Simulation On The Combustion Of High Pressure Common Rail Diesel With Pilot Injection

Due to the increasingly serious energy and emissions problem, improving the economy, diesel engine emissions and noise are the main topics. Diesel emissions and noise are impacted on many factors; the injection program has a very important impact on emissions. The program resulted in different cylinder pressure, temperature, heat release rate, etc. These parameters have a direct bearing on engine emissions and noise. Extensive studies show that pilot injection is one of the effective methods of controlling diesel engine emissions and combustion noise. The diesel we used has high-pressure common rail injection system that can control fuel injection pressure and injection quantity flexibly. Therefore, the accuracy of pilot injection controlling can be achieved.CFD simulation software for the use of internal combustion engine has the following advantages : the cost is small; Short design cycle; obtain parameters that difficult for the bed experiments, such as cylinder detailed concentration, velocity, pressure, temperature and other micro- fields. CFD simulation software STAR-CD was used in this study. Based on CA6DL-37 diesel 1400 r/min, 50% load the different injection programs were simulated. Fuel , NO and Soot concentrations, temperature and velocity fields were analysed , the main factors affecting the engine emissions and noise were found ,and the results of different pilot injection programs working on the combustion and emissions were obtained .CA6DL-37 diesel 1400 r/min, 50% operating condition load without pilot injection, referred to the original operating condition, Numerical simulated its combustion process, and then analyzed micro-fields of simulation results. NO are created mainly in three phases:①Premixed combustion stage, The NO was mainly found in high temperature flame of oil-beam roots lateral;②diffusion combustion stage, then the main NO was formed in the high temperature area of diffusion flame lateral;③late stage of combustion, The NO was formed in the bottom of combustion chamber and the gap. The peak of Soot's formation was in the region of wall near 364°CA. When the combustion is nearly finished, a lot of Soot gathered in the chamber bottom. From the macroscopical analysis we can find the heat release rate and pressure rise rate of the initial combustion was too high , it was detrimental to the noise decline.According to the original operating condition'injection as well as the features of common rail fuel injection system. Under the prerequisite of ensuring the center, based on CA6DL-37 diesel 1400 r/min. 50% load operating condition without pilot injection, we optimized the injection programs with 5% 10% 25% and 50% pilot injection and the spray interval of 5°CA 10°CA and 15°CA .Then we used the same model with the original operating condition to simulate these new injection programs, and got the following conclusions:Pilot injection of a small amount, for example, 5% pilot injection operating conditions, the pilot injection fuel didn't fired before the main injection started. The formation of the rarefied mixture, shortened the main injection ignition delay period,then reduced the first peak of heat release rate and pressure rise rate, thus lead a reduction of the NO production and combustion noise. As the main injection was put off, burning was also postponed, period of high temperature was shortened, so reduced the NO. Moreover, due to the late main injection, in the end of combustion excessive amount of fuel would bring Soot up. Because the amount of pilot injection fuel was so small, then increased spray interval between the pilot injection and the main injection made little influence on combustion and emissions, 5-10 operating condition only increased the initial combustion's pressure rise rate and heat release rate.The pilot injection amount of 10—5 and 10—10 operating conditions was so large, the fuel formed the mixture that can be ignited very early, so the combustion was earlier than the original operating condition, and so was the formation of NO, but only a little. Large amount of pilot injection led the mixture thicker, the main injection entrained the mixture so deduced oxygen concentration, slowed the pace of NO formation. The same as mentioned above, postponed the main injection led to a corresponding Soot increase. Widened spray interval between the pilot injection and the main impacted the NO formation. The injection time of 10—10 operating condition was earlier than 10—5 operating condition, it had enough time to mix, so the combustion area was larger and the temperature was lower. The formation of NO was a little later than that of 10—5 operating condition, and so the final amount. Moreover the initial pressure rise rate and heat release rate of 10—10 operating condition were higher than those of 10—5 operating condition.25% and 50% pilot injection fuel can combust fiercely before the main injection, which made the initial heat release rate and pressure rise rate higher than the original operating condition, led the combustion noise increased. The high temperature of the initial combustion caused large amount of NO and Soot generate. Because the pilot injection reduced the concentration of oxygen, and the main injection was so late, the pace of NO formation slower, but the amount of Soot larger than the small pilot injection operating condition. Widened spray interval of the pilot injection and the main injection increased the initial pressure rise rate and heat release rate, reduced the NO, but on increased spray interval of the 50% pilot injection operating condition, the Soot will increase intensely.Comprehensively compared the emissions and power loss of all operating conditions, 5% pilot injection operating condition was the optimal one, this operating condition can reduce the amount of NO and the combustion noise, also didn't bring a marked increase of Soot and power loss.In order to eliminate the influence of postponing the main injection, we adjusted the start moment of main injection to the start moment of the original operating condition. When adjusted the main injection start moment of 5—5operating condition to 12°CA BTDC, simulation results showed that the formation of NO and Soot changed a little compared to the original operating condition. And the initial pressure rise rate and the heat release rate reduced. So the adjustment made a certain sense of reducing the combustion noise. Adjusted the main injection start moment of10—5operating condition led a decline of the NO, but the reduced amount was smaller than the former 10—5operating condition, however the amount of Soot, initial pressure rise rate and the heat release rate were higher than those of original operating condition. So the adjustment had little practical significance.