Abstract
Keywords:
NOx, emission, model, calculation speed, EGR, lambda.
Abstract:
A very fast NOx model was developed. Using extensive Exhaust Gas Recirculation (EGR) sweeps on three different compression ratios it is shown how the NOx trends and levels are captured by the NOx model that uses table interpolation of local equilibrium in-cylinder conditions and a simple correction algorithm for low-temperature combustion. The calculation time of one engine cycle is approximately 0.3 seconds.
Also, a simple and extremely fast method to predict engine-out NOx levels is introduced and physically derived. It is shown how the Emission Index of engine-out NOx, plotted on a semi-logarithmic scale, forms a straight line in relation to a single parameter, called inert EGR, defined as EGR% over relative air/fuel ratio (Lambda). A correction can be made for compression ratio if the effective compression ratio is variable. This log-lin correlation holds irrespective of engine load and intake pressure, as long as combustion phasing and engine speed are kept constant. The straight-line relationship could be used as an extremely fast NOx model integrated in the engine control unit. Also, it could be used combined with the more detailed NOx model described above for fast and accurate targeting of a specific engine-out NOx level.