To understand why Euro 6 diesel cars must necessarily mount the SCR catalyst, previously used only on trucks, we must start from the basics of diesel engine operation and also consider some recent events in the automotive world.

How do diesel engines work?

The diesel engine is a type of internal combustion engine that uses a mixture of air (comburent) and diesel (fuel) as fuel, and that of compression. In fact, diesel engines are defined as compression ignition engines (unlike petrol which are defined as positive ignition).

The ignition of the diesel engine occurs in four stages:

• suction: the piston is lowered and the pressure difference thus generated inside the combustion chamber attracts the air molecules from the outside; the intake valve opens and atmospheric air enters through the cylinder;
• compression: according to the principle of compression, a gas, if compressed, heats up. When the intake valve closes again, the piston begins to rise towards the cylinder head, compressing the air and causing an increase in pressure and temperature in the combustion chamber;
• injection: when the piston reaches the cylinder head, an injector releases the diesel fuel into the combustion chamber in the form of small droplets and in a measured quantity; the fuel absorbs the heat of the air molecules and ignites, starting combustion. The thrust generated by combustion causes the piston to descend and the triggered movement rotates the vehicle wheels;
• exhaust: combustion generates a series of residual gases composed of unburned molecules and highly polluting substances, which are expelled from the combustion chamber through the exhaust valve and then released into the environment through the exhaust pipe;

This type of engine certainly has advantages, including lower fuel consumption and higher efficiency; but it also has its weak points, for example it is noisier, larger and produces more particulate matter (PM) and more nitrogen oxides (Nox), two highly polluting and dangerous substances for human health.

The regulation

To reduce particulate emissions, the manufacturers have been adapting for some time (Euro 4 – Euro 5) by setting up an exhaust after-treatment system on diesel engines, the DPF.

For NOx, however, the legislation has become stricter after the 2015 Dieselgate scandal: in that year the United States Environmental Protection Agency (EPA) denounced Volkswagen for having manipulated the diesel engine control units of some cars in such a way that detected acceptable Nox emission values ​​for type-approval purposes, although the real emissions on the road were significantly higher. The scandal then involved numerous other manufacturers around the world and together with various other consequences, led to greater attention on the controls and monitoring of diesel engine emissions.

From 2018/2019 (Euro 6d), the European Commission has introduced the obligation to adopt a Nox abatement system on all diesel engines.

What are Nox and what does the SCR do?

To understand what NOx are, let’s go back to the composition of the fuel + comburent mixture with which diesel engines are powered: air + diesel.

It is a poor mixture, that is, a mixture where the amount of air is higher than the ideal one to burn the fuel totally (the air-fuel ratio in diesel engines also reaches 25: 1).

78% of the atmospheric air entering the cylinder is composed of nitrogen, a gas which is inherently inert, 21% of oxygen, which is instead oxidizing, and 1% of other inert gases.

During the compression phase of the diesel cycle, due to temperature and pressure peaks, nitrogen and oxygen react together and give rise to different molecular forms: nitrogen monoxide (NO), nitrogen dioxide (NO2), nitrogen trioxide (N2O3) and nitrogen pentoxide (N2O5). Generally, the expression NOx is used to refer to these elements.

NOx have extremely serious consequences for both the respiratory system and the environment.

To eliminate them, the manufacturers have adopted a system of post-treatment of the exhaust gas specific only for NOx: it is called Selective Catalytic Reduction, or  SCR.

The Selective Catalytic Reduction system

This system is based on the triggering of a targeted NOx reduction chemical reaction. The reducing agent used is generally aqueous ammonia (NH3) or urea (CO (NH2) 2). Since in high concentrations these substances can be toxic or irritating, they are usually diluted. The highest quality and most popular product on the market is Ad Blue, consisting of 32.5% of urea [CO (NH2) 2] and 67.5% of deionized water.

The Ad Blue is contained in a tank located upstream of the SCR catalyst. It is pumped into the waste stream by means of a doser; the quantity of Ad Blue injected is established by the engine control unit which in turn communicates the information to the control unit of the Ad Blue tank. In contact with the high temperature exhaust gases, Ad Blue transforms into ammonia (NH3) which reacts chemically with Nitrogen Oxides (NOx) “by disassembling” them and generating two completely harmless substances: nitrogen (N ) and water vapor (H2O).

The reduction takes place inside a porous ceramic element coated with Vanadium, at a temperature of about 265-425 ° C, and has an efficacy of up to 80%.

In 2013, the European Federation for Transport and the Environment (T&E), a non-governmental organization whose objective is to raise awareness of Europe on environmental protection issues, commissioned a study to TUV Nord on the emissions of particulate matter produced from direct injection petrol engines.

In 3 years the Euro 6c regulation would come into force, which would have introduced a new limit on PM emissions of petrol engines based on the number of particles emitted: 600 billion particles per km. A big change considering that the Euro 5 limit value for diesel engines was 6,000 billion per km.

