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MOT Test of Diesel Particulate Filter

Diesel Particulate Filters and the MOT

A Parliamentary lobbying group called The Westminster Commission for Road Air Quality (WCRAQ) set up by Barry Sheerman, the Labour MP for Huddersfield, claims that up to 10 percent of diesel cars could have faulty or damaged diesel particulate filters (DPFs). The organisation has further suggested that stricter MOT Testing should be introduced to check on vehicles’ DPFs, on the basis that “MOT Testers only have to check for the presence of a DPF”. This was reported in ‘Motoring Research’ which also noted that the group says this is not good enough, pointing out that if the filter is damaged, more particulates will be emitted because of reduced filtration. It was also noted that the WCRAQ also cite Dutch research which estimated that 10 percent of diesel cars have damaged, faulty of DPFs which have been tampered with or removed.

Ill-informed and wrong!

So, let’s see if the claims of the new lobbying group stand up to scrutiny.

Most obviously, a reasonable question to ask is how the DPF could be checked during an MOT inspection. Scientifically the most obvious thing to do is to remove the DPF, and then test it to see if it is working properly.

So how would you do that? Well, you could remove it from the vehicle and put it onto a piece of equipment to see how well gas would flow through it, or set up a controlled test by passing particulate-laden gases through the DPF and see how effectively they are filtered out – but all this during an MOT inspection is clearly ‘mission impossible’ – there is neither time nor the resource to do this during an MOT Test, and in fact it would be in clear breach of the current MOT regulations to do so.

OK, so how else could the effectiveness of the DPF be checked?  Here’s an idea: why not measure the particulate density downstream of the DPF, to measure if it is acceptable. Well actually, that is precisely what happens during an MOT with the tailpipe diesel emissions test. Now if there isn’t a failure after the DPF has been removed, damaged, or tampered with, then there’s something wrong with either the test equipment, or the level of particulate density considered as acceptable by the authorities responsible for the MOT Test – which in the UK is the Government, who still follow the EU Directive on emission levels.

The Diesel Smoke Meter (DSM) test

As both Testers and Vehicle Testing Station owners well know, the diesel smoke meter test is a very exacting procedural process undertaken using equipment which must conform to strict manufacturing specifications. In addition, the acceptable level of smoke density emissions is set very precisely for each different design of diesel engine being tested. So, if that doesn’t detect a problem with the Diesel Particulate Filter, then the acceptable level for the engine being tested must have been set incorrectly.

So, the WCRAQ is wrong. The problem is not so much to do with the MOT not actually checking the DPF, but that the acceptable emissions levels have not been set to a figure that would detect whether or not the DPF is working properly. That is a matter entirely for the Government. The current level has been set by the European Union, and all indications are that since Brexit, the British MOT is, at present, likely to mimic any European changes.

It’s not about the DPF – its what’s in the gas!

The WCRAQ cite Dutch research with suggests that 10 percent of cars on Dutch roads have faulty or damaged DPFs, but would the same apply in the UK? Yet even if the same or worse figures apply in the UK, the issue here isn’t about the DPF; it’s about (as has just been pointed out above) what particulate contamination is in the gases coming out of the car’s exhaust pipes – which is measured by the Diesel Smoke Meter. Irrespective of whether the DPF is damaged or not, if the exhaust gases don’t meet the required diesel emission standards, as required for that vehicle, then the vehicle should fail the emissions test.

Yes, of course, maintaining a much cleaner air quality environment is vital, and vehicles’ DPFs plays a vital role in achieving that aim. However, by claiming that DPFs should be inspected somehow during an MOT Test is really totally impractical, and in any event is unnecessary when the actual emissions from the vehicle are measured.

The real issue is for Government to set engine manufacturers tighter emission targets on diesel engines, and make sure they don’t cheat the system (which many car manufacturers have done), and then to ensure both that the diesel emission testing equipment is sensitive enough to detect lower levels of emissions and that the level for each engine type is set so that any defect or change in the DPF will result in that engine failing the diesel emission test.

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