Predicting the Influence of Charge Air Temperature Reduction on Engine Efficiency, CCV and NO _x -Emissions of a Large Gas Engine Using a SI Burn Rate Model

In order to meet increasingly stringent exhaust emission regulations, new engine concepts need to be developed. Lean combustion systems for stationary running large gas engines can reduce raw NO_x-emissions to a very low level and enable the compliance with the exhaust emission standards without using a cost-intensive SCR-aftertreatment system. Experimental investigations in the past have already confirmed that a strong reduction of the charge air temperature even below ambient conditions by using an absorption chiller can significantly reduce NO_x emissions. However, test bench operation of large gas engines is costly and time-consuming. To increase the efficiency of the engine development process, the possibility to use 0D/1D engine simulation prior to test bench studies of new concepts is investigated using the example of low temperature charge air cooling. In this context, a reliable prediction of engine efficiency and NO_x-emissions is important. Furthermore, restrictions to the engine operation like increase of cycle-to-cycle fluctuations due to high excess air ratio or late combustion need to be predicted as well in the engine simulation.

For this purposes, a combustion model originally developed for on-road SI-engines was calibrated using four operating points at standard charge air temperature. To predict the cycle-to-cycle variations, an existing model was adapted to the new combustion model. The Zeldovich mechanism for the formation of thermal NO was used to predict NO_x emissions. The simulation results of the calibrated models have been compared to experimental data covering variations of air-fuel-ratio, spark timing and charge air temperature. In this comparison, the models showed a good predictive ability in terms of engine efficiency and cycle-to-cycle variation. To cope with the difference between prechamber spark plug and regular spark plug, an upscaling of the combustion model is sufficient. In order to match the measured NO_x emissions, the NO formation rate needs to be increased by a factor of six in the model. Furthermore, the increase of NO_x emissions with a reduction of air-fuel-ratio λ is overestimated. Nevertheless, the qualitative change of NO_x emissions with changing boundary conditions is reproduced well by the model. The simulation results prove the applicability of the combustion model to large gas engines and the possibility to use 0D/1D engine simulation for the evaluation of new engine concepts. Considering the NO_x model errors, the lack of a NO₂ model and a general deficit of the Zeldovich mechanism at lean conditions are identified as possible reasons, which will be the subject of future work.