Investigations On Aluminum Alloy Ablation For Diesel Engine Pistons

With t he increasingly higher specific power output diesel engine, thermal l oad problem is becoming i ncreasing acute. The main caus e of the fa ilure of the engine i s the piston ablation, which i nfluenc es the structural integrity of diesel engine. In this aspect, it is necessary to thoroughly understand the ablation properties of aluminum piston alloy at the typical ser vice temperat ure. In this work, the a blation behaviors and mec hanisms of al uminum alloy was inve stigated. An ablation model was established bas ed on the experimental result, which was validated in finite ele ment analysis. Simplified methods were used for t he pi st on ablation prediction, especially the piston working in harsh working condi tions, a nd the simulation results agr eed well with t he experiment al data. In a ddition,an investigation wa s also carried out to study the ablation behaviors of Al2O3f/Al composit e and aluminum borate whi sker composite. Results of ablation properties of thes e ma terials have been presented, and analysis of the r esults and numeri cal s imulation of expe rimental result s also be en provided. Fi nall y, the failure a ssessment method of a luminum a lloy abl ati on failure was studi ed.The whole ablation test system includes fire heater, thermocouples and softwa re for analysis t he testing data, which c an model the ablation of piston material in gas with high te mperature and high velocity. Experi ment re sults showe d that the surface t emperature of te sting sample had a very signific ant effect on t he mass ablation rate. The i nfluenc es of impact air flow angle and velocity on the mass ablation rate became less. The re lat ionship bet ween t hese factors and the mass ablation r ate was obtaine d base d on the expe riment al dat a. Further more, the ablation fail ur e behaviors of alumi num alloy were charac terized based on morphol ogy st udy, profile analysis and mate rial composit ion.The ablation me chanisms of alumi num al loy wer e studied bas ed on the experimental dat a. It was shown that the high content of eutec tic Si microstructure were fir st dissolution,which lead t o a volumetr ic expansion and pus hed through from the material inner, and then adhered to the surface of the sample in pa r tic le-like st ructures. These small parti cle s and compounds with low melting point on the ext ernal sur face of t he sample were blown away under the high temperature and high speed air flow, event uall y resulting in melted ablation of the al uminum al loy. As the ablation process continued, the volume fraction of pre cipi tat e inc reased and t he surface roughnes s of the sample would achieve critica l value. The convexparts on the surface of the sample would be further sheared off by the high speed air flow. It should be noted that the high speed air flow would accelerate the denudation rate of t he material.The abl ati on of aluminum alloy is a complex coupling pr oblem. The mas s loss during abl ati on process c ontains three sections: precipitate at high temperature,melting loss on the material surface and denudat ion by the high velocity a ir flow. Based on ablation mechanism analysis, consi dering me lting and denuda tion me chanism, an ablation model of aluminum al loy was established.The experiment al system wa s mode lled geometrically, and the fl ow field was simulated with Fl uent, obt aini ng the heat t ransfer coefficient at the sample surface. Using the se dat a as bounda r y conditions, the temper ature fie ld of s ampl e was c alculated with finite difference method. Meanwhile the ablation procedure was calculated using the abl ati on model. The simulat ion r esults a greed wi th experimental data rea sonably, i ndicating tha t the ablation model and calculation method we r e available for ablation pr ediction.The piston ablation result ing from wa ll burning in strengthened dies el engines was analyzed. The t hermal boundary conditions of piston head were modeled with AVL-FIRE.The ablation progress was predicted with simplified abl ati on mode l. Results showed that the ablation mass increased with t ime a ccumul ated, but the ablation rate reduced. The abl ati on would cease when piston tempera ture fi eld became steady. Considering the coking behavior of Oil-cooling channel, the piston ablation was a nalyzed numerically. All the se result s cont ributed to the pis ton struct ure design.In or der to asses s ablation properties of Al-Si piston alloy reinforced with Al2 O 3 fiber,the abla tion be haviors of Al 2O3f/Al composite and aluminum borate whisker composi te have been investigated. The ablation behaviors of Al2O3 f /Al c omposite were analyzed using the finite element model of piston. Resul ts reveal ed that Al2O3f/Al composite a nd aluminum borate whisker composite exhibited higher ablation prope rties and maintained its structural integrity under the hi gh temperature a nd speed airflow, which were contribut ed by the dense structure and high yield strength at high temperature of the compos ite s. Compared with aluminum alloy, the line ablation and mass ablation of these composites were lit tle.In order to assess mechanical pr operti es of ablated aluminum al loy, an investigation was ca r ried out to study the hardness of alumi num alloy after t he ablation process. The relationshi ps between Rockwell hardness and ablation temperature wa s obtained based on the experimental data. Ass essment standard and t echnical requirement s of the ablationfailure of aluminum alloy were also est ablished.