Understanding diesel exhaust aftertreatment systems

By David Bry

Diesels have long had an image problem. They were known to be smelly, slow, and dirty.

But that picture is very much out of date. Diesels don’t stink like they used to. Many are extremely powerful. And they’re remarkably clean compared to the old days.

Cleaning up diesel’s act took some work. The breakthrough came with the development of diesel exhaust aftertreatment systems.
All internal combustion engines produce pollutants such as CO2, CO, HC, and oxides of nitrogen (NOx). Diesel exhaust, when untreated, also contains significant amounts of particulate matter (PM). Regulations enacted worldwide required significant reduction in all diesel pollutant levels beginning in the mid-2000s.

For the U.S. market, the major emission reductions were implemented during the 2007 model year, on all diesel engines built after Jan. 1, 2007. Thanks to newly developed diesel exhaust aftertreatment systems, particulate matter was dramatically reduced.

Now that these systems are common and are making their way from dealerships to auto technicians everywhere, it would be good to review the basics of how these diesel exhaust aftertreatment systems work, how they can fail, and the effects of failures.

They are not preventive systems like exhaust gas recirculation (EGR) which reduces the formation of pollutants during the combustion process, or crankcase ventilation systems which prevent crankcase vapours from reaching the environment. Instead, diesel exhaust aftertreatment systems reduce pollutants after they have been produced.

The exhaust enters the diesel oxidizing catalyst (DOC) where HC and CO emission levels are reduced. This oxidizing process is similar to what occurred in the two-way catalysts on gas-powered cars of the mid-1970s. (Starting in the 1980s, gasoline vehicles employed a three-way catalyst that also lowered NOx emissions.)

In the modern diesel exhaust aftertreatment system, diesel exhaust fluid (DEF) solution is injected into the exhaust stream in a spray of tiny droplets. The diameter of these droplets range from 100 to 200 µm. (For reference, a human hair ranges from about 25 to 181 μm in diameter.)

From there, the exhaust works its way through the selective catalytic reduction (SCR) catalyst, which uses the DEF to create a chemical reaction to reduce NOx emissions

Finally, the diesel particulate filter (DPF) traps PM to prevent it from entering the atmosphere.

Throughout, the system is chock-full of sensors to confirm and regulate proper operation.

Exhaust components

While the DOC is not much different from the two-way catalyst on a 1977 Chevy Impala V8, the SCR is very different in construction and function from a typical catalytic converter. The SCR only reduces NOX and it cannot perform that function without the Diesel Exhaust Fluid (DEF) spray at the inlet side of the SCR.

The DEF solution spray vaporizes and decomposes to form ammonia and carbon dioxide. When the ammonia vapour is introduced to the NOx gas, it converts it to nitrogen and water – a harmless byproduct.

DEF is the reactant necessary for the functionality of the SCR system. It is a carefully blended aqueous urea solution of 32.5% high purity urea and 67.5% deionized water. (The DEF fluid is from the same family as urea fertilizer and bovine urine. But don’t try to collect your own fluid to save money. It won’t be of the proper purity or concentration. That’s no bull!)

DEF needs to be properly stored in a cool, dry, well-ventilated area, out of direct sunlight. At room temperature, DEF can be stored for two years. Keep it between -12 degrees C and 32 degrees C and it will last at least a year. Let that temperature rise above 32 C and the DEF shelf-life drops. So don’t buy a two-year supply at Costco and then store it in a greenhouse!

Clogged injector

To keep someone from putting diesel in the DEF tank, the standard nozzle diameter for dispensing DEF has been designed at 19mm versus the standard diesel fuel nozzle diameter of 22mm. In addition, the cap for the DEF tank is blue to further differentiate it from the diesel tank.

However, as you might expect, some people have made the mistake of dropping DEF into the diesel tank. This will result in a stalled engine and a very expensive repair bill.

Problem areas of the DEF system include the DEF reductant pump, the tank heaters (DEF won’t flow if it freezes – and it will freeze at -11 degrees C), and the spray nozzle which can clog up with crystalline DEF. Either issue will cause DTCs and continued driving with DEF issues can put the vehicle into several levels of reduced power; the final level of reduced power will result in a top speed of about 8 km-h.

So tell your clients to keep that DEF tank full of fresh DEF and they won’t get into a ‘DEF jam.’

The last stop

PM reduction – which most people think of as ‘soot’ – is probably the most visible part of the current diesel emission standards. Some of the particulates are reduced by precise control of fuel mixture, injection timing, and exhaust temperature. But any PM that makes it past everything else hits the diesel particulate filter (DPF).

That amazing piece of engineering can remove more than 90% of particulate matter carried in the exhaust gases. It uses a filter substrate consisting of thousands of porous cells to trap the bad stuff. And it keeps on being amazing right up until it gets plugged.

To prevent plugging – and the trouble codes and power loss associated with that – the system will automatically go through regeneration to “burn-off” the soot loading in the DPF.

The ECM/PCM monitors the amount of soot trapped in the DPF. Too much soot plugs the exhaust, triggers DTCs. If allowed to degrade too much, it will cause the vehicle to go into reduced power ‘limp’ mode.

On a scan tool you can see this soot accumulation expressed as either grams or percentage. When the number gets too high for the good of the system, the ECM/PCM will raise the temperature of the DPF. It will inject extra fuel into the exhaust stream via an extra injector. Or it will alter the injection timing. In some stationary-engine cases, it will employ electric heating elements. Either way, the elevated temperature will burn away the excess soot.

The automatic regeneration process typically happens with little fanfare. The driver may be totally unaware it is occurring. But if it doesn’t happen normally (engines that idle a long time or operate at low-power levels won’t be able to successfully perform a regeneration, resulting in high soot accumulation and DTCs), then you need to force regeneration via a manual “service regeneration.”

A scan tool is required for service regeneration. Commanding a service regeneration is accomplished using the output control function. The vehicle will need to be parked outside the facility and away from flammable materials, vegetation, other vehicles, and buildings, due to the high exhaust gas temperature at the tail pipe (around 315 degrees C).

The service regeneration can be terminated by applying the brake pedal, turning it off using the scan tool, or by simply disconnecting the scan tool from the vehicle.

Diesel exhaust aftertreatment systems have been a reality for more than a decade. With the information above and vehicle-specific procedures contained in Alldata’s OEM-level service information, you’ll be well on your way to servicing systems have been a reality for more than a decade. With the information above and vehicle-specific procedures contained in Alldata’s OEM-level service information, you’ll be well on your way to servicing today’s diesel cars and trucks.

David Bry has been an ASE Master automotive technician for 21 years and spent 14 years as an automotive educator at the high school and post-secondary level. He joined Alldata in 2016. This article originally appeared in Alldata News.