Active Particulate Filter: Operation, Standards and Hazards

How does the DPF work? Before delving into the specific topic, we believe it is appropriate to make some considerations on the subject of environmental legislation and underline that, even in this sector, Italy’s behavior, unfortunately, is not in line with the other countries of the European Union. This explains the different approach, certainly less realistic, logical, and coherent, adopted in our country about environmental protection in general. In the other EU Member States, environmental policies and laws are decided exclusively at the central government level and applied unambiguously throughout the country. In Italy, individual regions and municipalities can legislate autonomously on environmental matters. This great anomaly has led to a situation of absolute confusion and behavioural discrepancy as well as non-compliance with precise EU Directives. This is also the reason for the consequences triggered by the adoption of diesel particulate filters (DPF and DPF) and petrol (GPF) cars.

Article published by Bruno Pellegrini on May 22, 2016, and updated on January 3, 2024

1. Euro 4 car particulate filter
2. The particulate filter on the vehicle registration document
3. The health impact of particulate filters
4. How DPF works
5. The additive needed for DPF
6. The working temperature of the DPF
7. The DPF and fuel consumption
8. The DPF and engine oil dilution
9. Durability of the particulate filter
10. Regeneration of the particulate filter
11. Emissions with a particulate filter
12. Problems with DPF and DPF
13. The GPF petrol particulate filter

DIESEL PARTICULATE FILTER AND EURO 4

It is understood that in our cities most of the fine particles detected by the control units derive from oil heating systems and public transport, which, however, no one criminalizes. It cannot be admissible, from a European legislative, ethical, legal, and regulatory point of view, for a mayor of any Italian city to nullify precise regulations, established at a much higher level, and prohibit the free circulation of new vehicles. Vehicles that are in any case produced in full compliance with the European Directives in force and accepted by all EU countries. Directive 2003/76/EC-B (Euro4) establishes exhaust emission limits but does not prescribe a single method to obtain them, nor the mandatory adoption of exhaust filters for diesel cars.

In the case of Euro 4 diesel engines, the limits set by the aforementioned Directive are identical with or without a particulate filter. So much so that with the new standard there was no possibility of identifying, from the registration certificate, a car equipped with a particulate filter. Italian motorists and traffic policemen have noticed this following the first measures that discriminated against traffic in some urban centers. The former could not prove that they could circulate, but the latter also did not have evidence to the contrary. After the first wave of fines and appeals, the manufacturers have moved and urged the Ministry of Transport to clarify and introduce the necessary distinctions on the registration certificates.

PARTICULATE FILTER ON THE REGISTRATION CERTIFICATE

In the meantime, for cars with a “non-market” particulate filter, some manufacturers have provided customers with a specific declaration with which to make, through the Motor Vehicle Inspectorates, the update on the registration certificate. But all this has a cost that cannot be considered to be the responsibility of the buyer, who is once again a victim of the inefficiency of our bureaucracy. Currently, on the registration certificate (box 3) of Euro 4 (and later) diesel cars of first registration with a particulate filter, the wording “(Euro4 with disp. particulate matter)”.

HEALTH PROBLEMS AND THE PARTICULATE FILTER

This, at present, is the regulatory situation in our country, but the real usefulness of particulate filters is increasingly questioned by authoritative medical opinions. The conclusion is that the further reduction in the size of micro-dust is even more harmful to our health. Volatile nanoparticles, smaller than PM10, have a better chance of reaching the lungs and nesting inside them. The TUV in Germany has come out strongly against the use of particulate filters.

FAP PARTICULATE FILTER: HOW IT WORKS

The Active Particulate Filter (DPF) is a device introduced in 2000 by the PSA Peugeot-Citroën group to reduce particulate matter (PM10) emissions from diesel engine exhaust gases. Particulate matter is composed of soot particles (carbon residues from combustion) of 20-30 microns (1 micron = 1 thousandth of a mm). These incorporate smaller particles of polycyclic aromatic hydrocarbons (PAHs) and unburned sulfur compounds. It is the unburned deposits of HC, sulphuric acid (H2SO4), and sulfurous acid (H2SO3) that make particulate matter harmful.

