Research On Piston Slap Characteristics Of Motorcycle Engine

Compared with automobiles,motorcycles have neither engine mount with excellent vibration isolation performance,nor engine compartment with good sound insulation and noise reduction performance.the vibration and noise of motorcycle engines are more directly transmitted to the body and individual.Therefore,improving engine vibration from the source has more practical engineering value and important theoretical significance for reducing motorcycle vibration noise.Piston slap,as one of the main excitation sources of engine mechanical noise,has an significant impact on engine NVH performance.This paper took a two-cylinder motorcycle engine as the research object,analyzed the secondary movement,researched the vibration and noise of the whole engine caused by piston slap,the influencing factors were analyzed in detail,and carried out multi-objective optimization.The main research work is as follows:Firstly,the piston assembly dynamics model was established to analyze the secondary motion characteristics of the piston and the knocking characteristics generated by the contact between the piston and cylinder liner.In order to make the simulation model more realistic,factors such as piston elasticity and thermal and mechanical coupling deformation of piston and cylinder liner were considered,the working profile of piston and cylinder liner was taken as the constraint condition of the model.Took the measured gas burst pressure as the load,the kinetic results of the piston at full load 3000r/min operating conditions were analyzed.The results showed that the piston changed direction and knocked liner six times in one working cycle of the engine;under the gas burst pressure,15° after the piston reversal,the knock on the cylinder liner was most obvious,and the maximum piston kinetic energy was 0.100 N.m.Secondly,the rigid-flexible coupling multi-body dynamics model of the whole engine was established considering the elastic deformation of the engine body and crankshaft,and the vibration and noise response of the whole engine under the piston slap were analyzed.The finite element model of engine body and crankshaft were condensed by modal synthesis technique,then as flexible body to construct the rigid-flexible coupling multi-body dynamics model.The feasibility of the model was verified by engine surface vibration test.Based on the engine model,the vibration response of the main position nodes caused by piston slap were analyzed.The indirect boundary element method and MATV technique were used to analyze the engine radiation noise.The vibration response results showed that the thrust side(TS)and anti-thrust side(ATS)vibration of the engine surface of cylinder liner were obvious,and the maximum vibration acceleration level of the TS node was 146.4d B;The cylinder head and the cylinder cover were also more responsive to piston percussion.The radiation noise results showed that the sound pressure level(SPL)curves of all field points occurred peak value at 100 Hz of the second order engine frequency.The SPL of field point 2 at TS and field point 4 at ATS were larger than other points in 300-1100 Hz.The SPL of field point 9 at the top of the cylinder head cover was the highest in 1884-2418 Hz,with a maximum value of 75.7d B(A)at 2214 Hz,indicating that the radiation noise capability of the plane structure of the cylinder head cover was strong.All the field points showed obvious peak value at about 1244 Hz,which was close to the fifth natural frequency of engine body 1278 Hz and the fourth natural frequency of crankshaft 1230 Hz,indicating that it was caused by resonance.Finally,the influence of piston structural parameters on piston motion and contact characteristics between piston and cylinder liner were studied by control variable method,and multi-objective optimization was carried out.The main factors such as piston cylinder clearance and piston pin offset were selected,data points were selected by the Optimal Latin hypercube method.a response surface approximation model was established after correlation analysis between the optimized parameters and the objectives.Multi-objective optimization design was carried out based on the "NSGA-Ⅱ" algorithm.Compared with the pre-optimization,the optimized kinetic energy was reduced from 0.100 N.m to 0.073 N.m.the friction mean effective pressure decreased from 4.859 to 4.751 k Pa,the body surface vibration was also improved,the SPL of the overall radiated noise was reduced by about 3 d B(A).In this paper,the piston slap characteristics of motorcycle engine were fully studied by simulation and experiment,and the multi-objective optimization design was carried out by the method of approximate model,and better optimization results were obtained with lower time cost.