Doctor Diesel

Back to basics: Selective Catalytic Reduction

Web_2009 BMW 335d - NO START IN 999 mlsWe’re going to read a lot more about selective catalyst reduction, or SCR for short, in the coming years, as this technique appears likely to become the widely accepted method for meeting the latest, (Euro 6) tougher emissions limits for NOx, or nitrogen oxides, the adverse environmental and health consequences of which are particularly significant in urban environments. The conventional method of NOx reduction, using solely exhaust gas recirculation (EGR) appears to be nearing its limits and consequently meeting the new Euro 6 regulations (in effect for brand newly launched models from 1st September 2014, and existing models from 1st January 2015) may require more effective treatment. SCR is the route that many manufacturers are taking, most likely in conjunction with EGR though.

There are at present two ways of effecting SCR, both of which involve short term storage of NOx material in a catalyst box, usually employing barium coatings to store the NOx as barium nitrate. There are two ways of then processing the stored NOx, both of which convert it to harmless air, ammonia and water. Toyota’s D-CAT system consists of a number of elements, but at its heart is the DPNR (Diesel Particulate NOx Reduction system), a four-way catalyst that oxidises hydrocarbons and carbon monoxide, stores and reduces NOx and particulate matter (PM) simultaneously. It relies on the engine management system to vary the air-fuel ratio in the exhaust gases and, using a fifth fuel injector in the exhaust port, creates a fuel-rich gas to both remove particulate matter and destroy the stored NOx. More recently (although using a technique used in heavy trucks for some years), instead of using fuel-rich gas to neutralise the stored NOx, a solution of the chemical urea (often known as Adblue) is continuously injected into the exhaust gas, again in the exhaust manifold, and similarly converting the NOx into ammonia, air, and water.

Removal of particulate matter is handled separately in a conventional diesel particulate filter (DPF) and unburned hydrocarbons are eliminated by an oxidation catalyst box earlier in the exhaust gas route, just after the exhaust manifold. If and when the introduction of SCR possibly enables EGR to be eliminated, the problems of particulate matter control would be eased, since the EGR process itself reduces combustion temperatures and increases the production of particulates.

There is some resistance to the introduction of SCR using Adblue, mostly on account of the additive cost and inconvenience. It requires a sizeable storage tank, together with safety measures to ensure that it does not run out, although systems will generally have a life approaching 10,000 miles. Volkswagen engines, the latest Euro 6 181bhp 2.0-litre TDI unit, for example, now employ an additive-free form of SCR, with a NOx storage unit placed upstream of its DPF unit. It uses engine management and lambda sensors to control air to fuel ratios and create the rich mix needed to process the stored NOx. We lack the finer details of this system at the moment, which operates without the use of Adblue additive, (in contrast to US specification 2.0-litre TDI Passats, and European Sharans and Alhambras), but it’s worth adding that this new Volkswagen system now uses two separate EGR circuits, one lacking gas cooling, to aid fast engine warm-up and reduce cold engine emissions, and a cooled system to minimise particulate production, but lower NOx generation, at normal running temperatures. We’ll keep you posted on the latest SCR developments, but get used to hearing the name!

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