This paper surveys existing emissions models used in the prediction of NOx. The prediction of jet engine emission indices from fundamental principles have proven to be difficult due to the complex physical and chemical interactions occurring within their combustion chambers. Present day prediction of engine emission indices during engine development relies on published models, which are based on limited sets of data measured on older combustion chambers where minimizing pollutant emissions was not a major design criteria. Such empirical and semi-empirical models can, however, provide upper emission limits for new engine designs. A database comprising a wider range of experimental data (over 2000 measured points) taken from the literature was used to test the models. Advanced techniques were applied to optimize the coefficients of proportionality of governing equations of the best models in the literature. Most models tend to consistently over or under predict the measured values. In most cases, even though the standard deviation of the predicted values was not reduced, the correlation error was improved by removing this bias.

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