Study On Combustion Noise With Direct Common Rail Diesel Engine

With the improvement of living level, requirement for the performance of vehicles is getting higher and higher. One of the most important issues is to reduce the vibration and lower the noise. Engines are the key parts of vehicles, and they are the main noise sources in vehicles. Under the increasingly stringent limits and regulations on vehicle noise, engine noise and vibration problem has become more and more important. Noise index is important in evaluating engine performance. Combustion noise contributes the major part of diesel engine overall noise. Pilot injection is effective in reducing combustion noise. For a diesel engine with high pressure common rail electronically-controlled direct injection system, pilot injection enables a very high injection pressure and excellent control performance, thus getting accurate control on the pilot injection.This thesis reviews the origin, propagation and control approaches of diesel engine noise. With a DENSO ECU-U2 high pressure CR system as research object, the relative issues, such as the principle and characteristics of electronically control technique, the design and developments of embedded software for electronically control system, as well as the control strategy, are studied. According to the diesel engine combustion characteristics and the requirements of emission regulations, a suitable control strategy and its implementation for high pressure CR system are proposed. The running conditions of diesel engines are classified. For each running condition, a corresponding control strategy, including injection quantity, injection timing and common rail pressure, is obtained, which helps to meet the demand of quick start and steady running of engine.A test rig is set up based on a CA6DE direct injection diesel engine equipped with ECD-U2. MAP calibration is accomplished using the online calibration software, combustion analysis software and data acquisition software. The precision, feasibility and reliability of software design are verified by the tests. The experiments indicate that CR injection system has a great potential to decrease the emission and improve the performance of direct injection diesel engines.The experiments on function matching of CA6DE diesel engine are conducted. The structural parameters of injector, the structural parameters and swirl ratio of combustion chamber are improved and optimized. Using the high pressure common rail electronically control system and corresponding measurement instruments, the fuel consumption and exhaust parameters are measured. The results show that a higher injection pressure, smaller injector core diameter, as well as a lower swirl ratio, a proper combustion chamber shape, and a delay in injection timing, can improve the mixing and combustion of gas-air mixture, reduce harmful exhaust, and improve the overall performance of economy, power and exhaust.The effects of single injection and pilot injection on combustion noise are studied experimentally. It is found that: proper pilot injection amount and the interval between pilot injection and major injection is effective in reducing the maximum pressure rise rate and high frequency pressure oscillation, and thus controlling combustion noise; the effect of pilot injection on reducing combustion noise is significant under low, medium speeds and low, medium load conditions. With the increase of engine speed and load, the effect decreases gradually.The effect of heat release rate on maximum combustion pressure, maximum pressure rise rate and high frequency pressure oscillation under certain speeds are analyzed from both the theoretical and experimental aspects. The variation of heat release rate peak with respect to high frequency pressure oscillation amplitude, under the condition of pilot injection, varying common rail pressure and injection timing parameters, and its effect on combustion noise are obtained.The identification and separation of diesel engine noise are studied based on independent component analysis and time-frequency analysis. First, the independent components in diesel engine noise under different experimental condition are separated using FastICA algorithm. Then, the separated independent components under different condition are analyzed in joint time-frequency domain by means of continuous Morlet wavelet transform, and the time-frequency feature of each independent component is extracted from the Morlet scalogram. Finally, the effect of running condition on combustion noise is analyzed in time domain, frequency domain, and joint time-frequency domain respectively. It is found that pilot injection improves the fuel combustion, thus decreases the combustion noise.