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Diesel Particulate Filters

exhaustsDiesel fuel is extremely popular for automotive use, particularly in Europe where, for some manufacturers, diesel vehicles account for more than 50% of sales.

Part of the reason is that, in some countries, diesel fuel is much cheaper than petrol (gasoline). In the Netherlands, for instance, petrol is 30% more expensive than diesel. But even in countries where diesel comes at a premium compared to petrol, such as the UK, diesel vehicle sales continue to account for an increasing percentage of overall sales. That's because diesels have provided good fuel economy figures for many years and, even in the UK, the costs of running a diesel are likely to be less than for an equivalent petrol car.

So it's no surprise that the European motor manufacturers have pressed for regulations that make the diesel competitive against gasoline-powered racers. The obvious example of this is at Le Mans where, since the rules were changed, the gasoline competitors have been consigned to fighting over the minor placings in the premier LMP1 category.

The regulations are contentious, with gasoline engine suppliers complaining bitterly about disparities in the rules. With diesels enjoying a significant power and torque advantage, the gasoline manufacturers may have a point here.

One significant regulation concerning endurance racing, which is governed by the Automobile Club de l'Ouest, concerning all engines but aimed mainly at the diesels is regulation 5.5.3, which states, "The engine must not produce visible exhaust emissions under race conditions." As is obvious to anyone who has attended a truck race - the European type, where lorries are raced, rather than the NASCAR type truck races, which are something different altogether - diesel engines can produce a lot of smoke. Diesel particulate filters are therefore necessary to comply with this regulation.

exhausts diesel-particulate

These are commonly fitted to series production cars and commercial vehicles. But they're not a filter in the same sense as, say, an oil filter. The aim of an oil filter is to trap particles above a certain size permanently on the surface of a porous filter, while allowing the liquid phase of the flow to pass through relatively unimpeded. Over time, the filter becomes blocked and the pressure-drop across the filter increases, hence the need to change the oil filter periodically.

A diesel particulate filter (DPF) is different in the sense that its aim is not to trap particulate matter permanently. In appearance, the DPF is similar to an exhaust catalyst, although on closer inspection there are differences. The face of the filter looks rather like a chessboard, with half of the square passages blocked. Each passage through the filter is blocked by an impermeable wall at one end, while the axial walls of the filter are permeable. Figure 1 shows a typical filter element for a DPF.

The aim of the DPF is only to trap particulates on the surface of the filter until the temperature of the exhaust flow is high enough to oxidise them. The gas products of this oxidation can then pass freely through the filter. Filter sizing is therefore determined such that exhaust 'back pressure' is kept within reasonable bounds. A larger filter implies lower gas velocity across the porous media, so the associated pressure losses are lower. The filter only becomes 'blocked' if the temperature in the exhaust system is too low for oxidation to take place.

exhausts wall-flow-filter

Particularly sedentary drivers of road vehicles may sometimes see a warning light when the DPF has become clogged; the reason being that the filter has not been hot enough to burn the trapped particulates. The owner's manual will invariably encourage them to go for a drive under high load conditions, but while this may go against their instincts for economical driving, they harm the economy of their car by having a blocked DPF. Perhaps they should give their car to a Le Mans racing driver for a while?

Fig. 1 - Diesel Particulate Filter (DPF) element
Fig. 2 - Flow through a DPF. The particles are trapped on the wall of the filter until they are oxidised.

Written by Wayne Ward

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