The technology behind reduced diesel emissions

Mercedes-Benz Bluetec diesel exhaust system. Click image to enlarge

CANDIANDRIVER.COM - By Jim Kerr

You may have noticed some new labels on the diesel pumps at filling stations. Ultra Low sulphur diesel fuel is required for diesel vehicles produced after January 1, 2007 and this Ultra Low sulphur diesel has now made it though the supply system and is available at the pumps. Ultra Low sulphur diesel has less than 15 parts per million (ppm) sulphur content, compared to the maximum 500 ppm sulphur in regular low sulphur diesel fuel. Why do we need it? - to reduce the particulates in diesel engine exhaust.

When you hear about diesel particulates, you probably think about the black smoke you see shooting out the exhaust pipe as a diesel vehicle pulls away from a stop. Yes, those are particulates but much of what we see is large soot particulates that quickly fall to the ground. Most of the large particulates have been eliminated by the improvements in diesel fuel injection systems, but ultra fine particles (less than .1 micron in diameter) make up about 80-95% of diesel exhaust soot and that is what the new diesel emission systems are designed to control. The sulphur in diesel fuel doesn't create the particulates. Instead, ultra low sulphur fuel is needed to prevent damaging the exhaust converters and particulate filters that will prevent the particulates from entering the atmosphere.

Cars and trucks each have their own standards for emissions. For 2007 passenger cars, currently the only vehicle to meet their standards is the Mercedes-Benz E320 Bluetec sedan. Jeep will also use the engine and emissions equipment from this sedan to produce a diesel model for 2007. Diesel equipped pickup trucks, vans and commercial trucks also require new emissions equipment to meet the standards. Since 2004, these vehicles have required a catalytic converter to reduce emissions. Now they will have an oxidizing converter and a particulate trap in the exhaust system.

A particulate trap does exactly what its name suggests: it traps particulates. A fine honeycomb silicon carbide material captures the particulates in cells to prevent their release into the atmosphere. Half the cells are blocked at one end so the exhaust gases are forced into the open cells and through the porous cell walls. Particulates are trapped in the cells and the exhaust gases flow out the end of the other half of the cells to the tailpipe. In simple terms, the particulate trap is just a very sophisticated filter. The cell material is coated in precious metals such as Platinum and Pallidium to help burn the particulates when the particulate trap needs to be regenerated.

Over time, the cells in the particulate trap become loaded up and exhaust gas flow through the trap is restricted. Temperature and pressure sensors on the exhaust system monitor the efficiency of the trap. When the engine computer determines that the trap needs to be cleaned, it will operate the engine to burn off the carbon particles in the trap. That's when things start to heat up.

To clean out the particulate trap, internal temperatures must climb above 500 C. this is much hotter than normal exhaust, so a Diesel Oxidation Converter (DOC) mounted in the exhaust system in front of the particulate trap provides much of the heat. The computer will modify fuel delivery, injection timing, turbocharger operation and even throttle position (yes, some systems will use an electronic throttle specifically for cleaning the particulate trap) to create heat in the Diesel Oxidation Converter. The exhaust gases are heated by the catalytic operation of the DOC and then flow into the particulate trap where the catalytic operation of the trap burns out the soot. This can take several minutes to accomplish, and will typically be done while the vehicle is driving down the highway.

Using fuel with too much sulphur will poison the Diesel Oxidation Converter. The sulphur causes phosphorus in the exhaust to form deposits on the catalyst so that it doesn't work. The system can't generate enough heat to clean out the particulate trap, so you will need to replace expensive exhaust system components for the system to work again.

Ash from engine oils is another contaminant. Ash cannot be burned out of the particulate trap and eventually it will become plugged. Low ash engine oil meeting American Petroleum Institute (API) rating CJ-4 standard must be used to prevent plugging the particulate trap permanently. API CJ-4 engine oil has been on the market since 2002 so this shouldn't be much of a problem as long as the correct oil is used.

Diesel emissions controls have been phased in since 2004. January 1,2007 is just another step in the timeline to reduce harmful emissions, but it is one that will add new components to diesel vehicles and modify the way they sometimes operate.