By Mark Quasius, F333630
In the past 10 years or so, government efforts to reduce vehicle emissions and improve air quality have had an impact on diesel-powered motorhomes. As a result, coach owners need a working knowledge of diesel exhaust fluid (DEF), which plays a key role in ensuring that engines comply with emissions standards.
A quick review of pertinent U.S. Environmental Protection Agency (EPA) diesel engine emissions regulations will be helpful in understanding the importance of DEF.
- EPA 2007 regulations required ultra-low-sulfur diesel fuel and the addition of a diesel particulate filter (DPF) to the exhaust system to reduce soot in the exhaust. For the most part, those regulations took effect in the 2008 motorhome model year.
- EPA 2010 standards required major changes in the design of diesel engine emission systems and led diesel engine manufacturers such as Cummins to implement selective catalytic reduction (SCR) technology. SCR and the diesel particulate filter add more complexity to the emissions system while lowering emissions significantly below EPA 2007 specifications. In most motorhome brands, the changes began to appear in the 2011 model year.
SCR is “after-treatment” technology; it destroys harmful emissions after combustion, which allows engine manufacturers to fine-tune engines to produce
maximum power, efficiency, and fuel economy. SCR injects a decomposition agent — DEF — into the exhaust stream, where it forms ammonia vapor. The ammonia and atmospheric pollutants known as oxides of nitrogen in the exhaust flow into the SCR catalyst, where they react to form harmless nitrogen gas and water vapor, thus drastically reducing
emissions. The biggest impact on motorhome owners (aside from cleaner exhaust) is the need to deal with DEF.
DEF is formulated exclusively for use in diesel engines that use SCR technology. The nonhazardous solution, composed of 32.5 percent urea and 67.5 percent water, is clear and colorless and has a slight smell of ammonia. Don’t try to make
DEF yourself. Engine manufacturers specify that DEF must be certified by the American Petroleum Institute (API), the organization that rates engine oils and other petroleum products. The API Diesel Exhaust Fluid Certification Program allows DEF producers to display the API-certified label on DEF packaging.
DEF production must meet International Organization for Standardization (ISO) standards, specifically ISO 22241, which ensures that DEF is produced with a urea concentration of exactly 32.5 percent. That concentration provides DEF with its lowest freezing point, 12 degrees Fahrenheit.
Contaminated fluid can damage SCR injectors and catalysts, so production quality is monitored, and the level of impurities in DEF is strictly limited. Urea used in the manufacture of fertilizer is not allowed, and only distilled or deionized water may be used.
Even DEF containers are regulated by ISO 22241. DEF is corrosive to carbon steel, copper, and aluminum, so containers made of those metals cannot be used. A DEF container that bears the API DEF Certification Mark meets the ISO 22241 standard and is safe to use in a vehicle.
The amount of DEF a motorhome consumes varies with the size of the engine and how hard it’s being worked. It’s often said that DEF usage equals 2 percent of diesel fuel expended, but that’s a generalization. For lighter RVs with small displacement engines that are driven leisurely, the figure may be as low as 1.5 percent; for large, heavy coaches with the 15-liter Cummins ISX engine, the figure can be as high as 4 percent, depending on the vehicle’s load and how hard the driver accelerates.
We’ll use the 2 percent figure in the following example. Let’s say a motorhome has a 150-gallon diesel fuel tank, gets 6 miles per gallon, and has a 13-gallon DEF tank. In that case, 3 gallons of DEF would be consumed per full tank of diesel fuel (150 x 0.02 = 3). All 13 gallons of DEF would be used up after burning through 4 1/3 tanks of diesel fuel (13/3 = 4 1/3), which equates to traveling about 3,900 miles (4 1/3 x 150 x 6).
DEF isn’t difficult to handle, but some things should be kept in mind. Bulk DEF is available at truck stops from dedicated DEF pumps located on the driver’s-side fuel island, often right next to the diesel fuel pump and behind a rubber flap. DEF also is available in 2.5-gallon containers at auto parts stores, gas stations, and most large retailers. For a fairly comprehensive listing of DEF retailers, visit www.discoverdef.com.
DEF can evaporate if stored at high temperatures for prolonged periods, but tests have shown no significant risk of evaporation from DEF tanks as long as the tank or container is kept securely closed. DEF’s shelf life of two years is reduced if DEF is exposed to direct sunlight or if the temperature remains above 86 degrees Fahrenheit for sustained periods. DEF packaging includes an expiration date, so remember that if you stock up. Ideally, DEF should be used within one year of purchase, and it should be stored where temperatures do not drop below freezing or exceed 85 degrees.
DEF is always stored in its own tank on the motorhome and must never be put into a diesel fuel tank; nor should diesel fuel ever be put into a DEF tank. Fortunately, there are safeguards in place to prevent cross filling. Standard diesel fuel nozzles are 0.87 inch (22 millimeters) in diameter, while standard DEF nozzles are 0.75 inch (19 millimeters) in diameter, which should prevent accidentally inserting a diesel fuel pump nozzle into a DEF tank. Also, DEF tank caps are blue to differentiate them from diesel fuel tank caps. Diesel fuel is lighter than DEF and will float on top of DEF if it gets into the tank. But if that happens, even a small amount of diesel fuel will damage the SCR system, so do not run the engine. Instead, call a service center immediately and do not drive the vehicle until the diesel fuel is removed from the DEF tank.
On the other hand, it is possible to insert the smaller DEF nozzle into a diesel fuel filler neck. If this happens, do not start the engine. Since DEF contains 67.5 percent water, disastrous effects — such as exploding fuel injector tips — could result if it’s added to the diesel fuel tank. Call for help to have the fuel system drained or cleaned to remove the DEF.
