The following is an article written by the journalist Johannes Winterhagen for the “Der Sonntagsingenieur” series in the Frankfurter Allgemeine Sonntagszeitung of February 4, 2018.
Selective catalytic reduction (SCR) systems are an efficient technology for greatly reducing the nitrogen oxide emissions of diesel vehicles. However, the system mustn’t be sloppily retrofitted.
Anyone who has looked the happily whistling Gyro Gearloose over the shoulder knows that nothing is too hard for an engineer to handle, at least as long as he remains within the bounds of physics and the money doesn’t run out. As a result, mankind was able to reach the moon with an amount of computing power that is inferior to that of today’s smartphones. This also makes it possible to technologically retrofit older diesel vehicles in order to substantially reduce nitrogen oxide emissions. So why is there such a lengthy debate about the retrofitting of hardware?
The most efficient technology for reducing nitrogen oxide emissions from diesel combustion is the SCR catalytic converter. Selective catalytic reduction basically works as follows: Ammonia is added to the exhaust gas. In the catalytic converter, the ammonia reacts with the nitrogen oxides contained in the exhaust gas, causing only harmless nitrogen and water to come out of the tailpipe. However, this basically good idea faces a number of problems. The most important of these is that ammonia is an irritant gas that, unlike nitrogen oxides, can be rapidly lethal in high concentrations. That’s why SCR catalytic converters use an aqueous NOx reducing agent that, under the name Adblue, can now be obtained at any filling station. Adblue consists of 32.5 percent harmless urea, which is injected directly in front of the catalytic converter so that it first transforms into ammonia at that point. This conversion is brought about by the injection pressure and the temperature in the exhaust system, which causes the urea to react with the water in the solution and evaporate.
Although this sounds simple in theory, it’s no easy feat for engineers to accomplish in practice.
Although this sounds simple in theory, it’s no easy feat for engineers to accomplish in practice. The ammonia has to be distributed in the right concentration across the catalytic converter’s entire cross-section if it is to be completely converted into the harmless exhaust materials rather than being released into the environment. The thing that exhaust gas technicians fear the most is ammonia slip, and its prevention is the yardstick by which every retrofit solution is measured. Ammonia slip can be completely avoided by combining the SCR system with a slip catalyst that reduces the ammonia again but at the same time increases complexity. Complexity is extremely high as it is, because the dosing has to be precisely coordinated with the respective engine operating point.
The next challenge is to integrate the SCR catalytic converter into the exhaust system in such a way that it quickly reaches its minimum operating temperature of about 200 degrees Celsius after a cold start. The vehicles with the lowest nitrogen oxide emissions stay well within the Euro 6 limits in real-life operation and almost all of them have the SCR catalytic converter installed close to the engine or also have a storage catalytic converter that temporarily stores the nitrogen oxides. In such a setup, a retrofitted SCR system always has to be installed behind the oxidation catalytic converter, which not only removes carbon monoxide and unburned hydrocarbons from the exhaust gas, but also converts part of the nitrogen monoxide into the much more reactive nitrogen dioxide.
Most production vehicles that could be retrofitted don’t have enough room for such a solution. Instead, the focus is on removing the particulate filter and replacing it with a system that combines an SCR unit with a filter. Such a setup could work. However, the particulate filter has to be regularly “burned free” of material to prevent it from getting clogged if the vehicle is only driven short distances. The particulates are eliminated by injecting some fuel into the engine after the combustion process is finished. This fuel then ignites inside the filter. This burning process is controlled by a virtual reflection of the particulate filter in the engine management system. If the particulate filter is replaced without adjusting the engine management system (as some retrofitters are advocating), the engine model and the filter will no longer match. A real Gyro Gearloose would not only want to calculate such a system, but also extensively test it.
As if that weren’t enough, Adblue must be present in sufficient amounts, for which a separate tank is needed. So can the system simply be moved into the spare wheel well and the problem is solved? It looks as if there is no other space available for a retrofit solution. A glance at today’s Euro 6 production vehicles reveals that the tanks are always installed outside the vehicle interior. Automakers do this in order to spare the customers the need to handle liquids there and save themselves the trouble of installing pipes in the interior, which would cause a whole range of difficulties. It’s probably the best option for a retrofit solution anyway, because changes to the body design would require the entire accident safety system to be rearranged and necessitate new crash tests.
All of that doesn’t speak against retrofitting hardware, but for a realistic assessment of the costs involved. Some online suppliers promise to retrofit vehicles for €1,500 and some media even mention prices as low as €1,300. The latter price is supposedly contained in an expert opinion commissioned by the German Transportation Ministry. However, this expert opinion is under lock and key and the author of the report contradicts the claim in his reply to an enquiry by the F.A.S. “You can more than double the price,” he says. Moreover, the amounts do not take development costs into account, which are considerable. Thomas Koch from the Karlsruhe Institute of Technology estimates that the cost of developing a serious solution would be almost as high as that of a new system. Koch calculates that the total cost would amount to as much as €5,000 per vehicle, depending on the model in question.
The motorists’ own price-to-performance ratio would differ, even if the drivers don’t have to bear the costs of a retrofit solution themselves. Not only does the vehicle have to spend two days in a workshop, it subsequently consumes up to five percent more fuel. People will only accept such a calamity if the residual value of their vehicles is high enough and if they will continue to be permitted to drive their cars into the inner cities. Unfortunately, they still have no legal certainty of this.
Author: Johannes Winterhagen
From: Frankfurter Allgemeine Sonntagszeitung, February 4, 2018, No. 5, page 57 / Technik und Motor
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Original article (German)