| Massive nitrogen oxide (NOx) emissions is one of the most important problems facing marine diesel engine, which is drawing more and more attention. IMO (International Maritime Organization) has issued a series of regulations to control NOx emissions from marine diesel engines. The NOx emission limit of IMO Tier ? regulation is about 80% lower than that of IMO Tier I regulation. Therefore, the in-cylinder technologies for reducing NOx emissions are studied on a 240 medium speed diesel engine.The research consists of three main parts. First, quick-mixing combustion experiment on testbed is performed based on the new type of combustion chambers and injector nozzle. Then in-cylinder NOx reduction technologies to meet the requirements of IMO Tier III regulation are explored. Finally, the solution of quick-mixing and low NOx emissions combustion which is achieved by combining the technologies researched above, and it can reduce soot emission and improve fuel economy without power loss.In order to expand the fuel spray distribution, improve the mixing rate of fuel and air, realize the quick low temperature combustion, two kinds of new combustion chambers are proposed for the 240 diesel engine:big guided angle combustion chamber and small guided angle combustion chamber. For the first chamber, the collision guiding effect of the combustion chamber on the spray is enhanced by setting the collision ring at the spray droppoint, and the fuel secondary diffusion and the mixing speed of the fuel and air are increased. For the second chamber, the wall slope at the spray droppoint is decreased to reduce the resistance of the spray development in the injection direction, and the mixing of the fuel and air is promoted, and the distribution range of the spay is expanded. Based on the above ideas, three kinds of combustion chambers are designed for 240 diesel engine, and a new type of high disturbed injector nozzle is designed to improve the spray characteristic. The combustion characteristics of the new type of combustion chambers and high disturbed injector nozzle are identified preliminarily on the testbed. The experiment results show that when the big guided angle combustion chamber is adopted, the NOx emissions in E3 cycle conditions reduces by 10%, and the fuel consumptions decrease by 2% to 6% at some cycle points. When the small guided angle combustion chamber is adopted, the NOx emissions in E3 cycle conditions reduces by 8%, and the fuel economy in E3 cycle conditions improves by 3%. By using the high-disturbed injector nozzle, the NOx emissions of E3 cycle conditions reduce by 19%, and the fuel consumption rate reduces by 3%. By using the high-disturbed injector nozzle and the big guided angle combustion chamber, the spray volume increased ratio and the collision guiding effects are both enchanced. It provides the larger fuel distribution range at the peripheral, radial and axial directions, then the local high temperature regions decrease, and the NOx emissions of E3 cycle reduces by 32%, but the fuel consumption rates raise slightly at some E3 cycle points. By using the high-disturbed injector nozzle and the small guided angle combustion chamber, the spray volume increased ratio increases, and the resistance of the spray development decreases. It provides the larger fuel distribution range at the spray injection direction. The results show that the NOx emissions of E3 cycle reduce by 15%, and the fuel consumption reduces by 4%.For the exploration of the in-cylinder NOx reduction technologies to meet the requirement of IMO Tier III regulation, the verified simulation models of 1-D engine working cycle by AVL BOOST and 3-D CFD in-cylinder combustion process by CONVERGE are conducted to predict the engine performance with different intake valve closing timings, geometric compression ratios, injection timings, and EGR rates. The numerical results show that, the in-cylinder temperature of compression stroke reduces in Miller cycle schemes, then NOx emissions can be controlled, but when the intake closing timing of the baseline engine (T58?A) is too much lower, the premixed combustion heat release rate increases rapidly, and the rough working of diesel engine tends to be occured. When T580?A is lower than the stagnation range (300K-310K), there is even the miss-fire phenomenon. The NOx emissions can be reduced by 41% when the M50 shceme is adopted, and the stability proformance of diesel engine can be ensured. The reduction range increases to above 50% by enhancing geometric compression ratio and combining with longer injection timing delay. On this basis, the EGR technology with constant intake pressure and intake air is simulated respectively. The results show that the NOx emissions reduction can be enhanced to 80% than the baseline, when 20%-30% EGR rate is adopted. When the intake pressure keeps the same as the baseline, the fuel sonsumption rate increases 4%; when the intake air keeps the same as the baseline by enhancing the intake pressure, the fuel consumption rate improves slightly. Based on it, the soot emission can be reduced by 7% when the geometric compression ratio is increase properly, then the NOx, and the soot emission can be reduced simultaneously, and the fuel consumption rate is improved by 4%.Based on the quick-mixing combustion experimental study and the in-cylinder NOx purification technologies exploration, the "quick-mixing combustion and low NOx combustion" concept is proposed. Firstly, the quick mixing combustion system is formed by the big guided angle combustion chamber or the small guided angle combustion chamber and the high-disturbed injector nozzle. Secondly, a technical route combined moderate Miller cycle and moderate EGR with higher geometric compression ratio and longer injection timing delay is proposed. Based on the above researches,5 NOx emission reducing schemes are analyzed in E3 cycle, and the results show that quick-mixing combustion and low NOx combustion makes the NOx emissions meet the IMO Tier ? regulation, and keeps the baseline engine power, and also achieves a better fuel economy. |
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