Some bulk DEF pumps have magnetic switches built into the nozzle. The DEF tank has a magnet in the filler neck that allows the DEF nozzle to open and dispense DEF. The nozzle will not allow any flow without that magnet, such as when inserting the DEF nozzle into a fuel tank filler. However, not every DEF pump has these magnetic switches, and neither do the 2.5-gallon jugs, so pay attention to the tank when you are adding DEF. If you want to use a DEF pump with the magnetic nozzle to refill plastic storage containers, use a device such as a DEF Magnevator, which slips over the pump’s nozzle and allows DEF to flow.
DEF is not hazardous to handle, but it can stain clothes. If you spill it on clothing, just wash it away with water. A small amount on the ground can be rinsed away with water or wiped up with a paper towel or rag. Residue that remains will turn to crystals, which can also be rinsed away with water. As mentioned earlier, DEF can be corrosive to carbon steel, copper, and aluminum, so rinse those metals if DEF comes into contact with them.
Because DEF corrodes certain metals, the DEF tank in a motorhome is made of polypropylene. There are limits on the length of the hoses that connect the DEF tank to the engine’s emissions system, so the tank is located at the rear of a diesel-pusher chassis and at the front of a front-engine vehicle such as a Super C or Sprinter chassis. Most side-radiator chassis don’t have enough room to place the DEF tank on the driver’s side of the coach, so they are located on the curb side. Unfortunately, DEF pumps at truck stops are always on the driver’s side (to accommodate the driver’s-side DEF tanks on large trucks), which doesn’t always work well for an RV. However, chassis manufacturers have made efforts recently to add driver’s-side DEF fills as a convenience for motorhome owners.
As noted earlier, DEF freezes at 12 degrees Fahrenheit. In cold temperatures, DEF will not flow until it is warmed sufficiently. The warm-up occurs when the engine’s cooling system moves heated engine coolant through a heat exchanger in the DEF tank. DEF begins to flow quickly enough that emissions controls do not issue a warning code.
When the engine is shut down, you may hear an electric pump running for approximately 60 seconds. The pump drains all the DEF from the hoses and returns it to the tank to prevent damage to the lines and valves from freezing. DEF expands about 7 percent when frozen, so there must be a bit of air space above the DEF in the tank to allow for expansion during cold weather. The filler neck in DEF tanks generally is low enough to prevent overfilling, but if the curbside tank also has a second driver’s-side filler cap, you will want to keep an eye on this.
Here’s a bit more information about how SCR works. Exhaust, containing atmospheric pollutants known as oxides of nitrogen, flows out of the diesel particulate filter (DPF) and into a decomposition reactor. There, the hot exhaust comes in contact with a light mist of DEF, which is sprayed from an electronically controlled dosing valve. Ammonia then forms through a series of chemical reactions. The oxides of nitrogen and the ammonia pass from the decomposition reactor to the SCR catalyst chamber where they react to form nitrogen and water vapor. The end result is exhaust with near-zero hazardous emissions.
DEF is a critical component; without it, the emissions system will not work. The EPA requires that the emissions system be fully operational at all times, so certain safeguards are in place to ensure that the vehicle won’t operate without DEF.
A gauge on the instrument panel shows how much DEF is in the tank. In some cases, the gauge consists of four LED bars within the diesel fuel gauge. Four green LED bars mean the DEF tank is full; three green bars indicate it’s three-fourths full, and so on. Typically, when the level of DEF in the tank reaches 10 percent, the last green bar turns amber, “LOW DEF” is displayed on the instrument panel’s information center, and a warning icon appears. The vehicle continues to operate normally, but more DEF should be added as soon as possible.
If the DEF level drops to 5 percent, the LED bar turns red, and the information center display reads “ENGINE PERFORMANCE DERATE IMMINENT.” When the level drops to 3 percent, the display says “ENGINE PERFORMANCE DERATE ACTIVATED,” and engine torque output is reduced by 25 percent. You’ll be able to limp off the road at reduced power, but you’ll be burning DEF as you do.
When no DEF remains in the tank, the red LED bar remains illuminated, and the information center display says “SPEED RESTRICTION ON. DEF REQUIRED.” Engine torque is now at 60 percent, and the vehicle speed is limited to 5 mph, just enough to pull off the road. At that point, DEF must be replenished to at least 10 percent of tank capacity in order to drive to a location where the tank can be filled. It’s wise, therefore, to carry a 2.5-gallon jug or two of DEF, even if you normally refill with pump DEF at truck stops. Carrying extra DEF is especially important if you travel in Mexico, where it is not widely available.
DEF systems aren’t high maintenance, but DEF can age past its shelf life if a motorhome is stored for long periods or the engine is run only briefly and infrequently. In that case, it’s best to drain DEF from the tank and replace it with fresh every year.
A DEF filter, located near the bottom of the DEF tank, should be replaced every 200,000 miles or two years, whichever comes first. The filter can be removed easily with a 1 1/16-inch 12-point socket wrench and extension. The filter is at the base of the tank, so you’ll be looking up at it. Just be sure that you are off to one side when you remove it, because a bit of DEF will dribble out of the filter housing. The Cummins DEF filter is a small cartridge that comes with a small tool to yank the filter out of the housing when necessary. Owners should not forget the filter when the chassis is being serviced. A plugged or restricted filter can lead to a failed DEF pump, which is an expensive repair.
EPA emissions regulations have resulted in cleaner-burning diesel engines that are more economical and efficient. Aside from having to refill DEF tanks and change a filter biannually, the impact on motorhome owners has been minimal. All in all, both coach owners and the environment have benefited.