| The poor engine cold start performance and emission problems are urgent to be solved.With the help of optical GDI engine,this paper study about the combustion cycle-to-cycle variations by inducing different forms of flow field,then explore the effective methods for solving cold start problems.This project focus on tumble and swirl plane by analyzing the flow features and cyclic variations,combined with the diagnosis of in-cylinder combustion flame images,to insight the mechanism of combustion fluctuation under different flow patterns.In this study,relying on visible optical engine,the high speed high power double pulses laser and high frequency double frame camera are utilized to measure the velocity vector field of two important flow,and the particle image velocimetry method is introduced.The proper orthogonal decomposition method is conducted to quantitatively decompose the multi-cycle large variation flow velocity fields,then reconstruct the decomposed vector fields based on different flow scales to separately analyze the large-scale patterns and micro-scale turbulences,finding out the cause of in-cylinder flow fluctuation.Besides,the flame image features,such as flame area,flame kernels distribution and flame center diffusion paths,can be used to clarify the processes of combustion variation.The research shows that,the engine cyclic variations is determined by the dominant induced flow structure,the stronger the large scale flow is,the smaller the flow fluctuation will be,and these result in decreasing the combustion variation.But if the eddy intensity is too strong,the excessive flow velocity near the cylinder wall will Hinder the flame spread to the periphery,then the combustion performance will be limited.Enhancing the in-cylinder bulk flow intensity can make the flame kernels distributed more concentrated,the flame propagation will be more stable,then the cyclic variation decreased.Besides,improving the strength of bulk flow within a certain range,it can not only reduce the volatility of large-scale flow field,but also strengthen the small-scale turbulence,which benefits improving combustion speed,the efficiency can be improved.Other research shows that the combustion heat release rate has a strong relevant index with the 3/2 power of the flame area,the correlation coefficient can reach 0.992,therefore,the in-cylinder heat release rate can be accurately predicted by analyzing the early flame image.Based on direct injection engine cold start and emission optical diagnostic tests,it can be concluded that,under low-speed cold start conditions,enhancing the bulk induced flow,not only can reduce the variation of engine cycles to improve combustion efficiency,but also can inhibit the formation of soot particles effectively. |
