Internal combustion engine noise is a complex noise composed of multiple sound sources. According to the nature of the noise, it can be divided into aerodynamic noise, mechanical noise, and combustion noise. The generation of these noises is related to the working process of the engine and the movement of the components. When analyzing the combustion process of the engine and the motion analysis of the valves, pistons and other components, the crankshaft angle is usually used as a parameter. The change law of these parameters with the crankshaft angle is studied, that is, the angle domain analysis. Therefore, the angle domain analysis of the vibration and noise signal can correlate the engine operating conditions with the vibration and noise problem, and conduct an in-depth analysis of the engine vibration and noise. In the early stage, the noise source of a single-cylinder gasoline generator set has been identified by the sound intensity method, and the main noise sources of the unit are exhaust noise, intake noise and cylinder head noise. This paper analyzes these three noise sources based on the angle domain, and determines the relationship between them and the combustion process and the valve movement process, and determines the factors that affect their size.

Test system and procedure

. 1 Test system

The test uses the SCADAMobile data acquisition front end of LMS company in Belgium, and simultaneously collects crankshaft angle signal, speed signal, cylinder pressure signal, intake valve and exhaust valve vibration, cylinder body vibration, exhaust noise, intake noise, and cylinder head noise. The main test quantities and sensors are shown in Table 1. Test using LMS company's Test.1ab software for signal acquisition, sampling frequency of 25.6kHz,F frequency resolution of lHz.

. 2 Test steps

Connect the test system, set the test parameters, and calibrate the sensitivity of each sensor; connect the encoder to the crankshaft, and accurately adjust the top dead center position signal: replace the original spark plug with a spark plug integrated cylinder pressure sensor; arrange the acceleration sensor according to the spatial position. In this test, the acceleration sensor is arranged on the intake and exhaust valve rocker arm to characterize the vibration of the intake and exhaust valves, and the cylinder vibration sensor is installed in the center of the cylinder plane; the sound pressure sensor is equipped with a hood. Reduce the influence of intake and exhaust flow and cooling air on the sensor. The sensor for measuring cylinder head noise is arranged at a distance of 1cm from the cylinder head, and the intake and exhaust noise are measured at a distance of 30cm from the intake and exhaust ports. The sensor is arranged at a position with an angle of 45 degrees to the axis of the intake and exhaust ports to reduce the influence of the intake and exhaust flow on the sensor. The test is carried out in a semi-anechoic laboratory. According to the requirements of EU noise standard, the power of the unit is set to 3.75kW, and the rotating speed is controlled by the generator set at 3 000 150r/arin. Wait for its stable work. When the oil temperature rises to the normal operating temperature, start the test. The collected time domain signal processing, according to the encoder output pulse signal (1./pulse), complete the time domain signal to the angle domain signal conversion, according to the top dead center position of the angle domain data alignment.

Test results and analysis

The time domain signal and encoder pulse signal are collected in the test. After software calculation, the time domain signal can be converted to the angle domain, and the cylinder pressure, intake and exhaust rocker arm vibration, cylinder body vibration, intake and exhaust noise, cylinder head noise, etc. The angle domain signal of the crankshaft angle change. Through the analysis of the angle domain signals of cylinder pressure and intake and exhaust rocker arm vibration, combined with the top dead center position signal and valve phase parameters, the working state of the engine in each working cycle can be obtained. Figure 1 shows the engine in two cycles (Within 4 weeks of engine operation). The working process of the engine.

Corresponding the cylinder vibration signal to each working process of the engine through the angle relationship, and obtaining the relationship between the cylinder vibration signal and the working state of the engine. As shown in Figure 2, it can be seen from the figure that the cylinder vibration is when the valve is opened and closed There is an obvious shock peak, and there is an obvious vibration peak when the cylinder pressure is maximum, indicating that the cylinder vibration is affected by the impact of combustion and valve opening and closing.

Correlate the intake and exhaust noise and cylinder head noise with the engine working state. The relationship between intake and exhaust noise, cylinder head noise and engine working state and cylinder block vibration is obtained. As shown in Figure 3, it can be seen that the body vibration has a large vibration peak after the maximum cylinder pressure point, but there is no combustion noise. The corresponding large noise peak, it can be seen that the relationship between intake and exhaust noise and cylinder head noise and combustion noise is not obvious. The exhaust noise is clearly related to the exhaust valve opening process. The maximum noise occurs during the overlap process of intake and exhaust: the intake noise and cylinder head noise are related to the opening process of the intake valve, and the peak noise is related to the peak vibration of the cylinder block. Therefore, the intake noise and cylinder head noise are related to the valve mechanism. The mechanical noise or aerodynamic noise generated by the machine has a greater relationship.

Conclusion

In this paper, through the angle domain analysis of gasoline generator noise, the noise is corresponding to the working process of the engine and the movement process of the mechanism, and the intake and exhaust noise and cylinder head noise are analyzed in depth, and the influencing factors closely related to these noises are obtained.

Through analysis, it is found that the cylinder vibration of this generator set is mainly affected by the excitation generated by combustion and the impact when the valve is opened and closed: the intake and exhaust noise and cylinder head noise have no obvious correlation with the combustion noise, and the combustion noise is not the main noise source: the exhaust noise is mainly related to the opening process of the exhaust valve; the intake noise and cylinder head noise are closely related to the mechanical noise or pneumatic noise generated by the air distribution mechanism.

The angle domain analysis can easily and effectively obtain the relationship between the vibration noise signal and the motion process of the mechanism. The vibration and noise of rotating machinery can be analyzed in depth.