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Relationship between intake air temp and NOx formation

  1. Sep 16, 2010 #1

    This is currently an academic question - I don't have a 'spare' motor to risk at present :)

    Would NOx catalytic converters become unnecessary on a 'spark' engine if only relatively cold air was made available at the intake? My thinking is that NOx formation needs high high peak temps and a lower initial temp could reduce (or stop) those conditions.

    I know there are issues with fuel vaporisation and condensation if air is too cold, but I hope we can come back to that later.

    Also I realise power is going to be required for refrigeration and I hope to discuss some ideas of how the cooling can be done for 'free' later.

    Initially I would just like to know what flaws my concept for negating NOx has (?).

  2. jcsd
  3. Sep 16, 2010 #2


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    Depends how low you want to get the engine-out NOx. Practically it becomes very difficult if you still want to run at sensible air fuel ratios.
  4. Sep 17, 2010 #3
    Thanks brewnog. So you're saying that after-treatment is realistically unavoidable (with a gasoline engine) if vehicles are to meet current NOx air quality standards?

    Regarding a/f ratios, can you explain the problems?

    I know that traditionally weak running engines have had a tendency to run (counter-intuitively) hotter and I believe this is mostly down to the flame speed being slower, so not properly completing the necessary gas expansion before the end of the power stroke (Is this correct?). .... However, my gut feeling is that this won't lead to higher than ideal peak temps., but rather to a more distributed loss of lower heat into the cooling system and down the exhaust.

    I still am unconvinced that if given a cool enough charge a S.I. engine in practically all cases would be immune from NOx formation.

    Thanks again...
  5. Sep 20, 2010 #4
  6. Sep 22, 2010 #5


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    Practically, yes.

    Well, you might theoretically be able to run at extreme AFRs to get engine-out NOx low enough. However, you might approach practical limitations on lean misfire, detonation, stability, or metal temperatures before you achieve this.

    Not explicitly. Exhaust temperatures may be hotter due (in part) to the reasons quoted, and also because (in a gasoline engine) you lose the cooling effect from fuel evaporation with leaner mixtures. However, exhaust temperature trends don't really reflect peak cylinder temperature trends, and this is where NOx is formed.
  7. Sep 23, 2010 #6
    So holding down peak cylinder temperatures is key to retarding NOx formation, I understand that.

    Surely the incoming air temperature (if the volume of air is unchanged) is quite key to what these peak temps can reach (?). I'm sure there are other factors even a 14.7:1 ratio, like the quality of mixing and the humidity, but initial temp must play a part, no?
  8. Sep 24, 2010 #7


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    Yes, it absolutely makes a difference. However, (as I alluded to in my first post), the amount by which you can practically cool the air isn't that great. Cool a little and you'll notice a power increase (due to charge air density) and some NOx reduction. Cool further and your combustion stability will suffer, because you're chilling the combustion chamber too much. Cool further and your fuel won't evaporate, so you'll have no combustion. There are lots of practical considerations here which limit the NOx reduction effect you can obtain.
  9. Sep 25, 2010 #8
    Thanks again, Brewnog.
    I appreciate your patience and sorry if I've made you repeat yourself a little.

    If using Natural Gas can some of these low temp issues be avoided?
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