Technology

Distributed coating is used on open foils, to get a monolith to be assembled with mandrels.
The injected coating on the other hand, is used on closed braze-welded substrates.

A catalytic converter (improperly called “catalyst”) is a thermal-chemical reactor. It contains precious metals, which produce a chemical conversion of the exhaust gases that flow through it.

An oxidising catalyst works by oxidising CO and HC only, while a three-way catalyst can simultaneously oxidise the three most important polluting compounds: CO, HC, and NOx.

With the typical three-way catalyst, the catalysis of NOx in diesel engines is not very effective because of the low temperature of the exhaust gases and the excess of air.

In such cases, substrates with TiO2 and VO5 with urea as reagent can be used, as they can reduce NOx emissions by 80%, by transforming it into N2 and H2O.

These catalysts are known as SCR (Selective Catalytic Reduction), and are used on heavy vehicles and stationary plants.

A good catalyst contains at least 1-2 gr of Pt and 0.1-0.2 gr of Rh.

When it is coupled with an injection system equipped with a lambda closed-loop sensor, and in the presence of a stoichiometric combustion (ie. the amount of air is just enough to burn all of the injected fuel), a catalyst is able to reduce polluting agents (CO, HC, NOx) by 98%.

A catalyst requires at least 200°C to reduce polluting agents by 50%, while 400°C are necessary to guarantee the maximum catalytic performance.

A ceramic antiparticulate filter is a monolith where its channels are alternatively open and closed. When the exhaust gas flows in, it is forced through the porous walls towards the exit. During this process, the particulate remains trapped inside the filter.

When the pores are blocked by an excess of dust, the filter is regenerated by an electronically-controlled superheating process.

Whenever counterpressure in a closed-channel ceramic filter is too strong, or there is no FAP, but a reduction of particulate emissions is still desired, BRAIN’s metallic filters can be used.

The filtering matrix is obtained by winding a special micro-perforated platinum-coated oblique steel foil. The result is a 200 CPSI substrate, where the particulate-rich gas passes through from one channel to the other, leaving its dusts in the micropores. These in turn will be unblocked during the high-temperature regeneration phase.

These types of filters have an efficiency of up to 70%.

Silicon carbide (SiC) is a hard and very porous material and is therefore particularly suitable for filters. On the other hand, it can be quite expensive.

Cordierite has been used in ceramic substrates for a long time. It is slightly less porous than SiC (about 10% less), but it is widely used thanks to its lower cost. Cordierite is also more fragile than SiC, with a lower specific weight.

The duration of a good catalytic converter will depend on how it is used.

Normally, after 60-70,000 Km, the catalytic power will be less than half, and the abatement of emissions can decrease from 98% to 50%. When that happens, it should be replaced.

Its loss of effectiveness is caused by a gradual ablation of the precious metals and its sintering, which is caused by high temperatures. The dispersion of the precious metals therefore fails, dramatically reducing their catalytic power.

Impregnation is the act of applying the catalytic metal on the porous ceramic support, using as much available surface as possible.

The active catalytic metal is typically part of the platinum group, but it can also be non-noble, such as cobalt and iron.

Depending on the chemical reaction, each type of catalysis will require a specific metal/support combination.

Precious metals are the real catalysts, while the ceramic or metallic matrix simply provides a physical structure.

The degree of dispersion of the noble metal will determine the performance of the catalytic converter: the higher the dispersion, the better its performance will be.

For the chemical reactions to happen, the catalytic converter needs to heat up to 400°C as quickly as possible.

Such a rapid increase of temperature will cause thermal, chemical and mechanical stress to the substrates. The materials used to build the substrates should therefore be light, resistant, not bulky, and allow for large surfaces.

In this respect, a metallic substrate has several advantages over a ceramic one.

A catalytic converter is composed of a metallic shell with a catalytic support inside, also known as monolith.

This monolith in turn is composed of a metal or ceramic matrix with a honeycomb structure and is fully coated with a porous mix of refractory oxides, impregnated with precious metals such as platinum and rhodium.

In the first case, the foils are braze-welded together, while in the latter the foils are fixed by perpendicular mandrels.

Braze-welded substrates are more rigid and resilient, while those with mandrels are more flexible and allow for more control when applying the washcoat, which improves the conversion.

In order to drastically reduce all possible emissions from a diesel engine, different solutions are required.

In the CRT EURO 5 systems, an oxidating catalyst is placed before the DPF, to activate filter regeneration and reduce CO-HC emissions.