Dear Doc, 

After 38 years of driving Citroëns and Peugeots, I have changed to a Subaru, for the simple reason that there is not a suitable match for our caravan in the current Peugeot and Citroën line up! So the Peugeot 4007 has gone, and I now have a new Subaru Outback, and so far I am very pleased with it. I went to the Subaru, not only for its kerb weight and the caravan nose weight of 90kg that it can accept, but because the horizontally opposed engine has always appealed (memories of the Citroën GS and GSA, I suppose), and, like Citroëns of years ago, the manufacturers seem to do their own thing. 

My question though! For some years Citroën and Peugeot have fitted a bag containing a fuel additive (for DPF regeneration. Doc) which normally lasted 60,000 to 80,000 miles before a replacement was needed. More recently to conform to Euro-6, they have added a tank containing AdBlue (for NO_x removal. Doc) as a further additive. What I don’t understand is how Subaru apparently achieve Euro-6 emissions standards without any additives. Any idea Doc? How do they do that? Regards 

John Austin

Hello John.

Like some other manufacturers, Subaru are a bit shy about revealing their methods for NO_x control, and particulates as well, come to that! Regarding the lack of any tank for diesel particulate filter (DPF) regeneration fluid, as you had with your Peugeot 4007, this is because Subaru uses the same system as all of the VW group, Mercedes-Benz and BMW, for example, where a higher temperature catalyst is used in the DPF to burn off the collected soot without the necessity for any catalyst fuel. I have also noticed that when Subaru launched the Euro-6 diesel engines, they mentioned using a smaller, more compact DPF unit. I would imagine that this is mainly to achieve faster warm-up, and that it is located closer to the exhaust manifold, such that the conditions for regeneration will be attained sooner after start-up from cold, and also be more effective when the engine is working less hard. As I say, Peugeot/Citroën facilitate a lower temperature regeneration by squirting in some of the “Eolys” catalyst fluid, which lowers the combustion temperature of the process. See below for the PSA Peugeot-Citroën information:

Filter operation: a two-stage process – the Peugeot/Citroën system. The additive diesel particulate filter uses a two-stage process. To start with, the filter fills gradually with the soot produced by the engine. Soot build-up in the filter walls is measured by pressure sensors, and once it reaches a given threshold, corresponding to a certain mass of soot, the filter is cleaned (or “regenerated”) automatically without any input being required from the driver. The regeneration interval is between 300 and 1,000 kilometres. To eliminate the particulates stored in the filter, the engine exhaust temperature is increased by applying a special engine calibration that triggers fuel post-injections. This has no impact on vehicle response or noise. Regeneration then burns off the stored particulates to produce water and carbon dioxide. Once it is regenerated (as measured by the pressure drop across the filter. Doc.), the filter starts a new filtration cycle from 300 to 1,000 kilometres (according to driving conditions and engine temperature Doc). Regeneration is assisted by introducing an additive to the fuel. This lowers the soot combustion temperature by a hundred or so degrees Celsius, which substantially reduces the amount of fuel injected during the post-injections. Additive-assisted technology regenerates the filter four times faster than the catalysed competitor system.(As used by others, including Subaru, but I’m not sure about the “four times faster!” Doc)

Regarding the NO_x removal without the use of the AdBlue additive, it seems that Subaru has achieved this by using a refined twin circuit EGR (exhaust gas recirculation) system – one for hot engine conditions, and one for cooler engine situations. How well this operates outside the very mild conditions of the current EC emissions test cycle remains unknown – or will do until the real life, RDE on-road monitoring commences when the second part of Euro-6 regulations is activated, supposedly in 2017.

Subaru are probably keeping their cards close to their chest! Renault, for instance, has quietly improved their NO_x removal system to cope with engine temperatures encountered outside the EC test cycle, in the knowledge that the forthcoming real-life on-road emissions testing might have found some shortcomings of their previous system.

I hope that this clarifies things for you John. Regards

Doc Diesel