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Emissions Explained

Catalytic Converter:  Looks much like what is found on spark ignition vehicles (gas cars and trucks).  The purpose of this catalyst is to aid in the reaction of the remaining O2 (Oxygen) in the exhaust with the CO (Carbon Monoxide) also found in the exhaust.  Part of this reaction splits the O2 molecule and attaches an O to each of the CO molecules, resulting in CO2 (Carbon Dioxide).  The other part splits HC (Hydrocarbon) into H (Hydrogen) and C (Carbon), which reattaches to the remaining  oxygen, resulting in H2O (Water) and  CO2.  This is a very simplified version of what happens, but helps bring some understanding of what the catalytic converter's function is.  You can tell very easily which trucks have had their catalytic converter removed by the smell of the exhaust.  A truck with a catalytic converter will have an almost "sweet" smelling exhaust, while a truck without one will have that very distinctive "diesel tractor" smell .  In order to effectively begin the chemical reactions that take place in the catalytic converter, it requires the exhaust  to come into contact with a large surface area of the material.  In order to do this, the inside looks like a honeycomb with holes so small  you could barely fit a pin in.  

Exhaust gas recirculation (EGR):   When the EGR valve opens, this system uses the back pressure of the stock exhaust to force exhaust back into the intake manifold where it can be "reburned" in the engine.  This reburning reduces the emissions of nitrogen oxide (NOx).

Diesel particulate filter (DPF):   Substantially reduces the emissions of the black soot often associated with diesel trucks. This system filters out the soot and must enter an "active regeneration" mode where the engine injects more fuel than is necessary, closes down the air intake, and reduces boost levels causing the exhaust gas temperatures to rise to the point that it will burn the soot out of the filter therefore cleaning it.  This happens every 700 to 1000 miles and significantly reduces fuel mileage and power output during operation.

The increased exhaust gas temperatures (EGT) required to regenerate the filter are the main reason why you will see trucks that have this system also have an interesting tail pipe that looks like a trumpet.  This trumpet allows colder air to enter the exhaust before it leaves the tail pipe so that it does not damage the truck, people, or property.

Selective catalyst reduction (SCR):   In an effort to further reduce nitrogen oxide (NOx) emissions, this system injects diesel exhaust fluid (DEF) into the SCR.  The tank must be filled approximately every 5,000 miles with DEF, which can be purchased at nearly every truck stop location.  

It is highly recommended that an extra bottle be stored in the truck at all times as the truck will enter into LIMP mode if the tank becomes empty.

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