Main diesel engine is the most important part of propulsion shafting system on the ship. It decides the productive and safety of the ship in any weather condition on the sea. Therefore, engine's state is always concerned and all information of engine during operation should be monitored. One of the necessary information always concerned by many researchers as well as designers is the engine vibration. There are some types of engine vibration caused by many activities and process such as mechanical vibration, sound vibration and acoustic emission (AE) signal. Based on the engine vibration analyzing results, the situation and quality of engine can be determined. Depending on the substance of each vibration's type, the suitable sensors as well as analysis methods are applied to measure and analyze the vibration.
There are some types of conventional method normally applied to measure the engine vibration, such as the torsional vibration is normally measured by strain gauge while axial vibration is typically obtained by gap sensor or Linear variable differential transformer (LVDT); the velocity sensor and accelerator sensor are applied to measure the engine's body vibration; the magnetic pickup sensor and encoder are employed to measure the vibration of angular velocity; sound vibration is detected by microphone. However, the AE signal is the energy emitted under the elastic waves in wide frequency range from 0 to 1MHz and propagates in solid material. So, it is impossible to apply these conventional methods to measure the acoustic emission signal. In this case, the exclusive sensor, called AE sensor, is utilized to detect the AE signal.
Nowadays, the condition diagnostic technologies of general machineries and their structures using AE sensor are widely used. Many studies were carried out to prove that AE sensor is suitable and powerful tool to measure the high frequency vibration (AE signal) of reciprocating and rotating machineries. Especially, acoustic emission technique (AET) has been widely employed to evaluate the state of diesel engine's components and focused on the AE signal at cylinder area. AE signals at cylinder area are radiated from many sources such as the friction and wear due to the slide between piston rings and cylinder liners, cylinder pressure, flow through the orifice, combustion, gas-sealing force applied on piston rings, blow-by and the valve activities. Based on the AE signal obtained from experiments, the characteristics and the quality of some mechanical components as well as processes can be determined. Due to the complicated of the generating AE signal by these sources, they should be individually investigated. There were many researchers focused their researches on the frequency range less than 50 kHz to determine the characteristics of diesel's components such as injector, valve, piston wear, etc. Moreover, these researches were almost applied on the four-stroke high-speed diesel engine with the experimental condition. Therefore, the lack of these researches on the frequency range higher than 50 kHz for these sources on two-stroke low-speed marine diesel engine in real-working condition.
Recently, some manufactures succeed in developing AE sensors up to 900 kHz. Additionally, a new analog to digital converter (ADC) device with sampling rate supported up to 2 MS/s and 8 analog input channels has been developed by National Instrument company. So, this is a good chance to develop an acoustic emission measuring system with frequency range up to 900 kHz. This system can be applied for diesel engines which have maximum 7-cylinder (1 channel to measure shaft speed and 7 channels for 7 cylinders) or other measuring systems. And, of course, to complete this system, it is necessary to design a data acquisition and analysis software for AE measurement with high frequency up to 1MHz. So, in this thesis, a software has been firstly developed by C# language for monitoring simultaneously 8 channels using National Instrument (NI) hardware with 2 MS/s per channel. Then, it was applied in some experiments on low-speed 2-stroke, 6-cylinders, 7-cylinders and 11-cylinders marine diesel engine to investigate the AE signal in frequency range from 20 kHz to 900 kHz real-working condition on cylinder body as well as injector location.
Engine/Rotor Vibration Analysis and Monitoring System using Acoustic Emission Sensor (EVAMOS AE) software has been designed to combine with NI USB - 6366 BNC and AE sensors to measure high frequency vibration. In this study, every experiment of each case study is carried out at same speeds to make sure that other working conditions are not changed. Base on raw data file, researcher also designs some additional functions to help analyst handle these data file. It can cut file based on tachometer signal with one or few revolutions. Additionally, this software supports a common analysis method in frequency domain named Fast Fourier Transform (FFT).
With 2-stroke low-speed marine diesel engine, to investigate vibration in one engine cycle, signal of one revolution of every operation modes will be cut to analyze and compare. Then, some analyzing theories will be applied to find out the results. The study results revealed that the AE signal generated by some sources with different frequency ranges can be detected by this measurement. The signal emitted from combustion process is higher and clearer than remain processes. Six basis theories are suitable and powerful to analyze AE signal in high frequency range. Some processes as well as activities could be mapped on angular - frequency domain as friction, combustion, injector process and exhaust valve opening.
These analyzed and compared results can be used to build the database as a library in computer in order to serve detecting and diagnosing abnormal operation in diesel engine. It also can be used as database for researches in the future as machine learning.