Effects of Combustion Characteristics on Combustion Noise in Diesel Engines

Abstract

The demand of diesel engines for passenger cars has increased due to their higher thermal efficiency, however, diesel engines have weaknesses

namely, the engine noise and the vibration. Especially, the combustion noise is significantly louder than that of gasoline engines due to their different combustion modes. In diesel engines, some fuel, which are already mixed with air, are rapidly burned after the ignition delay in the premixed combustion phase and the components of the engine block experience a rapid pressure rise, which is known as the main source of combustion noise. In this study, the relation between combustion characteristics and in-cylinder pressure excitation was investigated. In addition, the combustion characteristics of lower level excitation were studied. Finally, the closed control system using combustion noise index was developed to reduce the combustion noise. In this study, a 1.6 liter diesel engine was used. The in-cylinder pressure, exhaust emissions and smoke were measured. Rapid prototype controller was used for the real time control system. Furthermore, a vehicle, which was equipped with the same engine, was used for bench test as well as the vehicle test. The main source of combustion noise is from the in-cylinder pressure variation. The pressure is also affected by the change in heat release rate. Thus, in this study, the relation between the shape of heat release rate and the pressure excitation characteristics were studied. Through an engine experiment, it is hard to change the shape of heat release rate as we wish. Thus, the heat release rate was simulated by Wiebe functions. The 3 Wiebe functions are generally used to describe the diesel combustion. The pressure curve was reproduced by using the simulated heat release rate. Frequency analysis, FFT and 1/3 octave band analysis, were conducted to evaluate the excitation of the pressure curve. When more fuel was burned rapidly, the excitation level increased. When the combustion duration increased or the fuel quantity decreased, the combustion noise decreased. However, the start of combustion did not affect the combustion noise much. Pilot injection, which was used to reduce the combustion noise, decreased the combustion excitation of main combustion injection. However, the combustion of pilot injection considerably affected the total excitation. The shape of heat release rate for reduction of combustion noise was investigated. When the combustion duration became long and the fuel was burned moderately, the combustion noise decreased. In addition, the split injection of the main injection effectively reduced the excitation. When 30% fuel was burned early, the rest of fuel continuously burned and the combustion noise was dramatically reduced with the same IMEP. The injection strategies for the engine noise reduction was examined via engine bench tests. When injection parameters, SOI, swirl, injection pressure and EGR rate, were changed, the heat release shape and excitation characteristics were studied. The combustion noise could not be reduced from changing them without an emission deterioration. To prevent a formation of more emission and to reduce the combustion noise, early pilot injection strategy was applied. When the pilot injection quantity increased, the main injection fuel was burned smoothly. However, an increase of pilot injection quantity was anticipated to produce more smoke. To solve this dilemma, the early injection concept was applied to pilot injection because the combustion noise can be reduced without an emission increase through an early pilot injection. Finally, a closed loop control system using CNI was developed for the reduction of combustion noise. The controller, which can measure the in-cylinder pressure and calculate CNI in real time, were developed using NI-PXI and LabVIEW. The combustion controller, which can modify the injection strategies, was developed using ES1000 and ASCET. SOI, injection pressure and pilot quantity were used as control parameters. The system applied to a vehicle. The current CNI level could be controlled to follow the target value in a transient operation. At that time, 1.2~2 kHz frequency of the cabin noise in the vehicle was reduced down to 4 dBA.