What is DPF in Cars?

DPF is the short form for Diesel Particulate Filter and it is a crucial component in the exhaust system of modern diesel vehicles. A Diesel Particulate Filter helps reduce the emission from diesel engines by capturing and storing soot, also known as particulate matter, present in the exhaust gases. Since the implementation of BS6 emission norms in India, a DPF has become an essential part of diesel engines. Without it, meeting the strict emission norms for diesel engines would not be possible. In this article, we are taking a comprehensive look at DPF, going over its purpose, how it works, the different types of DPF regeneration, common  problems with DPF maintenance and its overall significance in diesel cars.

Why are DPFs Essential in Modern Diesel Cars?

Year after year, diesel engines have plundered plaudits for their high fuel efficiency and torque output which is significantly more than that of petrol engines. These advantages had made diesel engines the popular choice of engine not just for commercial vehicles, but in SUVs and smaller passenger vehicles such as sedans and hatchbacks as well. However, diesel engines have always had an inherent drawback. They are known for emitting particulate matter (PM), more commonly known as soot.

There are microscopic particles that are made up mostly of carbon atoms that are a major air pollutant. PM or Particulate Matter is of different sizes. For example, PM 10 refers to particles that are smaller than 10 microns in size. These particles can enter the lungs when we breathe in air, and accumulate over time, creating health hazards. Furthermore, PM 2.5 particles are less than 2.5 microns in size, and not only can we breathe them in, but such small particles can enter the bloodstream as well, reaching different parts of our body. Particulate Matter can also contribute to smoggy conditions or reduced visibility.

Over the years, the growing environmental awareness has brought new laws into place, curbing exhaust gas emissions. The Indian government made serious inroads to curb vehicular emissions when it transitioned from BS4 to BS6 emission norms in 2020, which saw a strict clamp down on the amount of permissible NOx and PM emissions for diesel vehicles, bringing them in line with global standards. To meet these stringent emission norms, the use of a Diesel Particulate Filter or DPF became absolutely mandatory for modern diesel vehicles.

How does a DPF Work?

On a very basic level, a DPF is a ceramic filter designed to trap the particulate matter in the exhaust gases. Engineers usually place it in a stainless steel container, and it is usually placed close to the engine to maintain high operating temperatures. We shall learn more about why this is necessary further in the article.

At its core, a DPF is a ceramic filter designed to trap particulate matter from the exhaust stream. It is typically housed within a stainless steel casing and integrated into the exhaust system, often located close to the engine to maintain optimal operating temperatures.

Most commonly, Diesel Particulate Filters are made out of cordierite or silicon carbide. These elements are moulded into a honeycomb structure through which the exhaust gases are pushed. The honeycomb structure consists of alternating channels, some of which are plugged are open at the front and plugged at the back, whereas others are open at the back and plugged at the front. This forces the exhaust gases to flow through the walls of the filter, which are porous in nature, rather than simply passing straight through the filter.

Here’s a detailed breakdown of the working of a DPF:

1. Soot Trapping: The hot exhaust gases are pumped out from the engine and into the DPF. As they enter the ceramic filter, they are forced to flow through the walls of the DPF. The soot particles, which are larger than the pores of the filter, get stuck in its internal structure whereas the cleaner exhaust gases, largely free of particulate matter, exit from the DPF.
2. Accumulation: Over hundreds and thousands of kilometres, the soot is continuously deposited into the DPF. As this build up of soot increases, it starts to create a back pressure on the engine. This pressure is monitored by the ECU typically using pressure sensors which are positioned before and after the Diesel Particulate Filter.
3. Regeneration: Once the soot has accumulated beyond a point, it can start to choke the DPF. This is referred to as the DPF being ‘loaded’ or ‘clogged’ and is a natural part of the entire DPF cycle. However, allowing the soot to build beyond this point can cause excess back pressure on the engine, reducing its power output, increasing fuel consumption and even damaging the engine. To avoid this, the DPF is cleaned periodically through a process called ‘Regeneration’.

DPF Regeneration Explained

Regeneration is the process of burning the soot accumulated inside the DPF filter. The soot is oxidized and converted into less harmful substances such as ash and carbon dioxide. Regeneration of a DPF is important for its proper functioning and maintaining the overall health of the engine. Here’s a look at the different types of Regeneration:

1. Passive Regeneration

This is the most ideal scenario for cleaning a Diesel Particulate Filter as Passive Regeneration occurs automatically while driving. In fact, even the driver will be unaware of this process as there are no external signs of Passive Regeneration. It relies on the use of continuous hot exhaust gases from the engine.

• Mechanism: When a vehicle is driven at higher speeds, typically between 60 and 80 km/h, for an extended period of time, the temperature of the exhaust gases can rise to anywhere between 250 degree to 450 degree celsius. Such conditions are often met when cruising on the highway or driving on wide open city roads. At such temperatures, the soot in the DPF slowly burns away and is converted into ash and CO2.
• Benefits: Passive Regeneration is the most efficient type of DPF cleaning as it requires no intervention from the driver or the onboard computers of the vehicle. Instead, it simply relies on consistent driving conditions.
• Challenges: In urban environments, where the commutes are short, stop and go traffic is frequent and the speeds aren’t all that high, the temperature of the exhaust gases doesn’t climb as high as required for Passive Regeneration. This is when Active Regeneration becomes necessary.