Given this, the Tuv Nord report, published on November 28, 2013 on the T&E website, brought to light data of fundamental importance:

1. direct injection petrol engines emit particulate particles 10 times more than diesel engines equipped with DPF
2. produce 1000 particles of particulate matter more than indirect injection petrol

The test, carried out on 3 direct injection petrol all compliant with the Euro 5 standard, showed that all cars also fell within the Euro 6b standard, but none in the Euro 6c standard, while the diesels were all already compliant.

The conclusion of Tuv Nord was clear: the use of the particulate filter solves the problem of high emissions from direct injection petrol engines and makes it possible to meet the requirements of the Euro 6 regulation.

Here GPF entered the scense.

(You can find TUV Nord report here: https://www.transportenvironment.org/sites/te/files/publications/TUV-Technical_report.pdf)

What’s GPF?

The GPF is the exhaust after-treatment device for direct injection petrol engines.

It consists of a monolith (filtering element) with a honeycomb structure, that is, formed by open and closed ducts alternately. This structure means that the gases must pass through certain porous channels, inside which the particulate matter is “captured”. 

Upon reaching a certain temperature (about 600 °), the particulate thus accumulated is automatically eliminated through combustion (self-regeneration of the particulate filter).

The difference between GFP and DPF lies in the material with which the monolith is made and in the times of self-regeneration.

In the GPF the material used for the filtering system is cordierite (AIMg), while the DPF is made of silicon carbide (Sic).

This difference is due to the fact that cordierite better resists to high temperatures, which direct injection petrols reach much faster than diesels (this is also the reason why they pollute more, as also highlighted in the Transport & Environment study).

Furthermore, precisely because the direct injection petrol engines quickly reach the temperature necessary for self-regeneration to take place, the GPF lends itself well to driving in urban stretches, unlike the DPF which in these circumstances clogs up faster.

Attention to maintenance!

It has been said that for all these intrinsic characteristics, GFP, in theory, presents fewer problems from the point of view of maintenance.

However, based on our twenty years of experience in cleaning particulate filters, we remind all consumers that every combustion generates residues, therefore it is always advisable to periodically regenerate the particulate filters, DPF or GPF, at specialized centers.

We summarized with an infographic the evolution of the EU regulations on pollutant emissions released by motor vehicles, to clarify what has been done and what is going to happen. Just these days the European Commission is meeting in Brussels for a consultation on the new EURO 7 standard, an initiative that is part of the Green Deal, the community strategy to make Europe’s economy sustainable.

The first standard

European standards on polluting emissions have been introduced since 1992 on approvals, and since 1993 on registrations. 

Before these dates, emissions of carbon monoxide (CO), unburnt hydrocarbons (HC), nitrogen oxide (NOx) and particulate matter (PM) were not regulated at all and air pollution from road traffic was out of control.

From the first Euro-standard onwards, however, there has been a continuous succession of new regulations, more and more specific and cumbersome, which have made it difficult and expensive for manufacturers to adapt and have created confusion and inconvenience among drivers. In addition to this, the European Commission has identified 2 other major obstacles that prevent the Euro 6 / IV from bringing about an effective reduction in emissions: 

• first of all, the fact that the limit values ​​adopted so far are obsolete and do not take into account all the polluting substances that endanger the health of the environment and man; 
• secondly, emissions are not currently measured in all driving conditions and are not monitored for the entire life cycle of vehicles.

What comes next to Euro 6?

The European Commission therefore proposes to develop a new (and hopefully last) emissions standard that will solve some of these problems and that will specifically aim to:

1. simplify the legislation in order to make the adjustment by manufacturers and consumers easier, faster and less expensive. One of the strong points of the proposal is the application of the standard to all categories of vehicles without distinguishing between cars / vans and trucks / buses
2. tighten current restrictions and broaden the range of measured pollutants in order to include other contaminants that are not currently regulated, for example, nitrogen oxide, carbon dioxide, formaldehyde and acetaldehyde, and many others;
3. integrate control and data collection measures throughout the life cycle of vehicles and not only during the homologation phase;

The plan is complex and should come to a concretization by the end of 2021. Euro 7 will accelerate the transition towards sustainable mobility and will bring Europe closer to achieving the climate neutrality target by 2050.

Why a particulate filter on petrol cars?

The particulate filter for petrol engines is called GPF (Gasoline Particulate Filter), while the one mounted on diesel units is internationally identified as DPF (Diesel Particulate Filter) or FAP (Filtre à particules).

