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NORA: a way to control NOx emissions produced by combustion


Nitrogen oxides (NOx) are pollutants produced during combustion by combination at high temperature of oxygen and nitrogen present in the air. Controlling their formation is an important research field for the transport industry since a significant reduction in NOx at source may potentially make it possible to reduce the complexity and cost of pollution abatement systems.

In the framework of the Groupement Scientifique Moteur (GSM*), a multidisciplinary research team at IFPEN, bringing together experts in combustion chemistry and fluid mechanics, recently developed an effective approach for addressing this industrial challenge. Dubbed NORA (Nitrogen Oxide Relaxation Approach), the approach combines detailed nitrogen oxide formation models with advanced turbulent combustion models with an optimized calculation cost. It describes the formation and reduction of NOx during the combustion cycle and determines the different types and their proportions.

NO and NOx tracking in combustion chamber

The adopted method is to determine the concentrations of nitrogen oxides in thermodynamic equilibrium, then calculate the characteristic time of relaxation to this balance through a chemical kinetics model developed in partnership with the CNRS. Tabulation of this information based on a limited number of parameters (temperature, pressure, air-fuel ratio) can effectively and quickly predict the NOx concentration in combustion engines and their interaction with the formation of other pollutants such as soot.

Designed to be generic, this method allows a precise prediction of the impact of fuel on the NOx formation and interactions of different pollutants produced by combustion. It is also a tool that can be used to ensure a better match between the fuel and its usage.

* Economic Interest Group bringing together IFPEN, PSA and Renault SA

Source: IFPEN
Romain’s opinion:

NOx are really hard to model and to predict using simulation tools, even more when you want to perform fast simulation, close to real time. This kind of model is very useful for understanding the NOx formation process at a research level. Do you think OEM will buy this model and use it for application purpose? Do you think it will be used to save some engine test cell experiments or only for knowledge purpose?


  1. Roshan Jayachandran12-19-2013

    Two questions come to mind;
    When you say that the tool is generic, in what way is it so?
    And if the tool is an accurate predictor of the NOx formation process, does it hold good for transient operations of the engine as well, or is accuracy only assured in steady state operating points.?

    Unless these questions are answered with high certainty, I don’t believe this tool could be used industrially to gain test bench time. My reasoning for this being that even when your tool is generic, you still need to validate its results, and unless the tool provides a reasonable accuracy over transients, it may not reduce project length it cost by a large enough margin for OEMs to adopt it as part of their calibration process.

    • Romain Nicolas12-22-2013

      Very good questions!
      It is generic in the way that it can be used for several types of fuel, in several shapes of combustion chamber with several injection patterns.
      For the second question, I didn’t try out the tool, but like any modeling tool, accuracy is always better in steady conditions. We know that NOx are mostly formed during transient phases, so I guess that the originators of the tool have focused on these conditions.

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