2006. A major milestone was reached with the introduction of the 2007 Environmental Protection Agency (EPA) regulations that limited PM to the point where the industry had no choice but to install diesel particulate filters (DPFs) on pickups and commercial vehicles.

The development of the DPF made PM relatively easy to deal with, leaving NOx as the major technical challenge. At that time, most domestic diesel engine manufacturers chose to reduce NOx using in-cylinder techniques such as cooled exhaust gas recirculation (EGR). EGR acted as a heat sink in the engine cylinders, soaking up heat that otherwise caused nitrogen to combine with oxygen to form NOx. EGR also was inexpensive, certainly cheaper than aftertreatment techniques.

The 2010 EPA regulations forced everyone's hands, as now NOx was reduced to the point where in-cylinder techniques were being pushed to the limit. All on-road diesel engine manufacturers continued to use EGR, but now most adopted an aftertreatment technology called selective catalytic reduction (SCR) to meet the new NOx regulations. SCR was already being used in stationary power plants and in on-road vehicles in Japan and Europe. SCR worked very well in reducing NOx emissions.

The rub with mobile applications was that it required the customer to periodically purchase and install a somewhat exotic additive to keep the emission control system working. This was not lost on the EPA, whose main concern was if the truck ran out of the additive, it would run the same but would be out of compliance regarding NOx emissions. It took years before the EPA granted a waiver so SCR could be used in mobile applications in the U.S.

SCR Technology

The basic idea behind SCR is to install a special catalyst in the diesel exhaust system, and then inject ammonia (NH3) into the exhaust stream ahead of the catalyst. The term selective in SCR means that the ammonia prefers to react with the oxygen in the NOx and not with the oxygen in the exhaust gases. NOx molecules (NO and NO2) that leave the engine cylinders enter the SCR catalyst along with the ammonia, where NOx is broken up into nitrogen (N2) and water (H2O). Ammonia is the critical ingredient, and in the SCR reaction it is known as a reductant because it reduces NOx into less complex (and less harmful) components.

While ammonia works best as a reducing agent in the SCR process, it has some serious downsides in mobile applications. Ammonia is an aggressive and highly toxic fluid that requires special handling during shipment and storage.

A friendlier alternative to ammonia that works very well for mobile SCR is AUS32, or aqueous urea solution. AUS32 is non-toxic and is made up of 32.5 percent (by weight) high purity urea and 67.5 percent deionized water. In Europe, it is known as AdBlue®, while in the U.S. it is known generically as diesel exhaust fluid (DEF). So what is urea, and how is it able to take the place of ammonia in the SCR process?

Urea essentially is a nitrogen fertilizer that can be made from a number of different materials, the most common being natural gas. An aqueous urea solution will naturally decompose into ammonia and carbon dioxide, and this process takes place more rapidly with increases in temperature. Thus, urea acts as a carrier for the ammonia that is required for the SCR reaction.

When DEF is injected into the hot gases (>400° F) in the diesel exhaust stream, ammonia is released in two different processes: thermolysis and hydrolysis. The released ammonia then flows along with NOx molecules into the SCR catalyst where the reduction reaction takes place, preferably at temperatures between 570° F and 750° F. SCR catalysts are constructed similar to many other automotive catalysts, but they do not use precious metals. Instead, copper or iron zeolites are used as the catalyst material. Presently, copper zeolites appear to be the preferred technology for mobile applications.

DEF Challenges

A major downside of using DEF as a reductant is that it freezes at 12° F and will expand by 7 percent when frozen. While freezing does not degrade the DEF itself, the SCR system would be disabled temporarily and even could be damaged. The most vulnerable components are the DEF feed lines to the injector, which would be the first to freeze in cold weather.

Most SCR systems are designed so that either the injector or a purge valve opens and the dosing pump runs backwards during a key-off cycle in order to clear the lines. The feed lines often are insulated and use electric heat to prevent freezing during vehicle operation. The DEF tank also will have heat available to it, using either electric heaters or engine coolant lines that loop through the tank.

Some SCR systems are designed with two tanks: a larger storage tank and then a smaller active tank that supplies DEF to the injector. The large tank pumps DEF to the small tank as needed during vehicle operation, but only the small tank is heated. The idea here is that the small tank could be thawed relatively easily, bringing the SCR system online quickly after the DEF freezes. Once the vehicle is in operation, the large tank would thaw due to heat from nearby exhaust components, etc. and then would be able to continue supplying DEF to the small tank.

SCR System Service

The greater part of SCR system service will involve keeping your customer's trucks filled with high-quality DEF solution. SCR systems are calibrated to use anywhere from 1 to 5 percent of the vehicle's diesel consumption in DEF, depending on the system design and the drive cycle. Most DEF tanks are designed with enough capacity to take the vehicle to the next oil change before requiring a refill. If the vehicle is operated under moderate load conditions, it's possible that the driver could leave all DEF service to whomever is doing their scheduled service.

However, the vehicle gives the driver warnings when the DEF tank is getting low, starting around 800 miles to empty. The system will not let the driver forget about it, as the warnings get more pronounced as time goes on. If left long enough, the typical SCR vehicle will tell the driver that it is going to derate the engine if the DEF level is not increased, and then start by limiting road speed.

If the driver still doesn't get it, the vehicle either will not come off idle after a restart, or might not start at all. Regardless, the system will tell the driver that all they have to do is refill the DEF tank to make the truck run properly again. Let's say even the most stubborn operator will be trained quickly if they don't take the system seriously the first time.

So what happens if the driver puts something other than DEF (such as water) in the tank? While that might serve to get the rig started, the system is going to know in short order that you didn't put the right stuff in. The driver will be issued another warning that the DEF quality is poor and the warning cycle will be reinitiated. At this point, the driver will have to take the vehicle in for service and have the DEF tank flushed and filled with the correct solution.

If chemicals such as diesel fuel or washer fluid have been put into the DEF tank, this might require all DEF system components to be replaced. Again, it won't take more than one or two of these episodes to make the driver realize that it is easier to just leave the system alone and to keep the DEF topped up.

Your shop might see enough of these trucks that you will want to keep DEF in inventory. If so, keep a few important points in mind. First, only buy DEF that is clearly marked as API Certified, as this will ensure that you are getting a product that meets ISO 22241 standards. The DEF should be stored in and dispensed from its original container to prevent contamination. You also should keep in mind that DEF has a finite shelf life, and this gets shorter as ambient temperatures rise. DEF needs to be kept cool (under 86° F) and must be stored away from direct sunlight. It is good practice to rotate your DEF inventory as new product is brought in.

Technicians should receive some basic training on refilling DEF tanks. Emphasize the importance of putting the DEF in the correct tank, and to prevent spillage on the vehicle and themselves. Use of proper personal protective equipment (PPE) should be expressed. While DEF is non-toxic, it is a mild irritant and should be rinsed immediately with clean water if the technician gets it on their skin. It also is corrosive to aluminum and copper, so be sure to not spill it on electrical connections or other parts of the vehicle.

Parting Thoughts

SCR systems are new in the U.S., but like any new technology they represent an opportunity to expand your customer base and increase revenue. If you keep an open mind, simple filling of DEF tanks could mark the beginning of a move into the profitable world of diesel engine repair.

Tony Martin is an associate professor of automotive technology at the University of Alaska Southeast in Juneau, Alaska. He holds Canadian Interprovincial status as a Journeyman Heavy Duty Equipment Mechanic. He also has 19 ASE certifications, including CMAT, CTT, L1 and L2.