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HomeFeatured ArticlesWhat Is DPF And SCR In BS6 Diesel Cars? | Explained

What Is DPF And SCR In BS6 Diesel Cars? | Explained

Since the adaptation of stricter emission norms, the manufacturers are looking at many options to comply, especially when it comes to diesel engines. Most of them in India have completely stopped production of diesel engines themselves owing to the huge investment they have to make towards the R&D of developing engines that comply with the norms.

And for those who have complied with the norms, they use 2 methods in order to reduce the emission. They are Diesel Particulate Filters (DPF) and Selective Catalytic Reduction (SCR). Below is a little explanation of what they are and how they work.

What Is A DPF (Diesel Particulate Filter)?

A diesel particulate filter, or DPF, is an exhaust after-treatment device that traps particulate matter such as soot and ash. A DPF typically uses a substrate made of a ceramic material, that is formed into a honeycomb structure.

History Of The DPF

Diesel particulate filtering was first considered in the 1970s due to concerns regarding the impacts of inhaled particulates. Particulate filters have been in use on non-road machines since 1980, and in automobiles since 1985. Historically medium and heavy-duty diesel engine emissions were not regulated until 1987 when the first California Heavy Truck rule was introduced capping particulate emissions at 0.60 g/BHP Hour. Since then, progressively tighter standards have been introduced for light and heavy-duty diesel-powered vehicles. Similar regulations have also been adopted by the European Union and some individual European countries, most Asian countries, and the rest of North and South America.

How Does DPF Work?

The most common diesel particulate filters in widespread use are cellular ceramic honeycomb filters with channels that are plugged at alternating ends as shown below :

The ends of the filter plugged in a checkerboard pattern, force the soot-containing exhaust to flow through the porous filter walls.  While the exhaust gas can flow through the walls, the soot particles are trapped within the filter pores and in a layer on top of the channel walls.  The honeycomb design provides a large filtration area while minimizing pressure losses, and has become the standard, so-called wall-flow filter for most diesel exhaust filtration applications. Ceramic materials are widely used for particulate filters, given their good thermal durability, with the most common ceramic materials being: cordierite, silicon carbide, and aluminium titanate.

Issues With DPF

The most basic issue with DPF is that it gets blocked due to the accumulation of soot over time. Typically, when the filter becomes clogged or an error occurs in the system, an orange light will appear on your dashboard. This light varies based on the manufacturer, but commonly appears similar to the images shown below. When this lights up, you know your filter is most likely blocked, and regeneration may be required.

Oftentimes, blocked diesel particulate filters are caused by short journeys at low speeds. Vehicles operating at low speeds on short journeys are unable to meet the requirements for the filter to clean itself. DPFs may fail sooner if they are not well maintained. Additionally, filter blockage can be caused by the use of the wrong type of oil, performance modifications, using low-quality fuel, or even running the car frequently on a low fuel level

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Resolution To DPF Issues

There are 2 methods to resolve the DPF clogging and they are:

Passive Regeneration

Inside the after-treatment 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 the 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.

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

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Selective Catalytic Reduction

What is SCR (Selective Catalytic Reduction)?

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

SCR technology is designed to permit nitrogen oxide (NOx) reduction reactions to take place in an oxidizing atmosphere. It is called “selective” because it reduces levels of NOx using ammonia as a reductant within a catalyst system. The chemical reaction is known as “reduction” where the DEF is the reducing agent that reacts with NOx to convert the pollutants into nitrogen, water, and tiny amounts of CO2. The DEF can be rapidly broken down to produce the oxidizing ammonia in the exhaust stream. SCR technology alone can achieve NOx reductions up to 90 per cent.

Issues With SCR

One unique aspect of a vehicle or machine with an SCR system is the need for replenishing Diesel Exhaust Fluid (DEF) on a periodic 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 typically occur around the time of a recommended oil change, while DEF replenishment for heavy-duty vehicles and off-road machines and equipment will vary depending on the operating conditions, hours used, miles travelled, load factors, and other considerations.

DEF is an integral part of the emissions control system and must be present in the tank at all times to assure the continued operation of the vehicle or equipment. Low DEF supply triggers a series of escalating visual and audible indicators to the driver or operator. Once the tank reaches a certain level near empty, the starting system may be locked out the next time the vehicle is used, preventing the vehicle from being started without adequate DEF.

What Is DEF (Diesel Exhaust Fluid)?

Diesel Exhaust Fluid (DEF) is a non-toxic fluid composed of purified water and automotive grade aqueous urea. DEF is available with a variety of storage and dispensing methods. Storage options consist of various size containers such as bulk, totes, and bottles or jugs.

DEF tanks range in size from 5 to 22 litres depending on the vehicle’s application. The DEF tank fill opening is designed to accommodate a DEF fill nozzle to ensure only DEF is put into the tank. A diesel fuel nozzle will not fit into the DEF tank opening. In addition, the DEF tank has a blue lid to differentiate it from the diesel tank.

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Gautam Ganesh
A believer in "Four wheels move the body, two wheels move the soul." An avid car and bike enthusiast. Anything that has a handle or a steering wheel, excites me.

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