Let’s clarify the matter: petrol cars are equipped with trivalent catalytic converters, which reduce emissions of the three main components CO, HC and NOx.
Euro 1-4 diesel engines on the other hand, use oxidation catalysts that can reduce CO and HC emissions, and NOx as well, but only to a lesser extent.
Euro 5 regulations impose restrictions on the diesel particulate emissions so it was necessary to use anti particulate filters; Euro 6 has severe limits on NOx emissions as well, so they need to be equipped with SCR CAT and/or Denox.
For the above reasons, FAP filters are used on both Euro 5 and Euro 6 engines.
A typical Euro 6 Diesel exhaust is equipped with a pre-cat, a post-cat, a ceramic FAP filter and an SCR using urea. Alternatively, a trap to cause a forced regeneration of NOx denox can be used.
The reduction of particulate can then be obtained either with cordierite ceramic filters, or SIC or a different type of special metallic filter, where the gas is forced through a crisscrossed route, with particular spiral-shaped channels.
The walls of each cell have micro-pores that act as filters for the dusts in the exhaust gas.
The gas moves continuously from one cell to another, leaving more than 60% of particulate behind in the filter, and coming out cleaner.
Compared to a ceramic FAP, metallic filters won’t force gas through the cell walls, so they ì won’t oppose too much counter-pressure.
Any time the pores of the walls are full, a slight increase in pressure occurs on the exhaust collector. Consequently, there is an increase in temperature, which triggers a regeneration, which in turn cleans the filter.
From an Euro 5 and 6 regulations perspective, ceramic FAPs are unmatchable (they provide a 99.7% abatement of particulate), while the ceramic filter turns out to be the best compromise between limited emissions (reduced up to 70%) and a virtually unlimited life.