Filter particles

Geometry

On the diesel engine light vehicles, the particle filters consist of an extruded honeycomb out of sintered ceramics . The channels of the honeycomb are stopped alternatively in entry and exit of the filter in order to force the passage of gases through the porous walls to collect the particles. The various geometrical shapes of the channels are possible, usually of square section, but of the channels of triangular section can also be considered.

In the beginning, these filters were produced in Cordiérite. The specific constraints associated with the operation embarked on light vehicle led the various manufacturers of filters to replace cordierite by another ceramics. The Carbure of silicon lays out in particular of a thermal Conductivité higher which makes it possible to better dissipate the Chaleur released by the combustion of soots; that makes it possible to obtain a much better resistance to the cast iron and cracking.

Operation: Phase 1 - Filtration

The capture of the particles in the filter is obtained by Filtration. The accumulation of the particles leads to the formation of the layer of Suie on the walls which, initially, improves the effectiveness of filtration (95 to 99% in mass of the particles can be collected in the filter). However, this porous layer strongly increases the Pressure loss imposed in the Muffler. This tends to harm the good performance of the engine by involving a loss of power in particular. A cleaning (or regeneration) of the device becomes essential then after a few hundred kilometers.

Operation: Phase 2 - Regeneration

Several methods of regeneration were considered, most traditional is based on the Combustion soots by a rise in the temperature of exhaust fumes at the entry of the filter. This operation requires the presence of a Catalyze. The recent diesel engines (of type common rail for example) make it possible to control the quantity of fuel finely injected as well as the moment of the injection: pre-injection, principal injection and post-injection. It is the post-injection which helps with the regeneration of the filter by sending a great quantity of unburnt gas oil in catalyst where it will ignite, in order to raise to a significant degree the temperature of exhaust fumes to activate the oxidation of the soots trapped in the filter. Certain models of filters, in particular those of the group PSA require the addition of a compound catalyst in the fuel (containing Cérine) in order to lower the temperature of combustion of the soots contained in the filter to facilitate the regeneration of this last. Group PSA having deposited a patent for this process of regeneration, the other manufacturers had to study other processes from where the invaluable material introduction (Platine, palladium…) deposited directly in the bread of the filter. The temperature of combustion of not additivées soots being higher, the regeneration of the filter is more difficult especially when the vehicle circulates only downtown.

Limitations

The effectiveness of the particle filters decreases with the size of those. Even if only 10% of the particles escape the filters (see the report of France 2 of the 10/23/2007), their number is very important since their size varies from 0,1 to 1 micrometer. The problem is that these are precisely the particles of less than one micrometer which are most harmful. It is thus astonishing to see that these filters will be obligatory in a few years.

Rejections

According to the figures of certain manufacturers, the total mass of the particles rejected by a diesel engine after filtration does not exceed 0,004 G per km. But of course, that can represent billion particles, all depends on their mass.

On the basis of the fact that the Density of the particles is of 100 kg/m³, by supposing that the average diameter of the particles which cross the filter is of 0.5 micrometer and which they are spherical, their volume is of $\ textstyle \ frac {4 \ pi (0,25*10^ {- 6}) ^ {3}} {3} = \ mathrm {} \ scriptstyle 6,545*10^ {- 20} \ mathrm {m^3}$ , which gives a mass of 6,545×10^-18 kg per particle.

By dividing the mass of the particles escaping the filter by the mass from a particle from a half micrometer, one obtains 6×10¹¹, that is to say more than one half Billion of particles by km.

The particle filters are thus far from being effective at 100% in many filtered particles , in particular for the finest particles like the Benzopyrène S.

Remarks

the evolution of materials used, the optimization of the geometry of the device and the progress brought to the operation of the engines made it possible to strongly improve the durability of the particle filters. At the origin of: 80000 km out of the 2.2 HDi, first engine in the world equipped with a FAP, their lifespan for summer have strongly increased, so much so that today certain manufacturers speak about FAP without maintenance.
the difficulty of development of the integration of this system to the engine is before all the safeguarding of a neutral behavior of the engine for the driver, because to degrade operation is not done without nuisance.
the particle filter produced, by the diesel oxidation of the particles, gases with greenhouse effect, thus contributing to climate warming. Its long-term impact is thus far from being negligible and the question of its real interest must thus be raised (this without counting the energy expenditure necessary to its production)

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