Investigation On Diesel TR Combustion System Based On Disperse Low-Temperature Premixed Combustion Conception

The development of diesel engine technology is facing the challenges from both the increasely stringent emission regulations and the shortage of petroleum fuel, so to organize high efficient and clean bombustion on diesel engine, or to replace the conventional diesel diffuse combustion with premixed combustion, becomes an international research focus today. Furthermore, some progress has been achived in this research field, such as the realizing of homogeneous charge compress ignition (HCCI) combustion at low and medium loads, which can reduce both PM and NO_X emissions to almost zero. Howerver, such problems as how to expand HCCI combusution to high loads need to be sovled before application. Recently, some theoretical and technical research aobut how to realize premixed combustion on diesel engine was carried out by the author, and is introduced in this paper.Firstly, the paper presents the disperse low-temperatured premixed combustion concept, through combining the analyse of the process control and emission reduction principles of HCCI combustion and the research about the heat release characteristics of both diesel diffusive combustion and gasoline sprak-ignited premixed combustion. For the new combustion concept, the heat release process should be both spatial and temporal disperse, for the reason that the former is effective in reducing local high-temperarured zones, and the latter can slow down the combustion and avoid the over-high increase of the average combustion temperature and cylinder pressure, which is very important in realizing premixed combustion at high load. In addition, the maximum combustion temperature should be less than 1800K, the critical temperature for thermal NO_X formation, then the PM and NO_X emissions could be simultaneously reduced greatly. The paper also presents a combustion process control strategy mode. For this mode, several premixed fuel air mixture blocks of different temperature and concentration form in different chamber space zones before ignition, and each mixture block ignites at different time and burns one by one, then the heat release process has characteristics of both spatial diverse and temporal diverse simultaneously.Secondly, diesel TR (Three-Rapidity) combustion system was designed directed by the strategy model mentioned above. The system adopts a multi-hole nozzle with a centric hole, and there is an oriented arc set on the chamber wall and a plane convex at the chamber bottom center. The oriented arc is modified form the reflex edge of TRB chamber for the sake of reduing oil film formation. The spray images in high pressure vessel acquired by an high-speed camera show that the impingment spray strips off the chamber wall at the orinted arc and forms into spatial subspray, which penetrates towards the chamber center. So there is hardly fuel distrubuting in the near-wall zone of the lower chamber cavity wall. In addition,after stikeing the top plane of the bottom convex the centric spray diffuses radially and develops into a symmetrical disc spray.Experiments were carried out on a sigle cylinder 135 diesel engine modified with TR combustion system to investigate the performance and influence factors of the new system. The experiment results show that TR combustion system adopting a multi-hole nozzle with a centric hole of 0.20mm diameter can reduce both exhaust smoke and effective fuel consumption rate greatly. When injecting timing is 20°CA BTDC, exhaust smoke is only about 0.4BSU at full load, while NOX concentration is about two times more than that of the prototype engine. With the delay of fuel injection timing, NOX emission decreases gradually with the increases of smoke, but in certain range smoke increases slowly. When injection timing is 11°CA BTDC, at full load exhaust smoke is still less than 1BSU and the effective fuel consumption rate is 225 g/(kw ? h) or so. When injection timing is further delayed to around TDC, both exhaust smoke and NOX concentration decrease sharply under low and middle loads, while exhaust temperature and fuel consumption rate are very high.Investigation on ignition control shows that the ignition phase of side sprays delays with the diameter increase of the side holes, and both the rule injection duration and combustion duration shorten with the increase of the total holes area. In addition, the research also shows that the centric spray ingites before the side sprays indeed, which is helpful to realizing the temporal disperse of heat release process.A series of experiment including lowering compression ratio, fueling diesel-ethanol blend fuel and adopting cooled EGR was carried out to clarify the emission reduction potent of TR combustion system. Experiment results show that with the decrease of compression ratio from 17.6 to 16.8, the ignition delay period prolongs and NOX emissions decrease somewhat. But the combustion duration also prolongs, both exhaust smoke and fuel consumption rate increase. When the engine is fueld with 20% (vol) ethanol-diesel blend fuel, ignition phrase delays apparently, exhaust smoke decreases sharply, especially at low and medium loads it is almost zero. NOX emissions also reduce somewhat at low and medium, but the reduction decreases gradually with the load increase. At 90% load exhaust smoke is only about 0.4BSU while NOX concentration is less than 450 X 10~6. Furthermore, in certain load range the combustion duration does shorten with the load increase. It is analyzed that the combustion process is dominated by premixed combustion, so the air-fuel mixture concentration becomes leaner and cylinder temperature becomes lower with the load decrease, which slow down the combustion reaction rate. In addition, when TR combustion system adopts a little amount of cooled EGR NOX emissions at all loads decrease averagely about 40%, while fuel consumption rate keeps almost unchanged and exhaust smoke increasessomewhat.Finally, in order to understand the characteristics of in-cylinder mixture formation and combustion process of TR combustion system, the combustion process is numerically simulated. The simulation results show that with an oriented arc set on chamber wall the impinging sprays are switched to chamber center when they steak off chamber wall, so that there is hardly fuel distributing in the near-wall zone of the lower chamber cavity wall, which prevents the accumulation of substantial dense mixture in this zone and even the formation of oil film on the chamber wall. During the combustion process mainly the temperature of chamber center part is very high, and the temperature of the near-wall zone of the lower chamber cavity is so low that this zone functions as heat insulation. Furthermore, the oriented arc also influences the gas movement in the chamber cavity in such a way that the gas eddies distribute in more reasonable zones, which helps to promoting the air and fuel mixing process in the lower cavity during the middle and later period of the combustion process, and the evaporation of the fuel deposits on the upper chamber wall. Consequently the post-combustion duration shortens and the fuel economics increases. In addition, the simulation results also show that during the ignition process of TR combustion system several self-ignition points emergency at different location of centric spray and side sprays sequently, which makes the heat release process be of double disperse characteristics.