What are the specific challenges and potential solutions for reducing NOx emissions in diesel engines without sacrificing fuel efficiency, considering the trade-offs between various aftertreatment technologies and in-cylinder combustion strategies?

  • $\begingroup$ There is SO much available to read about this topic. Chapman, Judge, Ricardo should be on your "read and understood" list... $\endgroup$
    – Solar Mike
    Commented Mar 19, 2023 at 19:41
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    $\begingroup$ Your question is "how to design a modern diesel engine." I suggest this is overly broad and not well suited to this format. Maybe a graduate level engineering course. $\endgroup$
    – Tiger Guy
    Commented Mar 20, 2023 at 5:49

1 Answer 1


NOx is formed at high combustion temperatures, in the presence of abundant nitrogen. Diesels get high combustion temperatures by virtue of their high compression ratios, which are necessary for autoignition. Reducing the temperature at which heat is added in the Otto cycle will reduce NOx formation but also reduces the Carnot efficiency of the cycle. N2 concentration in the combustion chamber can be reduced by diluting it with exhaust gas but this decreases the concentration of oxygen available to support combustion and thereby reduces the power output of the engine.

The best way we currently have for reducing diesel emissions is by aftertreatment of the exhaust or, for the ethically-challenged, Volkswagen-style cheating.

For those of you who did not know about this, there's an exhaustive (pun intended) article about the VW defeat device cheat on wikipedia.

  • $\begingroup$ All VW really did was partially automate what nearly every mechanic does to a car before conducting an emissions test. Sabine Hossenfelder described it best as a typical example of German efficiency. Now they are back to doing it all manually, and the price is fixed by regulations. So there is flourishing business of beating the tests that is a pure drain on the economy with absolutely zero environmental benefit. But that doesn't make a good headline. $\endgroup$
    – Phil Sweet
    Commented Mar 19, 2023 at 21:09
  • $\begingroup$ What VW did was build that defeat into the system interrogation hardware so that the EC system wasn't just defeated when a mechanic was testing the system, but all the time the system was in actual use. So when a VW diesel was on the road, its EC system was always defeated $\endgroup$ Commented Mar 19, 2023 at 23:47
  • $\begingroup$ @phil sweet, ...that is, never operating, and of course VW did not inform the EPA that there were a quarter of a million VWs driving around the USA with their EC systems locked out and that this had been going on for nine years and that the lockout was implemented on the firmware level of the engine management computer and that VW's own regulatory compliance division wasn't told about it. $\endgroup$ Commented Mar 19, 2023 at 23:52
  • $\begingroup$ @nielsnielsen but the US government have been dodging environmental issues for many more years than VW... Having a Petroleum Stooge making "science" decisions about what is and is not driving global warming likely did more harm. $\endgroup$
    – Solar Mike
    Commented Mar 20, 2023 at 8:14
  • $\begingroup$ @solar mike, that is a different issue. Defeat devices attached to mandated emissions control systems are specifically prohibited by law. VW was caught red-handed in flagrant violation of that law. prosecution was a slam dunk. $\endgroup$ Commented Mar 20, 2023 at 17:49

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