By Jim Kerr
I was walking through a new vehicle auto show recently looking at the latest in big pickups when my eye (and many others as well) was drawn away from the usual shiny bits. What was the big attraction? It was the size of the tailpipe on a new Chevy diesel pickup truck.
Now I have seen large exhaust pipes before. Most of the diesel engine equipped trucks are using four to five-inch diameter tailpipes right from the factory. In some minds, if big is good, bigger must be better. Aftermarket six-inch tailpipes are so common in our neck of the woods, they hardly draw any attention anymore. But this factory Chevy tailpipe was not just big: further under the truck it looked like something from a rocket ship.
The rest of the exhaust system wasn’t as big as the tailpipe. The large tailpipe with its internal venturi is designed to draw air from beneath the truck and mix it with the hot exhaust gases before they exit the tailpipe. Most of the time the truck is operating, the exhaust temperatures will be normal, but when the truck’s particulate filter needs to be cleaned, exhaust temperatures skyrocket. Without the special tailpipe the hot gases could easily burn people or objects. Welcome to the world of diesel emission controls.
Particulate filters are required on all diesel pickups manufactured after January 1, 2007. Because of the time it takes to build and ship vehicles, we are just recently seeing these vehicles on the street. As the name would imply, particulate filters filter out exhaust particulate so they don’t enter the atmosphere. Black exhaust smoke is now a thing of the past. The filter looks like a large muffler in the exhaust system. Inside, it is filled with thousands of small cells that trap particulates but let the exhaust gases flow through the sides of the chamber.
Eventually, the cells in the particulate filter will become full and start to restrict exhaust gases. There are some industrial and large commercial diesel engines that have particulate filters too. When these become plugged, the filters must be removed and cleaned or replaced. Instead of removing the filters on light duty trucks, the filter is cleaned in place by putting the system through a burn off (sometimes called a regeneration) cycle. Just like the self-cleaning ovens for kitchens, the particulate filter is cleaned by getting it so hot inside that all the soot and particulate is converted to a very small bit of ash.
Exhaust back pressure is monitored by the engine computer and when the filter becomes restricted, a warning light or message will appear on the instrument cluster. The truck must be driven over 50 km/h for about 20 minutes for the burn-off cycle to complete. Because the exhaust system gets extremely hot during the burn-off cycle, it only occurs when the vehicle is in motion so moving air beneath the vehicle will help cool the exhaust. On average, the particulate filter needs to be regenerated once for every tank of fuel used.
To create the internal heat necessary for burning the particulate, the engine computer modifies how it operates the engine. An electronic throttle is partially closed to restrict the amount of air entering the engine. Turbocharger boost pressure is changed and additional fuel is injected into the cylinders as well as injecting it later. The combination of these controls allows fuel to continue to burn in the exhaust system, adding heat to the catalytic converter, which operates extremely hot and passes this heat on to the particulate filter.
After many years of driving, the particulate filter will eventually have enough ash deposits left over from the burn off cycles that it will start restricting exhaust flow and then the filter will need replacing.
A couple other things will plug the filter much quicker: regular diesel fuel and engine oils. Ultra Low sulphur diesel fuel (15 ppm sulphur maximum) and low ash CJ-4 engine oil are required to prevent permanent damage to the filter.
No smoke and a better environment are benefits of particulate filters on diesel engines, and that huge tailpipe looks pretty neat too.