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Le Mans exhausts

Not so many years ago, there was a united cry from all the teams in LM P1 not running diesels at Le Mans: “We can’t win”, and on the whole they were right. The rules are more balanced now though, and the diesel battle no longer exists. Audi is the only diesel LM P1 entrant, and competing with it are gasoline hybrids from Toyota and Porsche, to be joined soon by Nissan.

Audi needs to remain competitive in the face of new competition, and its latest diesel engine seems to be a match for the gasoline competition, although Porsche is surprisingly quick in its latest foray into the top division at Le Mans.

We last covered diesel exhaust systems specifically in 2010, when the focus was on diesel particulate filters. In that article we explained why such filters were required, how they work, how they affect noise and so on. The press pictures of the latest Audi R18 diesel do not clearly show where a particulate filter might be, but comments by Audi engine chief Ulrich Baretzky about being able to eliminate a second particulate filter seem to point to the fact that Audi still needs to use one to prevent visible smoke.

Why one filter instead of two? There is no magic in this – the R18 engine is based on a V6 block with a wide-angle vee, and the inlet system feeds the heads from the outside of the vee. The exhaust system is therefore in the centre of the vee, and the exhaust manifolds on each bank feed into a single twin-inlet turbocharger. One exit pipe, one particulate filter.

This exhaust system architecture makes sense for an engine with a single turbo, and is possible because of engine rules that are not too restrictive. Formula One, on the other hand, has a single turbo mounted to a V6 engine, and the exhaust routing is far less elegant because the rules mandate that the inlet system is in the centre of the vee and the exhausts on the outside.

From the available ‘spy’ pictures, there does appear to be a ‘racetrack-shaped’ particulate filter housing – that is, a block with round ends – immediately following the turbocharger. There has been speculation that the filter is housed within the single cylindrical exit pipe, but the pictures of the engine from other angles tell the true story.

It would be possible to make such a small-diameter long filter, but that would present too great a restriction to flow and an unacceptable pressure loss that would give reduced efficiency and increased fuel consumption. It is unlikely that there would be significant blockage of the R18’s filter by unburned soot owing to the high exhaust gas temperatures because of the high load.

During extended safety car running this might be a concern, but the carbon should be burned off quickly, either when returning to racing speeds or by using a controlled regeneration cycle that increases exhaust gas temperature in order to achieve the same effect (thanks to Paul Cole for his comments on my previous online article).

The racetrack-shaped filter elements are more expensive than the cylindrical types to manufacture and to house, but these are often used when packaging requirements won’t allow a sufficiently large filter face area with a cylindrical form.

Written by Wayne Ward

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