Particulate filters and their impact on performance

"Getting particular about your filter!"

If you've read our diesel tuning article then you probably want to know more about particulate filters. The particulate filter is a stand alone unit. It is located upstream of the cat in the exhaust system and its purpose is to remove soot particles from the gas stream.

The cat the proceeds to do its usual job of converting CO to CO2 and is in no way dependent or relied upon by the particulate filter.

In diesels the cats are two way devices and run in unregulated mode (ie. no O2 sensor like a petrol car). Petrol cats are 3 way closed loop controlled devices, although the principle is much the same.

Diesels are much more efficient that their petrol counterparts and release less CO2 however diesel exhaust fumes are black and sooty which is what the particulate filter attempts to address.

What does a particulate filter do and does it affect performance?

Most particle filter systems require some kind of fuel catalyst to be mixed in with the fuel. This is done automatically each time the filler cap comes off; the car registers the amount of fuel added and then administers the appropriate does of additive to the fuel tank directly.

This additive is stored in a vessel adjacent to the fuel tank and needs periodic refilling, typically every 50,000 miles. PSA vehicles use an additive made by Rhodia Inc (formerly Rhone-Poulenc); its brand name is Eolys. The additive lowers the burning temperature of carbon so that the soot can be cleaned out of the particle filter periodically. This process is known as regeneration.

Regeneration is triggered automatically when pressure sensors either side of the FAP (particle filter) register a pressure differential due to partial blocking (by the soot). During this process a number of things happen:

1. Intercooler is bypassed

2. Glow plugs turn on

3. Extra fuel is injected into the combustion chamber during the expansion phase.

This creates lots and lots of heat. So much in fact the the accumulated soot is literally burnt away. To my mind it's the equivalent of a horizontal chimney fire and probably as dangerous as one.

The process can be induced manually by a technician who has access to the appropriate diagnostic tools. This is called forced regeneration and can occur with the vehicle stationary in the workshop. In this situation, in addition to the intercooler/plugs/post injection the engine revs itself up to about 4500rpm for up to 40 minutes to burn off the soot. There's a rumor flying about that this creates so much exhaust heat that it's been known to burn the workshop's painted floor.

I'm told that some systems operate without the fuel additive but the principle is much the same.
So, no, a particle filter is not a cat. No, you can't get a sports one - its job is to primarily filter out larger airborne particles from your exhaust fumes.

Technically you can remove it but you'd have to re-program the ECU to ignore the pressure sensors otherwise the system is likely to go into limp home mode with fault lights all over the place.

My car is fitted with a FAP, and to my mind it's technology taken too far. What's the point of getting rid of a bit of smoke and replacing it with very very carcinogenic cerium compounds?

As far as particulate filters and performance go they're OK - the Peugeot LE Mans car used them. I am told though that if I removed it from my remapped 406 (192 bhp currently) it would go to nearly 200! So to summarise they are a necessary evil. They're not ideal for performance. But I wonder how much crap mine would belch out without it? I at least have that warm fuzzy feeling that I am not contributing to the suffering of Asthmatics around the country. Article submitted by HDI-fun a TorqueCars forum member and our resident Diesel head.

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