DPFs and DOCs

Comprehended into the exhaust stream as emission control systems of numerous diesel powered off-road machines you purchase today, is either a diesel oxidation catalyst (DOC) or a diesel particulate filter (DPF) or both

Now, Ultra-low-sulfur diesel must be used in DOCs to avoid the sulfur from poisoning the catalysts. Low-ash, API CJ-4 lube oil must be used to avoid ash from building up in the DPF.

While there has been much talk and concern over these changes, the truth is that these engines and their emissions control systems are here to stay. Tier 4 Interim engines are more costly. But for many of these engines, the amplified costs will be partly offset by advances in fuel efficiency and power. They are more complex but massively healthier for the air we all breathe.

The maintenance protocols will alter a little on these fresh engines.


Subject to the horsepower group, some engines can get by without using either a DOC or DPF, some use just a DOC, and a good many will use both plumbed composed in line on a single canister.

In this arrangement, the DOC is the first component to receive exhaust gas from the engine. Under normal conditions, the DOC responds with exhaust gases to decrease carbon monoxide, hydrocarbons and some PM.

The DOC also guards the DPF. Hydrocarbon liquids or vapour can interfere with the DPF’s capacity to trap and eliminate particulate matter, so manufacturers direct the exhaust through the DOC first, then into the DPF.

Some after-treatment systems, though, require only a DOC. And some manufacturers use a more deeply catalyzed DPF without a separate DOC. This type of system efficiently operates as both a DOC and a DPF.

The DOC is a flow-through device, compelling the exhaust over a honeycomb ceramic structure coated with precious metal. This chemically alters carbon monoxide and hydrocarbons to carbon dioxide and water.

In a combo unit, the exhaust gas leaves the DOC and enters the DPF. The DPF powers the exhaust gases to move through porous channel walls, trapping and holding the remaining PM.

Trapped particles on the DPF filter element are oxidized through a continuous cleaning process called passive regeneration. The heat of the exhaust burns away most of these elements but when that isn’t sufficient, extra heat can be produced in a process known as active regeneration. This may be activated with a change in timing or turbocharger output or a grouping of these to upsurge the DOC inlet temperature before a small amount of fuel is injected into the system.

Active regeneration will typically activate a light on the machine’s dashboard but will be transparent to the operator and not disturb operation. Cold temperatures and extended idle times upsurge regeneration frequency, but most manufacturers are adjusting the engine’s electronic control modules (ECM) to application-specific limitations to keep the number of regenerations as low as possible. Active regens use some added fuel, but since most of these engines gain fuel efficiency from electronic fuel inoculation, the impact on fuel consumption should be insignificant.

One thing that roots to the confusion is that DPF is occasionally used as a common term for the whole DOC/DPF assembly. In most cases, the DOC is used in combination with the DPF, particularly because the DOC supports heat formation to burn off the PM collected within the DPF. And because the DOC generates this heat, it is positioned in front of the DPF.

Maintenance requirement

DOCs are maintenance free and planned to last the life of the engine, as long as you use ultra-low sulfur diesel fuel. The sulfur in non-ULSD fuel will poison the catalytic coating on the inside of the DOC and decline its effectiveness.

In a combination of DOC/DPF, exhaust first arrives in the DOC where catalysts react with the gas to decrease carbon monoxide, hydrocarbons and some particulate matter. The gas then enters the DPF which entraps most of the residual particulate matter.

New Technology for emission control systems

The new DOC/DPF technology also needs the use of low-ash oils, sometimes called low SAPS (sulfated ash and phosphorous) oils. Ash, dissimilar to the PM or hydrocarbons, can’t be burned off, so it slowly collects in the DPF. The lower the ash level in the oil, the longer it takes to amass itself.

With the exhaust flow going from left to right, the nozzle and flame produce heat working into the DOC/DPF to burn off soot trapped in the DPF.

Several other manufacturers have declared that they will also have an SCR solution in time for the Tier 4 Final rules. Now, let us know what SCR is.

Selective Catalytic Reduction(SCR)

Selective Catalytic Reduction (SCR) is an advanced active emissions control technology system that inserts a liquid-reductant agent from end to end a special catalyst into the exhaust stream of a diesel engine. The reductant source is typically an automotive-grade urea, otherwise known as Diesel Exhaust Fluid (DEF). The DEF sets off a chemical reaction that alters nitrogen oxides into nitrogen, water and tiny amounts of carbon dioxide (CO2), natural components of the air we breathe, which is then ejected through the vehicle tailpipe.

SCR technology is designed to allow nitrogen oxide (NOx) reduction reactions to take place in an oxidizing environment. It is called ‘selective’ since it reduces levels of NOx using ammonia as a reductant within a catalyst arrangement. The chemical reaction is known as ‘reduction’ where the DEF is the reducing agent that reacts with NOx to renovate the pollutants into nitrogen, water and tiny amounts of CO2. The DEF can be quickly broken down to yield the oxidizing ammonia in the exhaust stream. SCR technology can achieve NOx reductions up to 90 percent.

Importance of SCR

SCR technology is one of the most cost-effective and fuel-efficient technologies available to aid in the decrease of diesel engine emissions. The objective is to reduce particulate matter (PM) and nitrogen oxides (NOx) to nearly zero levels. SCR can reduce NOx emissions by up to 90 percent while at the same time reducing HC and CO emissions by 50-90 percent, and PM emissions by 30-50 percent. SCR systems can also be collectively used with a diesel particulate filter to attain even greater emission reductions for PM. Moreover, off-road equipment, comprising of construction and agricultural equipment, should also meet the set standards and hence play their role in reductions of NOx, PM and other pollutants.

SCR usage

SCR has been used for decades to decrease stationary source emissions. In addition, marine vessels worldwide have been furnished with SCR technology. With its greater return in both economic and environmental assistances, SCR is also being recognized as the emissions control technology. SCR systems are also found in the increasing number of diesel passenger vehicles.

Things to take care while using SCR

One exclusive aspect of a vehicle or machine with an SCR system is the need for replenishing Diesel Exhaust Fluid (DEF) on a constant basis. DEF is carried in an onboard tank which must be periodically replenished by the operator based on vehicle operation. For light-duty vehicles, DEF refill intervals usually occur around the time of a suggested oil change, while DEF replenishment for heavy-duty vehicles and off-road machines and equipment will differ depending on the operating conditions, hours used, miles travelled, load factors and other aspects.

DEF is an essential part of the emissions control system and must be present in the tank at all times to guarantee continued operation of the vehicle or equipment. Low DEF supply activates a series of rising visual and audible indicators to the driver or operator. A growing DEF distribution infrastructure has quickly expanded to meet the needs of a developing SCR technology.


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This article was written by Vineet and Eeshan Bashir for any correction or guest article mail us at contact@automotivelectronics.com


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