Workshop: Dirty Diesel

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Words by Martin Pretorius

We all know that it’s a bad idea to fill your diesel engined vehicle with petrol. In a best-case scenario, doing so will cost you some money and inconvenience while the tank is being drained of the offending liquid; in the worst, it will cost you an engine rebuild. Consequently, you’re very careful at the service stations, making sure that your trusty steed gets only the highest quality, cleanest diesel: and, as long as you don’t venture too far outside of the major metropoles, it’s fairly easy to keep your modern diesel on a healthy diet.

But, as soon as you cross South Africa’s border, or even just venture into the platteland, you’ll encounter significant difficulty in feeding your vehicle good-quality fuel. Clean diesel is simply not as widely available as modern turbodiesels would prefer – 50ppm (parts per million) has only recently reached the hamlet of Beaufort West, for instance, and 10ppm ultralow sulphur diesel is as yet available at only a few service stations in Gauteng and Mpumalanga. The chances are that you regularly face having to give your vehicle’s high-tech engine a low-tech energy drink.

Why is the sulphur content of diesel such a big deal?

It all started when the European Union mandated stringent emission controls on all engines. Diesels received their own set of regulations governing maximum tailpipe emissions of soot (diesel smoke), as well as detailing the chemical composition of the gas itself. Engineers achieved compliance by introducing a host of new technologies, mainly by optimising the combustion process itself (with electronically-controlled injection systems) and by adding exhaust treatment systems (usually in the form of diesel particulate filters).

These exhaust filters capture the soot particles in the exhaust gas, and then eventually break them down into ash, thanks to sustained high exhaust temperatures and the presence of a catalytic substance. This process happens either as a result of a catalyst built into the particulate filter, or via the addition of a fuel additive.

So far, so good; but there’s a snag: the presence of sulphur dramatically reduces the efficiency of this break-down process because it affects the catalyst. If the filter can’t “burn off” the captured soot, it will eventually clog up. To keep this from happening, the sulphur content of the diesel had to be reduced – and quite dramatically, too, in South Africa, dropping from 2000ppm to the current European standard of 10ppm in less than two decades.

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