Research On The Cycle-to-cycle Variation Of A Two-stroke Engine With The Method Of Les

CCV(cycle-to-cycle variation) is one of the most important features of an IC engine during the combustion process. The occurrence of CCV has extremely negative effects on the power, economy and emission. Therefore, understanding and controlling CCV is necessary to improve the overall efficiency. Meanwhile, the advantages of LES(large eddy simulation) on the multi-cycle simulation become acceptable due to its special characteristics and it has provided a new method of the research on CCV.A 3D simulation was performed to study the CCV of a two-stroke engine from Leeds University with the method of LES in STAR-CD software. The maximum in-cylinder pressure of the simulation matches well with that of the experiment, while the corresponding crank angle is a bit earlier. The flow information of the plug section at spark time is quite similar to the PIV measurement results. Moreover, the energy fraction of the small eddy is always lower than 10%. That means the 3D model is accurate enough to make further research on the CCV.According to the simulation result, CCV is quite obvious in terms of the in-cylinder flow field, pressure, temperature, flame surface position, etc. The CCV intensity of the in-cylinder velocity after the closure of exhaust port is much weaker and as the piston moves up to TDC, it becomes smaller. The CCV intensity of the velocity at horizontal section reaches its maximum at the top of the cylinder, while the COV decreases as the axial distance between the section and the cylinder head increases. The CCV intensity of the velocity at transversal section decreases as the piston moves up. The swirl scale of the horizontal section first decreases and then increases as the axial distance between the section and the cylinder head increases, and reaches its minimum near the middle of the cylinder. The swirl scale of the horizontal section at spark gap remains virtually the same before 40°CA BTDC and deceases after that. The tumble scale of the transversal section decreases as the piston moves up. In addition, the initial flame kernel has a direct effect on the middle-early combustion process. The high TKE at the spark plug gap is beneficial for the formation of the initial flame kernel, while the fuel fraction, temperature or pressure do not show any obvious correlation.