2. Active Regeneration

When Passive Regeneration is not able to get the job done, the onboard ECU of a diesel vehicle steps in and initiates Active Regeneration. This happens when the soot level in the DPF reaches a predefined threshold, which is measured by the pressure sensors positioned before and after the Diesel Particulate Filter.

• Mechanism: During Active Regeneration, the temperature of the exhaust gases needs to be raised to 550 to 600 degree celsius to burn away the collected soot. The ECU uses different strategies to achieve the desired result.
  • Most commonly, fuel is injected into the exhaust system which then ignites near the DPF and raises the overall temperature of the system to burn off the soot.
  • The ECU can also adjust different engine parameters such as injection timing, EGR (Exhaust Gas Recirculation) valve operation, and the air intake to create hotter exhaust gases.
  • Although not as common, car makers can also use heaters in the exhaust system to raise the temperature of the DPF. 
• Driver Awareness: During the process of Active Regeneration, the driver could notice some subtle differences such as slightly increased RPMs, change in engine noise, brief burning smell, increase in fuel consumption and the engine start-stop system is disabled. It is important to know this as driver’s can sometimes mistake this for a fault with the car. 
• Importance of Completion: Once started, it is crucial that an Active Regeneration Cycle is completed fully. If the cycle is interrupted by switching off the engine, it can lead to further clogging of the DPF and will eventually lead to the DPF light coming on in the instrument cluster.

3. Forced Regeneration

At times, repeated attempts at Active Regeneration can fail or the DPF can become severely clogged. In such instances, a Forced Regeneration is required. This is also sometimes referred to as Service Regeneration.

• Mechanism: A Forced Regeneration is typically conducted by a service technician at the service centre, however, certain cars such as the Toyota Innova Crysta also have a switch for Forced Regeneration. For this procedure, the car needs to be stationary and the engine is held at a specific RPM to generate hot exhaust gases. Most cars require a diagnostic tool to conduct Forced Regeneration.
• When it’s Needed: It’s a sort of last resort before your mechanic suggests manually opening and cleaning the DPF or replacing it entirely. Forced Regeneration is also used when driving the vehicle for extended periods still doesn’t resolve a DPF issue and switches off the DPF light in the instrument cluster.

DPF Maintenance and Best Practices

While all components of an exhaust system, including the DPF, are highly robust due to the extreme heat stress they go through, their effective operation and life span can depend on proper maintenance and driving habits. Furthermore, ignoring DPF related problems can cause other components of the engine to fail as well.

1. Understand Your Driving Style: If you drive mostly in urban environments over short distances and at low speeds, then, your diesel vehicle will mostly rely on Active Regeneration to clean the DPF. You should watch out for signs of Active Regeneration which are listed above, and try to not interrupt the cycle. Alternatively, you should also take your car out on the highway once a month for an hour-long drive at around 80 km/h to clean the DPF through Passive Regeneration.
2. DPF Light: Modern diesel cars have a dedicated light for DPF and it is positioned in the instrument cluster of the vehicle. If the light comes on and continuously stays on, it is an indicator that the vehicle DPF is clogged and a regeneration cycle is required. In such a situation, you need to drive the vehicle continuously at highway speeds to clean the DPF. However, if the light is blinking, it can signal the requirement of a Forced Regeneration or an otherwise fault in the DPF system. It’s an indicator to immediately visit the service centre.
3. Use Correct Engine Oil: DPF equipped vehicles need low-ash or low SAPS (Sulphated Ash, Phosphorous, Sulphur) engine oils to function properly. Conventional engine oils have the above mentioned metals that can permanently clog the DPF.
4. Use Quality Diesel: DPF equipped vehicles are supposed to run on ultra-low sulphur diesel. Poor quality diesel can also lead to increased soot production and will quickly clog up the DPF.
5. Address Engine Issues Promptly: Underlying engine issues such as faulty injectors, turbocharger problems, or simply a malfunctioning sensor can lead to excessive soot production in diesel cars, leading to a clogged up DPF. Such issues should be checked and addressed promptly.
6. Avoid Excessive Idling: Idling for longer durations can lead to DPF issues. In such a case, the exhaust gases don’t reach the required temperature for DPF cleaning which can lead to the soot build up.

Common DPF Problems and Their Solutions

Despite being highly robust in their construction, DPFs are susceptible to certain issues. The table below lists the common causes, symptoms and solutions for such problems.

Conclusion

The enforcement of BS6 emission norms greatly elevated the role of DPFs for treating exhaust gases in diesel engines. The initial years saw many DPF issues crop up due to drivers being unaware of the new technology, however, such issues have steadily declined over time. The future of diesel passenger cars is also up for debate in India with government policies favouring greener alternatives.

However, as far as DPF is concerned, it is the cornerstone for emission control among diesel vehicles, not just in India, but globally as well. As long as diesel cars continue to be sold in India, DPF systems will continue to be a part of them. As far as their maintenance is concerned, correct driving habits and following the best practices related to DPF can ensure that the system functions flawlessly.

FAQs about DPF