The FAP technology, mainly adopted by Peugeot/Citroen, Volvo, Ford, and Toyota, involves mixing a catalyst additive with diesel fuel: cerium dioxide (CeO2). The “wax” is contained in a specific 5-liter tank placed next to the diesel tank and the mixing, using an electric pump, takes place automatically at each refueling. The limit calibration to obtain wax mixing requires at least 7 liters of diesel fuel to be injected into the tank.

THE RISKS OF WAX USED WITH FAP

Cerium oxide is toxic and to avoid, as far as possible, inhalation and contact with the body, it is supplied to dealers in bags with quick coupling to the relevant tank. All this must be done with disposable gloves and an activated carbon mask. We consider it useful to note that it is not advisable to transfer diesel fuel with wax additives from one tank to another already containing other diesel with wax. These clarifications seem to us to be more than appropriate since some press has reported erroneous information that the wax is separately injected into the combustion chambers. Cerium dioxide, which acts as a metal catalyst, also causes a decrease in the flash point of particulate matter to around 450°C. As we will see later, DPF (additive-free) technology, on the other hand, provides for a higher particulate combustion temperature (about 650°C). During the regeneration phase of the particulate filter, the agglomerate deposits trapped in the filter are periodically burned, reducing them to sizes in the order of 3.5 – 1.5 microns. These nanoparticles (which are hazardous to health) are then expelled from the exhaust.

WORKING TEMPERATURE OF THE FAP

It is therefore a mistake to assume that particulate matter is eliminated by this process. The amount of dust that can be measured with current instruments is only partially reduced. A good percentage is transformed into even smaller volatile nanoparticles that escape current legislation and therefore monitoring tools. The increase in temperature necessary for the combustion of the particulate matter inside the DPF is obtained by making the exhaust gases that reach the filter hotter. This is achieved by prolonging the combustion of diesel fuel well beyond the top dead center of the piston via one or two post-injections. In addition, the intercooler can be bypassed to get warmer air into the combustion chamber.

THE PARTICULATE FILTER AND DIESEL CONSUMPTION

These tricks result in an increase in fuel consumption and a slight drop in performance during the regeneration phase. However, these contraindications cannot be overlooked in the overall picture of vehicle running costs. The regeneration phase of the particulate filter is controlled by the difference in the inlet and outlet pressure of the DPF. As the filter becomes progressively clogged, the pressure jump increases until the electronic filter management module reads the “regeneration” limit value.

This should take place, depending on the conditions of use of the vehicle, every 300-1000 km (10-15 hours of operation). During extra-urban journeys at speeds of at least 90 km/h for a time of about 10 – 20 minutes. If the driver is unable to meet these conditions quickly, technical and economic complications could arise, as we shall see later.

FAP AND ENGINE OIL DILUTION

The post-combustion necessary to properly heat the DPF and DPF during the regeneration phase inevitably causes a progressive dilution of the engine oil. This undesirable phenomenon is directly proportional to the frequency of regeneration and, intuitively, it will be more severe in cars subjected to frequent journeys in urban traffic. When the percentage of diluted diesel in the engine oil reaches an estimated 7%-10%, a warning on the instrument panel will alert the user that the engine oil needs to be replaced.

On some cars, the dipstick has an additional notch indicating the maximum tolerable level following diesel dilution. This task, in addition to creating an unexpected inconvenience, greatly increases the costs of running the vehicle since the operation is considered normal paid maintenance.

SERVICE LIFE OF THE PARTICULATE FILTER

The DPF working with the technology of the catalyst additive (wax) must be replaced within certain mileage distances because the filtering element, over time, tends to clog irreversibly. The ashes, which are largely produced by wax, that cannot burn are trapped in the filter permanently. The deadlines for replacing the particulate filter range from 80,000 km for the first generation DPF to 120,000 km for subsequent DPFs, up to 160,000 km for the most advanced DPFs. The average price list cost of a new DPF is 500-600 euros; for some models, overhauled DPFs are also available at a much lower price.

