Friday, May 18, 2012

Diesel Particulate Filters

Bear with me on this one, it's going somewhere I promise you.

There’s a lot of stories doing the rounds of the Interweb about diesel cars that are fitted with particulate filters suffering with premature failure of this expensive part.

What's a particulate filter? Briefly, if you’re as old as I am you will associate diesel engines with lots of black smoke. Over the years, the manufacturers have cleaned up the exhaust emissions, and although you see the odd vehicle (often a black cab) belching out soot, they are generally a lot cleaner than they used to be. This is in part due to the fitting of a Diesel Particulate Filter (DPF). What the DPF does is trap the soot, thus cleaning up the exhaust. When the filter reaches a high enough temperature, the soot will simply burn off, like a chimney fire, thus cleaning the filter. However, if the vehicle is used for a lot of short urban journeys, the filter may never reach the required temperature to burn off the trapped soot. The engine management computer can detect the soot build-up, and can force the filter up to temperature by injecting extra fuel into the engine, or by various other methods, but this doesn’t always work, and if the driver ignores the warning lights indicating excessive soot build-up, the vehicle may end up needing a very expensive new filter, or worse.

Over 50% of new cars sold in the UK these days are diesel. No surprise there – with escalating fuel costs, buyers are tempted by the superior efficiency of diesel, plus there’s usually a cost advantage in the form of lower vehicle excise duty. But, being the car-dependent nation that we are, a significant number of those cars will be used for exactly the type of short urban journeys that are the enemy of the DPF. But for a lot of people, the car is the default mode of transport, and they expect to just be able to jump in their car and go without having to worry about dashboard warning lights. As a result, car manufacturers have taken to warning potentialcustomers of this issue, and steering them towards petrol cars. This is a shame in environmental terms, because although they emit lower amounts of certain pollutants than diesel vehicles, petrol-fuelled cars have significantly higher CO2 emissions.

In the quest for lower emissions, manufacturers have been coming up with more and more technological solutions, and the result is cars that are becoming more and more complex and less reliable. If people used a bike for their short urban journeys, that would reduce emissions, fuel bills, and reduce the probability of their DPF failing. (It would also reduce the probability of their heart failing, as an additional bonus.) Maybe manufacturers should include a free bicycle with every diesel car. Even a decent bike is cheaper than a DPF, and a bike will last a lifetime.


  1. This of course has nothing whatever to do with the absolutely undeniable proposition that short journeys do not need cars, but my personal anecdotal observation is that it is not private cars which are responsible for PM10 emissions in London. Rather it is taxis, in particular, and buses, and commercial vehicles large and small. And yet, all of these probably drive around enough to get their filters properly warmed up. How is it that cabbies can get away with such polluting vehicles? Could it be anything to do with Boris reducing the inspection regime on commercial vehicles generally, presumably in pursuit if that Tory obsession “cutting red tape”? Isn’t the real problem cheapskating businesses which fail to maintain their fleet adequately?

    I also have to wonder why diesel remains so much more expensive than petrol, despite the rate of duty being somewhat lower. There was a time when diesel was if anything slightly cheaper. Certainly it costs less to manufacture. Oil comes out of the ground with rather more of the heavy “fractions” than our economy wants, so to produce gasoline it is necessary to “crack” the crude oil in a fractionating column at the refinery, to make two short-chain molecules out one long-chain one. Diesel is also much easier and safer to store, as its flashpoint is considerably higher. You can safely and legally store a rather larger quantity of diesel in your garage than petrol, where the limit is 10 litres (?)

    Personally, I regard the electric/battery car s a complete evolutionary dead-end, on a par with Neanderthal man, but battery commercials, now there’s another matter. It is not for nothing that the old milk float of our childhoods was run on batteries – a fairly short, meandering daily journey with multiple stops, the parked overnight in a depot net to an electric socket. Buses, delivery vans, even taxis could be battery-driven these days. Of course, they are not “zero emission” really because they just export an albeit lower level of pollution to somewhere else like Drax or Dungeness B, but at least in theory the could be recharged from wind, wave or tide.

  2. I have heard that it's demand that makes diesel more expensive than petrol. Refinery capacity isn't keeping pace with demand, and diesel is basically what's burned in oil-fired boilers so if there's a cold or extended winter, the diesel price will go up further.

  3. The Snow Performance crew was right. Running one of their Boost Cooler kits on a Cummins DPF-equipped diesel truck still led to lower exhaust gas temperatures and increased horsepower. Most importantly, the water-methanol injection kits improved combustion efficiency so dramatically that the particulate filters on their test trucks just never got dirty enough to need to be cleaned. The exhaust stream was "scrubbed" so effectively by the water-methanol injection that the filters didn't need to go into regeneration mode. This of course led to even greater and consistent gains in fuel economy.

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