This may or may not help folk understand the "need" for and use of the filter.
I knicked this from a saab forum (same diesel 1.9 16v units), so thanks to them!
Cars with Z19DT and Z19DTH engine alternatives can be equipped with particle trap (market dependent / option) to trap soot, which is burned once the trap is full. This is called regeneration.
The particle trap comprises a porous body, similar to a catalytic converter with many small passages with porous walls.
Every other passage is blocked at the back (inlet passage) and every other at the front (outlet passage).
The exhaust gas is forced to flow into a passage that is blocked at the rear (inlet passage).
This passage is surrounded by outlet passages. The exhaust gas in the inlet passage will pass through the porous walls while the particles of soot are too large to pass through and will stick inside the particle trap.
ECM calculates the amount of soot in the particle trap. Note that the differential pressure sensor (744) is not used to measure whether the trap is full but for diagnosis purposes and to prevent manual regeneration using the diagnostic tool if the particle trap is too full. The calculated amount of soot is based mainly on engine load and speed.
ECM monitors driving style and selects a suitable time to employ regeneration. Cars driven a lot at idling speed and low load will attempt to regenerate earlier than cars driven more with high loads and engine speeds. In order for regeneration to take place, a certain temperature must be attained in the particle trap so that the soot can be burned. A temperature above +550°C (1022 F) is required in the particle trap if the soot is to be burned. A particle trap exhaust temperature sensor (602R) is used to measure the temperature. If the temperature cannot be attained, ECM assumes regeneration is not possible and will make a new attempt at the next opportunity.
To increase the temperature in the particle trap so that regeneration will take place, an extra injection is performed during the exhaust stroke. At around 160°C (320 F) a small amount of fuel is injected into the cylinder and as this is so late (the piston is almost at BDC with the exhaust valve open) the extra fuel will not contribute to the engine torque, nor will the exhaust temperature increase appreciably. Quite simply, the exhaust gas is enriched with HC (hydrocarbon), which initiates a reaction in the front catalytic converter. The gas temperature increases, which is monitored by the front exhaust temperature sensor (602F). The heated exhaust passes into the particle trap where it reaches the rear catalytic converter first and is further heated to at least +550°C (1022 F), the temperature required for regeneration, or the soot will not be burned.
Regeneration continues until ECM calculates that all the soot has been burned. This calculation is based on the value from the exhaust temperature sensor, the engine load and the engine speed. Regeneration can take up to 15 minutes but the driver will not notice any difference in the engine or performance.
In the event of the car being driven in such a way that regeneration is not possible, a diagnostic trouble code will eventually be registered and the "Check Engine" indicator come on. Regeneration must now be done manually using the diagnostic tool.
Regeneration is not possible in certain driving conditions, e.g. driving under extremely light loads and extensive idling. This will cause a DTC to be generated and the "Check Engine" indicator to come on. Regeneration must now take place manually using the diagnostic tool.
Regeneration must be done manually using the diagnostic tool whenever the ECM is changed, see Manual regeneration.