Employment of numerical methods for efficiency optimization of a large gas engine with regard to NOx-formation and knocking combustion

Abstract

The present work deals with the pressing need of reducing the engine fuel consumption while keeping the emissions below the legislations’ compulsory limits. For this purpose a numerical methodology is applied to the optimization of the combustion process. The simulative approach concerning a large gas engine operating at very lean conditions is exposed in the present work. The potential of simulating the combustion process is investigated by test-bed measurements to check the quality and the reliability of the results. The fractal combustion model is employed and since the combustion start in the cylinder is wrongly positioned, a parameter is implemented to represent properly the ignition delay. Additionally, a specific gas engine combustion model is examined because of its better representation of the in cylinder turbulence. However, considering the absence of an EGR depending parameter in its formulation and thereby the overestimation of the burning velocity, the model has been improved introducing the residual gas concentration dependence in the formulation of the laminar flame speed. On the basis of this investigation the optimal predictive combustion model for a large gas engine is detected. The quality of a simple predictive NOx-emissions model is investigated and compared to detailed kinetic mechanisms to evaluate its reliability compared to the very short required computational time. Since the models currently employed for the onset prediction of abnormal combustions are not suitable to the investigated working conditions, a specific high reliable knock model is developed for more detailed analysis. Moreover, it is exposed how the engine operating parameters air excess ratio, residual gas concentration, ignition timing, indicated mean effective pressure and compression ratio affect the efficiency improvement in the field limited by the NOx-formation and knock thresholds counting on the previously optimized numerical models. The second part of the work deals with the effects of the combustion chamber geometry on the burning process. To best accomplish the purpose, the operating parameters are kept constant since their variations would additionally affect the combustion behaviour. A CFD simulation method is introduced and the flame front propagation and its interaction with the chamber geometry are exposed in a detailed manner. A one-dimensional model is then indirectly combined with the 3D models in order to analyse the influence of the combustion chamber on efficiency, emissions and probability of knocking combustion. As a result of the described investigations the best engine setup was achieved. Low residual gas concentration, high air/fuel ratio and early ignition timing are the characteristics that match the targets of high efficiency within the legislative and technical limitations. Additionally a piston shape that boosts up the performance has been detected. The potential of the employed methodology is exposed as conclusive part of the work.