What is the best treatment for diesel engines
PollutantsClean diesel engine - possible, but expensive
Panayotis Dimopoulus Eggenschwiler believes that the good old diesel has generally fallen into disrepute after the VW emissions scandal. Because:
"In principle, diesel burns with better efficiency. That means: It delivers the performance you want, consumes less fuel and also emits less carbon dioxide, i.e. less greenhouse gas."
Compared to the gasoline engine, the exhaust gas aftertreatment expert at the Swiss EMPA research institute in Dübendorf near Zurich clarifies. But he also knows: Diesel engines have a small blemish - namely a significantly higher proportion of nitrogen oxides than gasoline engines.
"That is why up to Euro 5 there were also specifically higher limit values for nitrogen oxides in diesel vehicles than for gasoline-powered vehicles."
The new pollutant norm has been in force since September
However, since the beginning of September, the Euro 6 emissions standard has been in effect, which also stipulates a reduction in nitrogen oxide emissions in passenger car diesel engines - and good advice is literally expensive. Most manufacturers rely on the so-called AdBlue or SEC process, which stands for "Selective Reduction Catalyst": This is a chemical process in which the exhaust gas reacts with aqueous urea, which is carried in an extra tank. Ammonia is produced under heat. But this splits the harmful nitrogen oxides into the harmless components water and nitrogen. That’s the theory. But in practice it usually looks different - the devil is in the details, according to Panayotis Dimopoulus Eggenschwiler:
"If you inject too much of the urea solution, it leads to a lot of ammonia. And that can escape into the atmosphere, which you don't want either. Because: Ammonia doesn't smell pleasant and is also poisonous per se."
So use urea more sparingly when injecting into exhaust gas aftertreatment? EMPA researcher Panayotis Eggenschwiler cannot be enthusiastic about this either:
"With too little urea-water injection, you have too much nitrogen emissions. Setting up and regulating the entire system is quite complex."
And so complex that, according to the experience of the EMPA researchers, either too much nitrogen oxide or too much ammonia came out of the exhaust pipe in most urea-based exhaust gas aftertreatment systems, without any software manipulation.
"In addition, if you hit too much of this urea-water solution on the exhaust walls, you tend to create solid by-products. The solid by-products are not toxic per se, but they clog the exhaust."
"As big, as heavy and as expensive as the engine itself"
As a basis for the development of more effective diesel exhaust aftertreatment systems for cars, the Swiss EMPA researchers are examining in detail all processes that occur in the AdBlue process. You have found that the optimal addition of urea depends on many parameters: speed, operating temperature, engine speed, driving behavior and much more - parameters that, in contrast to tests on a test bench, change in fractions of a second while driving freely on the road. This is particularly true of cars, which travel much faster than trucks and buses. That is why the AdBlue technology there has to be designed for more complex driving modes, in contrast to trucks, in which such systems have been installed for ten years. However, it works much easier there than with trucks. Panayotis Dimopoulos Eggenschweiler:
"Roughly speaking, the nitrogen oxide aftertreatment plus the particulate aftertreatment in the truck is as big, heavy and as expensive as the engine itself."
No question about it: you won't be able to clear that much space in a car.
"We are working on new solutions. They make the system more robust, better, but also more complex and expensive."
And so much more expensive that, according to the Swiss EMPA researcher, diesel-powered small cars will in future hardly be competitive with corresponding cars with gasoline engines. On the other hand, the situation is different for upscale mid-range and luxury-class cars.
"Quite simply because we have very demanding CO2 limit values here in Europe. These 95 grams for 2020/2021 are hardly achievable if the diesel loses market potential," emphasizes Christian Bach, head of the vehicle drive technology department at EMPA.
It relates to the average CO2 emissions of a vehicle fleet: Without diesel cars in the luxury class, he is certain that a brand will not be able to meet the EU limit values. This makes it all the more important to him and his team to optimize exhaust gas aftertreatment based on Adblue, as is currently being promoted by EMPA, among others.
"This process will still take some time. But the potential is there. And that's why we are actually convinced that the diesel engine will drive around on the road with almost no nitrogen oxides in the future."
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