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The wsj has too few infos, however if you can read german, there are very detailed articles: https://www.sueddeutsche.de/auto/daimler-mercedes-glk-rueckruf-kr

by Glawen 7y ago

The wsj has too few infos, however if you can read german, there are very detailed articles: https://www.sueddeutsche.de/auto/daimler-mercedes-glk-rueckruf-kraftfahrtbundesamt-1.4495109 https://www.sueddeutsche.de/auto/daimler-mercedes-glk-rueckr... As i understand it, the motor seemed to artificially open the thermostat to keep the engine colder than usual. Now my guess, as an automotive engineer, is that they did this to reduce the time where the engine is in the normal operating range. It makes no sense to me, unless the emissions test only applies when the engine is in normal temperature. Or, second guess, as the engine is colder, it emits less NOx, because NOx appear when the combustion temperature is too high. Or simpler: the thing activates when the coolant is 20 deg celsius, the starting temperature of the test

toast0 7y ago
> Or, second guess, as the engine is colder, it emits less NOx, because NOx appear when the combustion temperature is too high. That's a major issue. NOx emissions are caused by engine heat interacting with nitrogen and oxygen from outside air, which gets you to a nasty trade-off. Run the engine hotter and get more efficient combustion, therefore less CO2 emissions; or run the engine cooler and get less NOx emissions. By design, Diesel engines combustion happens at much higher pressure and temperature than gasoline engines, which is why diesels produce much more NOx. Diesel emission fluid can reduce NOx, but also reduces efficiency, meaning more CO2 will be released.
- Glawen 7y ago
  True, it is a constant compromise that the engine is doing at each injection. For the article, i think the journalists just didn't understand the explanation, and it is probably the simpler solution : they enabled the emission cleaning algorithm when the coolant at startup is 23 degrees, so clearly a defeating device.

by Glawen 7y ago

toast0 7y ago

> Or, second guess, as the engine is colder, it emits less NOx, because NOx appear when the combustion temperature is too high. That's a major issue. NOx emissions are caused by engine heat interacting with nitrogen and oxygen from outside air, which gets you to a nasty trade-off. Run the engine hotter and get more efficient combustion, therefore less CO2 emissions; or run the engine cooler and get less NOx emissions. By design, Diesel engines combustion happens at much higher pressure and temperature than gasoline engines, which is why diesels produce much more NOx. Diesel emission fluid can reduce NOx, but also reduces efficiency, meaning more CO2 will be released.

Glawen 7y ago
True, it is a constant compromise that the engine is doing at each injection. For the article, i think the journalists just didn't understand the explanation, and it is probably the simpler solution : they enabled the emission cleaning algorithm when the coolant at startup is 23 degrees, so clearly a defeating device.