Generally, the noise of the cylinder pressure during combustion is transmitted to the engine body through the piston, the connecting rod, the main bearing, and the vibration of the surface of the internal combustion engine caused by the cylinder head or the like is called combustion noise. When the diesel engine is working, the combustion chamber burns at a high temperature and high pressure in a very short time, and the energy is rapidly released. This sharp pressure rise excites the structural vibration of the engine, thereby radiating noise. It is clear that the cylinder pressure is the force of combustion noise, so the combustion noise is a function of the cylinder pressure and is also related to the stiffness of the engine structure, the acoustic radiation effect on the engine surface and the transfer characteristics of the surrounding air.
The combustion process of a diesel engine is usually divided into four phases - a fire delay period, an emergency combustion period, a slow combustion period, and a post-combustion period. The study of the combustion process of diesel engines generally uses a pressure curve (P-?) analysis method. Figure 1 is a typical cylinder pressure curve.
Both the cylinder pressure and the combustion noise are periodic phenomena, and the frequency of the cylinder pressure is branched into the frequency component of the combustion noise. Fourier analysis of both cylinder pressure and combustion noise shows that the sound pressure level has a significant dependence on the cylinder pressure level in the higher frequency band. Regardless of the analysis from the pressure or spectrogram, it is clear that the key to reducing combustion noise is to control the rate of increase in combustion pressure. In other words, the diesel engine should be selected for a gentle working process. The rate of pressure rise depends on the ignition delay and the fuel injection law. Therefore, the general method of reducing combustion noise has two aspects:
1 Increase the compression ratio, appropriately delay the injection advance angle, and use a fuel with a high cetane number. Such measures are used to shorten the fire delay period.
2 Reduce the initial fuel injection rate, use the intake vortex to reduce the amount of combustible mixture before the fire.