Tuning can have a dramatic...
Tuning can have a dramatic effect on emissions. If you're running 800 hp at the tires like this Duramax is, and trying to spool a large set of twin turbos, then your tune is bound to be a bit smoky. Even without smoke, having advanced timing can raise NOx levels.
As our world moves into a greener tomorrow, emissions regulations for diesels have gotten much stricter. In many states there are still no emissions testing laws for diesel trucks, but prepare for that to change. Diesels have always had to meet certain emissions requirements from the factory, and since the modern diesel pickup's arrival in the '80s, those standards have tightened considerably. In fact, the United States has some of the most stringent diesel emissions standards of anywhere in the world, with California being the toughest of the tough.
In this article we'll explore the basics of the emissions equipment that keeps diesels on our roads. We'll tell you how they work, how they can fail, and how modifying your truck will affect how they function. A normal, good-running diesel will still emit carbon monoxide, hydrocarbons, nitrogen oxides, and particulate matter. All of these pollutants must be taken care of in order to meet modern emissions standards.
Catalyst
With all the talk about diesel particulate filters (DPFs), you'd think that diesels never had to go through this before. As far back as the mid '90s, Dodge and Ford fiddled with adding catalysts or EGR systems to their diesel-powered line of pickups. A catalyst changes harmful emissions such as carbon monoxide (CO) and hydrocarbons (HC) to less harmful versions. In the case of carbon monoxide, it is transformed into CO2 (carbon dioxide) and hydrocarbons are turned into H2O (water) and CO2 (carbon dioxide).
DPF
While catalysts are good at removing carbon monoxide and hydrocarbons, diesel particulate matter (soot) simply passes right through them. To take care of this, all new diesel trucks come equipped with DPFs. To combat soot, DPFs trap particles so the exhaust will exit the tailpipe without any hint of black smoke. However, these trapped particles must be cleaned out of the DPF somehow, or else it will become clogged. To clean the DPF, the engine must go into cleaning mode (often called regeneration, or regen) where fuel is sprayed into the engine very late in the exhaust stroke. This increases exhaust gas temperatures to the point where the excess soot can be almost completely burned off. The downside is the extra fuel burned during regeneration hurts fuel mileage considerably.
 When a new diesel goes into...  When a new diesel goes into regeneration mode, the exhaust gases can become extremely hot. For this reason, most newer trucks have some sort of vented exhaust (arrows) that mixes outside air with the exhaust gases to cool it before the exhaust exits the tailpipe. |  The catalyst is the object...  The catalyst is the object that looks like a small, round, muffler. While many people consider them a restriction, they do little to decrease flow in the exhaust until horsepower gets well over 400 hp at the wheels. |  One of the biggest changes...  One of the biggest changes in the '07-and-up trucks was the addition of a diesel particulate filter. Known as DPFs, these filters are a big reason why newer diesels are 90% cleaner than older models. |