Diesel has been on the receiving end of a lot of bitter barbs in the past, chiefly due to the black exhaust associated with older diesel trucks and buses. Things have changed significantly, however. As is well known, the black-smoke-belching smokestacks of Pittsburg stank the city a century ago are long gone and its beautiful, vibrant hills encircle it today. In the same way, diesel vehicles have been undergoing a image renovation, making diesel viable for years to come.
According to Hyatt, 2010 was a critical year for long-haul truckers and fleet managers when the EPA mandated the use of selective catalytic reduction (SCR) in diesel engines. The reason this development was unsettling is because what makes SCR work is a consumable called diesel exhaust fluid (DEF). This would not only add cost, but also add an added maintenance step. Not the end of the world, but t would be an additional inconvenience.
Things turned out better than expected, however. Instead of resisting change, engine manufactures embraced it and found they could continue to make reliable engines. Not only that, the new technology that was incorporated reduced emissions so much that topping off the DEF now and then wasn't such a hurdle after all.
In the next section of the article Hyatt explains how the SCR works and the role of DEF to make it happen.
He begins by noting that selective catalytic reduction isn't new. It's actually been around for half a century, initially used to reduce pollution from coal-fired power plants. The primary pollutants were nitrogen monoxide and nitrogen dioxide, which are also the problem with diesel combustion.
The author goes into detail on how SCR works. First, exhaust gas must be filtered to remove soot and ash. Then the exhaust gas flows past a nozzle that sprays DEF into the stream of gases. (DEF is primarily water and urea, which you can read more about here.)
The hot exhaust gas and DEF enter the catalytic converter where a chemical reaction takes place, magically converting the nitrogen oxides to nitrogen and water. You may recall from your chemistry class in high school that the air we breathe is nearly 80% nitrogen. In short, it's gas that is harmless to our environment.
The details are oversimplified here but will give you a sense of the chemistry and technology involved. It's a variation of what a car's catalytic converter does. Hyatt states that "Most modern diesel engines use SCR in combination with exhaust gas recirculation and a diesel particulate filter (DPF) to reduce emissions."
Exhaust gas recirculation (EGR) is now used in nearly all modern ICE engines as a means of eliminating unburnt fuel. But this process also has handicaps and engine manufacturers are striving to find the right balance between EGR and SCR. The target is the elimination of emissions in a manner that doesn't hamstring performance and fuel economy.
Are there any downsides to DEF? You might think the hassle factor and cost are downsides, but since you only need to refill the tank about as often as you change your oil, that's not much of an inconvenience. As for cost, it's not really expensive.
Because of the increased implementation of this technology it is helpful to understand why it has been adopted and how it works. Emissions systems are becoming increasingly critical. According to the WSJ today, the current stimulus package includes a section in which every public company in the U.S. might be required to report climate information such as emissions and greenhouse gases related to their products and their climate risks.
Near the end of this story Hyatt states that the achievements in diesel will prod U.S. truck manufacturers to offer more diesel models in traditionally gasoline-dominant market segments. We've already reported on some of this.
The author sums up his story with this observation of note:
"Where DEF really becomes critical is in big diesel engines. We don't mean like your Cummins 6BT, we're talking Class 8 semi-trucks. These vehicles do millions of miles over their lifespans, and their massive diesel engines go through a lot of fuel in that time. These vehicles go through a lot of DEF as you might imagine, so at truck stops, DEF is sold at the pump."
