Diesel Particulates

Monday 15th November, 2010
In my previous article, on CO2, we discovered that diesel fuel actually contains significantly more CO2 than petrol, yet diesel cars often have relatively low CO2 output. Some of the reason behind this is the clouds of soot they produce - the scientists call these diesel particulates.
In a perfect world, fuel would be 100% combusted and all the hydrocarbons turned to CO2 and water. Sadly we don't live in such a world and cold diesel engines, as with coal fires, leave us with a load of uncombusted carbon in the form of soot. This either clogs up the chimney or passes out into the atmosphere to create urban smog. In recent times we've discovered that this isn't at all environmentally friendly and a potential cause of many respiratory medical conditions, like asthma. As a result, manufacturers have been desperately trying to find ways to clean up the act for diesel, like the catalytic convertor did for the petrol engine.

Enter the Diesel Particulate Filter, or DPF.

Fitted to modern diesel exhaust systems, you can treat the DPF as an intelligent vacuum cleaner bag that catches these unwanted particulates on the way out of the tail pipe. As emissions regulations become stronger, they fit a smaller filter to catch the smallest particles required.

If you've ever driven a diesel car on a cold day, you'll know that they take ages to warm up - not so good on a frosty morning when you want to de-ice the car. This also massively affects emissions standards as, at low speeds (below around 40mph), there simply isn't enough heat in the exhaust to fully combust all the carbon. On older diesels this means clouds of smoke out the back when driving around town but, with a DPF, the soot gets caught in the "bag" until such a time when it can be disposed of. When the car speeds up, and exhaust temperatures (EGTs) rise, the excess soot is released back into the exhaust where it can be fully oxidised by a catalytic converter into CO2 and small amounts of ash. The difference between soot and ash can easily be visualised by casting your mind back to the BBQ you may have used this summer. The black charcoal briquettes you have at the start are compressed soot which you then burn, in the air, to small amounts of grey ash and CO2.

This process of cleaning out the DPF is known as "passive regeneration" and you can sort of recreate it in a petrol car by emptying your vacuum cleaner out of a rear window as you go down the dual carriageway. It can be witnessed as you see black cabs hit the Hammersmith flyover or the M4 on their way out of London. They drive around all day, clogging up the filter, then they hit the Chiswick junction of the M4 and clouds of white ash come flying out of the exhaust.

So the DPF indirectly reduces the amount of CO2 that comes out of the exhaust during a normal cycle. 44g of uncombusted carbon dioxide actually weighs just 12g. So if you can catch just 12g of soot in a DPF for each kilometer driven during a testing cycle, you can theoretically reduce those CO2 emissions by 44g/km - taking the car from tax Band F to Band A and making it exempt from the London Congestion Charge. All you have to do is make the diesel engine a little less efficient and find somewhere to hide a pencil lead amount of carbon until later.

But what if you often drive slow journeys or have regular stop/start and your DPF unit never gets hot enough? Well, to cut a long story short, you have a bit of a problem and passive regeneration simply won't work. One of my readers found this was when the oil warning light lit on his small diesel powered car after just 7500 miles, when it was less than a year old. After a £110 oil change, the warning light remained lit so he returned it to his dealer to investigate the issue. He discovered that, due to regular slow speed journeys, his DPF was becoming clogged up and was dangerously close to needing a £1000 replacement. As all he did most days was a 10mile journey from London Zone 4 to Zone 2 and this simply wasn't enough for his DPF to regenerate passively. To compound the problem, a software fault in his Italian car meant that the specific DPF warning light didn't come on when it should have, so the first he knew of it was when the oil had been poisoned by excess diesel fuel content. Diesel in the lubricant oil? Yes. This comes from "active regeneration".

Active regeneration takes place when the soot filter in the DPF reaches a set maximum limit. The car is programmed to react to this and do what it can to burn off the excess carbon. It usually does this by adjusting the combustion cycle. Some fuel may be deliberately added into the engine late, decreasing fuel economy but also making the EGTs hot enough for DPF regeneration. Diesel fuel is also added to the lubricant oil to help with this process, not unlike chucking a load of lighter fluid all over your charcoal on the BBQ to make it burn quicker. In extreme situations, the exhaust gases may be diverted around the turbo to ensure they are hotter when they hit the DPF. This will be noticeable to the driver as a serious loss of power.

Most DPFs come with a warning light to let you know when you’ve done too many short journeys and problems are starting. At this point, the owners manual will tell you to take the car for a good fast run at around 40mph+ for 15-20 mins. If you fail to do this, problems get worse and you get the diesel in oil thing and reduced power. So basically, your average British owner is expected to use an extra 3 litres per tankful to keep the DPF clean - they don’t use this in the CO2 calculations either.

Ultimately, if you do a lot of slow or short journeys, you should not buy a modern diesel engine. Don’t just take my word for it, look at what the AA said here. Dealers are now actually being instructed not to sell diesel cars to “city” users as can be seen in this technical document from VW. "Not recommended for sale in the Channel Islands and inner city driving". Sadly this wasn't the case for our reader above and he's now considering legal action against his dealer for inappropriately selling him a diesel car for use in the city.

So the big question is, why are diesel cars with “low CO2 emissions” and DPFs now exempt from the London Congestion Charge? Their fuel isn’t particularly economical and contains more CO2 generating carbon. How exactly is this going to reduce inner city pollution? It isn’t.

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