I was reminded of how far the automotive sector (both light and heavy duty) has come when I was driving beside a train along Moyie Lake. The train was just belching out a black plume. That just does not happen with today's diesel trucks and cars.
Emission controls have hit diesels hard. Unlike the late seventies when automobiles came under the emissions microscope and horsepower went down along with emissions, the diesel has maintained or gained horsepower with each new iteration while significantly reducing emissions.
In order to successfully accomplish this the diesel engine fuel control and emission control systems are very complex.
Diesel fuel injection used to be the epitome of a mechanical system. The pumps that squeezed fuel out through spring loaded fuel injectors were marvels of machining. One company (Bosch) held almost all the patents.
These mechanical systems soon succumbed to electronic controls. These electronic controls significantly reduced emissions and the diesel clatter from under the hood. More control over when and how much fuel was injected could seriously reduce noise and exhaust soot.
By its nature the diesel engine produces very little carbon monoxide or unburned hydrocarbons. It does however produce high amounts of nitrous oxides.
EGR (Exhaust Gas Recirculation) has been and still is used to combat the nitrous oxides issue.
Recent emission standards tightening (almost as tight as gasoline engines) has forced the diesel engineers to make some serious headway on emissions of nitrous oxides and that sooty plume.
Your most recent diesels now have what is known as common rail fuel injection. This allows even better control of the fuel injected into the engine. The injectors are now piezo devices in many cases. Google it for an explanation. The result is multiple precise fuel injections on each piston power stroke.
Since fuel control is as precise as it can be with current technology the engineers have resorted to after treatments in the exhaust system to do that last bit of cleansing to keep the government happy (also our skies much clearer).
The aftertreatment comes in the form of diesel particulate filters (DPFs) and catalytic converters. DPFs are what you might think, a filter in your exhaust system that is prevented from clogging by burning itself clean every once and awhile (regeneration). These DPFs seriously reduce soot.
Then, as in your car, there are catalytic converters. I have written about them before. Recent diesels have added a twist to them. Oxidation catalysts take care of excess CO and HC creating CO2 and H2O similar to the gasoline engine but then there is the reduction catalytic converter in charge of changing nitrous oxides to harmless nitrogen and water. Your new diesel truck is probably packing around a gas tank size load of diesel exhaust fluid (DEF). This fluid is injected just ahead of the reduction catalyst. It contains 30% urea and 70% water. Upon injection to the hot exhaust stream it vaporizes to ammonia and carbon dioxide. Through the catalyst and out comes nitrogen and water.
These extraneous emission control systems add even more complexity to the modern diesel.
Your diesel mechanic is now as versed in complex electronics as any other mechanic.