The primary components of a vehicle’s after-treatment system are the diesel particulate filter system (DPF), which reduces particulates, and the selective catalytic reduction system (SCR), which reduces nitrogen oxide (NOx).

The DPF system consists of the following:

Diesel Oxidation Catalyst (DOC) – The DOC reduces particulate matter and oxidizes carbon monoxide and hydrocarbons.
DPF – This filter is designed to remove more than 90 percent of particulates out of the exhaust.

The SCR system consists of these components:

Diesel Exhaust Fluid (DEF) – A solution made of urea and deionized water. DEF is introduced into the exhaust stream to reduce NOx emissions.
DEF pump – A part of the DEF delivery system that vacuums DEF from the tank, through the pump, filters the DEF and primes and maintains pressure between the DEF pump and doser.
DEF doser – The doser injects DEF into the exhaust system.
Decomposition chamber – This assists in mixing the exhaust stream with DEF prior to the SCR.
SCR – This is the catalyst for the chemical reaction that transforms the combination of the exhaust stream and decomposed DEF to meet regulations for NOx emissions.
The DOC and DPF work together at the front end of the system to reduce carbon monoxide and hydrocarbons, but more importantly, capture soot and ash in the exhaust system to reduce the risk of those particles reaching the atmosphere.

From there, the exhaust moves through a mixing chamber that introduces DEF … needed for the chemical reaction to reduce NOx in the SCR. The resulting ‘exhaust’ that gets reintroduced into the atmosphere is comprised of harmless nitrogen gas and water vapor.

In addition to the primary components of the aftertreatment system, there are also sensors. A differential pressure sensor estimates captured particles in the DPF. Exhaust gas temperature sensors monitor the system temperatures. NOx sensors measure the NOx conversion. PM (particulate matter) sensors, also known as soot sensors, measure particulates in the exhaust (mainly ash and soot). All of these sensors indicate when the after-treatment system needs repair or maintenance.

Diesel Particulate Filter Regeneration:
The key to maintaining a DPF is to ensure it’s able to regenerate itself when it fills with soot (triggering the warning light). The two types of generation include passive and active.

Inside the aftertreatment device (ATD), the exhaust first passes over the diesel oxidation catalyst (DOC), then passes through the diesel particulate filter, which traps soot particles. Passive regeneration happens when heat in the engine builds to the point where soot, or carbon, is combined with oxygen to create carbon dioxide. Since carbon dioxide is a gas, it can pass through the filter. Ash, on the other hand, is already a byproduct of combustion, so no amount of heat from the engine can convert it. Over time, the ash will build up to the point where the filter has to be physically removed and cleaned. This filter can then be reinstalled and reused. Passive regeneration occurs as the vehicle is driven normally under load; the driver is not aware that it is happening. It may not always keep the DPF clean over the course of the workday, so the filter may have to undergo active regeneration.

Passive regeneration is part of normal engine operation, however active regeneration requires the engine to take action. For example, a truck fully loaded with 80,000 pounds moving down the highway will create enough heat in the engine for a chemical reaction to occur—which is passive regeneration. Active regeneration takes place when the engine isn’t creating the heat it needs. For example, this may occur in a truck that’s not fully loaded. Once the soot level reaches a certain point, the engine injects fuel into the exhaust stream, which goes over the oxidation catalyst and oxidizes the fuel to create heat. The heat created from the fuel oxidizing is then used to convert soot to carbon dioxide.

Both active and passive regeneration happen automatically and without driver input. Active regeneration can occur automatically any time the vehicle is moving. The exhaust gas temperature could reach 1500℉ (800℃).

Active regeneration is unknown to the driver with the exception of some additional dashboard lamps. The biggest sign to look for to determine if it is taking place is the high exhaust temp light, which will turn on once the aftertreatment doser starts to inject, increasing the temperature in the aftertreatment device.

When operating conditions do not allow for DPF cleaning by active or passive regeneration, the vehicle may require an operator activated parked regeneration.

In order for this to take place, the vehicle must be standing still. The driver or technician brings the engine to operating temperature and initiates the parked regeneration by activating the dash controls. This may take anywhere from 20 minutes to an hour, depending on ambient conditions and type of engine or DPF system.

It’s critical for the driver or technician to ensure the exhaust outlets are directed away from structures, vegetation, trees, flammable materials and anything else that may be damaged or injured by exposure to high heat. Not all DPF systems have a parked regeneration feature.

How much does it Cost to Replace a Diesel Particulate Filter?
Replacing a diesel particulate filter can be pricey. A new filter from a car manufacturer can cost anywhere from $2,000 to $10,000.

It’s no secret that as cars and trucks age, their value decreases. Oftentimes, the price associated with replacing a DPF in an older, higher-mileage car or truck is more than the value of the vehicle itself. It’s much easier (and more affordable!) to clean a diesel particulate filter than it is to replace it, which is why understanding how these filters work and performing regular maintenance is so important.

If a parts supplier charges less for a DPF, be wary—the filter must be the correct type for your vehicle. Otherwise, you will most likely end up spending more on repairs.