Effect Of Cycle-to-cycle Variation Characteristic On The Knocking Combustion Of SI Engines

Spark ignition(SI)engine downsizing is a promising way to improve thermal efficiency and reduce the emission.However,further improvement in the downsizing degree is limited by knocking combustion.It is normal that the flow field varies between cycles,which leads to cycle-to-cycle variation(CCV)of SI engine combustion consequently.Further,such variations would lead to the difference in knock intensities between cycles under abnormal combustion conditions,which matters a lot to the engine performance.To study the effect of CCV characteristic on the knocking combustion,the possible reasons leading to the cyclic knock intensity difference is explored,the key physical-chemical process that affects the knock intensity is studied.This is meaningful to fully understand the knock phenomenon under realistic operating conditions.In this study,the phenomenon of the knock intensity variation induced by the flow field changing is first investigated,the effect of turbulence on the auto-ignition process and the intensive pressure wave formation induced by multiple auto-ignition kernels are then analyzed by integrating simulation,experimental study and theoretical analysis.Detailed research contents and main findings are listed as follows:First,the phenomenon of normal combustion CCV are numerically studied.By coupling G-equation combustion model with chemical reaction progress variable passive based adaptive mesh refinement,the balance between the accuracy of prediction and efficiency of the simulation was achieved in the large eddy simulation(LES)platform.Then,the study on the effect of flame structure and the flow pattern inside the cylinder on the cyclic variation of combustion is conducted.Results show that the difference between the flow patterns is the main reason causing the cyclic variation of combustion,a disorder velocity field with strong turbulent mixing helps to accelerate the overall flame speed and lead to higher peak pressure.Based on the validated LES platform,the auto-ignition kernel occurrence and the subsequent development during both high and low knock intensity cycles are investigated to further explore the potential effects of CCV on the knock.In addition,the correlation between the mean knock intensity of a certain operating condition and the statistic averaged flow pattern inside the cylinder is analyzed.According to the results,changing of flow field influences the local turbulence-combustion interaction process directly,which determines both the timing and location of auto-ignition kernels occurrence.Then,different modes of multi-autoignition kernels interaction is observed,and various knock intensity could generate under different operating conditions or between cycles of a certain condition.The results also demonstrate that knocking combustion always originates from the multi-kernel auto-ignition.To achieve a more comprehensive understanding of the effect of turbulent mixing on auto-ignition,a dynamic adaptive multi-zone(DAMZ)algorithm is proposed to accelerate the detailed chemical reaction solving process and improve the accuracy of knock prediction.By integrating DAMZ with linear eddy mixing model,the effect of turbulent intensity on the occurrence and development of auto-ignition spots is studied.Results indicate that,within a regime that has an initial temperature below the negative temperature coefficient(NTC),with the increasing of turbulence intensity,the autoignition formation in the core region is delayed and the amount of homogenously ignited mixture in the core region decreases at the onset of auto-ignition.In addition,a balance between reaction and diffusion is observed under strong turbulence intensity conditions.This verifies the existence of diffusive–reactive flame structure and indicates that the kernel expansion is governed by turbulent flame propagation.However,under the condition of higher initial temperature within the NTC regime and low turbulence intensity,the first auto-ignition kernel occurs in the boundary layer instead of the core region as in the low initial temperature cases.With increased turbulence,such “cold” auto-ignition is inhibited due to the enhanced mass and heat transfer.Moreover,the expansion processes of nascent kernels are mainly spontaneous ignition,and turbulence is not able to change the nature of auto-ignition flame propagation during the end-gas combustion.Also,the effect of turbulence on the autoignition process is closely related to the thermodynamic state of the ending mixture.At last,numerical research on the mechanism of intense pressure wave formation induced by multiple auto-ignition kernels in the primary reference fuel(PRF)/air mixture is conducted based on a 1-D solver for the compressible reactive flow coupled with the DAMZ.The appropriate initial condition(a small interval between hotspots,a low initial temperature 800.0 K and a large initial temperature inhomogeneity)for the detonation formation within a limited space is identified.When new autoignition,which usually is induced by the original auto-ignition kernels,occurs between the initial pressure waves,the multiple pressure waves will overlap with each other gradually.Finally,the detonation will be induced by the coupling between chemical reaction wave and pressure wave.As the detonation is usually generated far away from the cylinder wall,the intensity of the strongest pressure wave can be suppressed by accelerating the main flame speed through introducing appropriate turbulence.However,if the initial temperature is elevated,the most intense pressure pulse occurs between the hotspots that are near the wall.Hence,the knock becomes more difficult to be suppressed.This study contributes to the understanding of both the key factors that affect the intensity and the location of the peak pressure pulse inside the cylinder and strong pressure waves(detonation waves)formation under engine-relevant conditions.