Direct injection system on petrol engines is responsible for the change, in fact while on the one hand it provides more power with less fuel consumption, on the other it pollutes more. With direct injection the compression of fuel in the combustion chambers takes place at very high pressures and this affects the amount of particulate released into the environment (a quantity 1000 times higher than diesel engines according to the independent European association Transport & Environment).
The installation of a particulate filter is therefore the best solution to meet the increasingly stringent anti-pollution regulations, so that many manufacturers are taking this path, overall the PSA group (Peugeot, Citroen, DS and Opel) that since January 2018 has updated the 1.2 and 4-cylinder turbo 3-cylinder 1.6 engines, equipping them with the GPF.

How does GPF,Gasoline Particulate Filter, work?

PSA group explained the functioning of the Gasoline Particulate Filter saying that, unlike traditional diesel particulate filters, the GPF has the advantage of not clogging: the filter cleans itself continuously, and not only when it is clogged.

This result was achieved by changing the engine, the catalyst and the system that controls the air/gas mixture but above all working on the materials: unlike the diesel ceramic filter, made of silicon carbide, the GPF adopts cordierite, a mineral resistant to the heat that reacting with the particulate, triggers an internal cleaning of the filter, which is continuous and self-regulated.

Apart from the fact that cordierite is currently also used for Dpf, we must remember that according to the predictions of the manufacturers, the DPF and FAP diesel particulate filters promised an effective self-regeneration system, but the reality of the facts as everyone knows is very different: the majority of drivers do not meet the speed and mileage parameters necessary for self-regeneration to start. They are therefore forced to replace the particulate filter or rely on our cleaning service with Flash Cleaner Machine.

We’ll see what happens with the GPFs but let’s remember that every time a fuel burns, it produces ashes and the ashes will always have to be removed.

The first car equipped with the GPF is the Peugeot 308 , which makes use of the direct fuel injection like any car for which the GPF is required, and which according to the PSA will be able to cut particulate emissions by up to 75%.

EURO 6 is an European standard about the reduction of pollutant emissions of light passenger and commercial vehicles in force from Sept. 2014 for new models and from Sept. 2015 for any vehicle.

EURO 6 concerns mainly the emissions of nitrogen oxides (NOx) which can affect the human breathing.

In order to reduce NOx emissions, manufacturers are adopting the SCR (Selective Catalyst Reduction) system, the efficiency of which has already been proven since this system has been used on trucks and heavy vehicles in general since several years.

AdBlue and its risks

The SCR catalyst is integrated to the oxidation catalyst – which transforms the unburnt hydrocarbons (HC) and the carbon monoxide (CO) – and to the particulate filter (DPF) already mounted on the vehicle, and it works through a chemical additive made of ⅔ water and ⅓ technical urea: the AdBlue. This additive is injected directly into the SCR catalyst where, due to the contact with the exhaust gas and the high temperature, it turns into ammonia and neutralizes the NOx, reducing them to a mixture of oxide and steam.

For the SCR system to work properly it is essential that the quality of the urea used for the production of the AdBlue is high and complies with certain chemical requirements (technical urea).

Whenever the AdBlue is out of specification the risks for the SCR catalyst are really serious, starting from a progressive decrease of the operating capacity caused by the reduction of the usable area, to the instant and irreversible damaging caused by metals (i.e. copper, zinc) or pollutant substances, leading to one and only solution: the replacement of the the SCR catalyst.

Cleaning with Flash Cleaner Machine

Flash Cleaner Machine allows the recycling of SCR catalysts that have become clogged due to crystallized urea, which provokes the obstruction of the channels and the loss of efficiency.

Where all the other cleaning systems fail, Flash Cleaner Machine is able to intervene successfully and in a relatively short time, avoiding an extended downtime of the vehicle.

The combustion of the PM10 naturally takes place at around 600-650°C but exhaust gas only reaches 200°C maximum. It’s necessary to raise the temperature with a post-injection of fuel: this is the moment when the cerium comes into play to lower the ignition temperature of the soot particles to 450°C, speeding up the regeneration and protecting the filter from a potential dangerous thermal stress.

Note that cerium oxide, that is stored in a tank and must be periodically refilled (about every 80-120 thousand km), facilitates the combustion of the particulate but it does not burn together with it, on the contrary it sediments inside the filter.

Flash Cleaner Machine is the only technology on the market which is able to clean not only the PM10 residuals but also the cerium deposit trapped inside a FAP!

Particulate filters are devices which trap PM10 (up to 99%) and, as any other filters, need to be cleaned out regularly to prevent the blocking and resulting damages to the car: an overfilled filter can damage the engine and can itself be damaged or destroyed.

The cleaning of the particulate filter, that is the combustion of the PM10 trapped inside the filter, is called regenerationand it takes place automatically every 400-800 km more or less, depending on the quantity of particulate inside the filter and the driving behaviour.

The regeneration process is connected to the backpressure value, that is the difference between the incoming pressure of the air in the filter and the outcoming pressure: the more the filter is clogged, the higher is the difference, until the car recognizes the limit value and let the regeneration begin.

The most common particulate filters are FAP and DPF which are different in functioning but especially in the regeneration strategy.