Performance and emissions of a methane-fueled spark-ignition engine under consideration of its cyclic variability by using a computational fluid dynamics code

In this work cyclic variability is examined in a methane-fueled spark-ignition engine, focusing on its influence on performance and emissions. A recent numerical approach within the ignition model of an in-house computational fluid dynamics (CFD) code has been expanded, with the aim to increase the accuracy when estimating the effect of this highly-complex phenomenon in engines. Apart from focusing solely on the flame propagation process and the related small-scale turbulence, the effect of each cycle on the next one has also been included. This is accomplished by developing a single-zone thermodynamic model applied only during the engine open cycle, in order to accelerate the calculations, while the engine closed cycle is simulated by the CFD code. A methodology for estimating the coefficient of variation (COV) of indicated mean effective pressure (IMEP) is used based on the simulation of a large number of consecutive engine cycles, until the COV of IMEP converges. The variability of the engine performance and nitric oxide (NO) and carbon monoxide (CO) emissions is evaluated, showing that this methodology of incorporating the open cycle brings a significant effect only on the emissions, mostly to NO varying up to 70% compared to the average cycle one, unlike the engine performance where a negligible difference of COV of IMEP is disclosed compared to the use of only the closed cycle. Overall the calculated emissions show a large variability, especially for NO, revealing that the additional computational effort for this kind of multi-cycle simulations is justified to providing accurate details.