The main difference between DPF and DPF is the absence of wax as a catalyst to reduce the regeneration temperature. DPF technology, considered safer and less expensive, has been adopted by many car manufacturers (BMW, Audi, Jaguar, Mercedes, Mazda, Fiat, Opel). In the DPF, higher temperatures (approx. 600-650°C) are therefore required for the regeneration phase.

The strategies for increasing the temperature of the exhaust gases are similar to those already examined for the DPF. A second delayed post-injection prolongs the combustion in the exhaust manifold, all the way to the catalytic converter. In addition, other measures can be taken, such as, for example, blocking the EGR valve, partially closing the intake air throttle valve, and closing the variable geometry blades of the turbocharger.

THE REGENERATION OF THE PARTICULATE FILTER

The absence of wax creates less ash build-up in the filter and this is a big advantage since it avoids having to replace the DPF at the DPF mileage intervals. However, the accumulation of ash over time that cannot be eliminated can reduce the efficiency of the DPF to the point that it is necessary to replace it at very long distances. However, it remains to be clarified whether the cerium dioxide expelled, even in small quantities, from the exhaust can have long-term harmful effects on our health.

It would be a further case of a “remedy” worse than the disease, as was the introduction of benzene (carcinogenic) instead of tetraethyl lead. It is interesting to note that in the DPF there is also “passive” regeneration, which occurs when the vehicle is driven at high speed and the filter temperature reaches about 250°C. In this case, the DPF control module does not intervene.

FAP AND TAILPIPE EMISSIONS

Another overlooked aspect of the operation of particulate filters concerns the increase in CO2 emissions, as this gas is produced by the combustion of particulate matter according to the reaction:

C+O2 = CO2

Another positive effect that occurs in the DPF is the exploitation of nitrogen dioxide as an oxidizer for carbon, obtaining carbon dioxide and nitric oxide:

NO2+ C= NO+CO2

CO2 is not a toxic gas (it is industrially produced and we ingest it continuously with carbonated drinks), but it is a greenhouse gas.

FREQUENT PROBLEMS WITH FAP AND DPF

The described “active” regeneration does not take place if the vehicle is only used for short distances or at too low a speed (city traffic). If you do not have the opportunity to go on fast-moving roads, you enter the phase of “forced” regeneration to be carried out in the workshop with the help of diagnostic equipment. This procedure, considered normal maintenance, and therefore subject to a fee, may require the replacement of the engine oil due to excessive dilution from diesel. If the user does not arrange for the forced regeneration to be carried out in service, there is a risk of irreversible clogging of the particulate filter that will need to be replaced. This eventuality, resulting from lack of maintenance, is not recognized in the warranty.

As you can see, the picture that emerges is not the most rosy, especially for the pockets of motorists. Paradoxically, the urban use of cars with DPF or DPF entails serious functional, practical, and economic risks for the user. Added to this is the thesis supported by some authoritative scholars according to which the volatile nanoparticles emitted by filters would be even more harmful. We believe there is enough for the public administration and manufacturers to carefully re-evaluate the matter. It is equally clear that these potential complications, which should not be underestimated, further reduce the convenience of diesel cars compared to petrol (and hybrid) ones. This convenience is already largely compromised by the small difference in the price of diesel and petrol, the higher consumption of powerful diesel, and the average durability and reliability of these supercharged engines. The current market trend shows a decisive comeback of gasoline cars.

GPF PARTICULATE FILTER ON PETROL ENGINES

With evolving environmental regulations and growing awareness of harmful emissions, more and more gasoline-powered vehicles are adopting the Gasoline Particulate Filter (GPF) combined with the presence of gasoline direct injection (GDI). Gasoline particulate filters are based on the same technology as diesel particulate filters (DPFs) and have also become popular on engines with a small displacement (see the 1.2 3-cylinder on Stellantis cars). The particles in the exhaust gases collect in the petrol particulate filter, causing a gradual filtering limitation over time, so a regeneration phase is planned depending on the distance.