Condition Diagnostic of Two-stroke Low-speed Diesel Engine using Acoustic Emission Sensor

1. Introduction

The main engine of the propulsion system is the most important part of a ship. It determines the productive capacity and the safety level of the ship in any weather condition at sea. Thus, the engine’s state is a primary concern and all information regarding its operation should be monitored. In particular, the firing state of the cylinder is critical to normal operation. Currently, most diesel engines use pressure and thermal sensors to obtain this information. In this report, a new technique is introduced to evaluate the firing state of a cylinder as well as the engine’s status.

Recently, acoustic emission techniques have been used in a wide range of health monitoring applications. These approaches are commonly used to investigate the state of the components of diesel engines including injectors^(1,2), valves⁽⁴⁾, piston wear⁽⁵⁾, cylinder pressure^(1,3), gearboxes^(6,8,9), and bearings^(7,10,11). In an experiment conducted by Tian Ran Lin on a 4-stroke, 4-cylinder diesel generator engine, various loads were used to determine the fault state of the injectors⁽¹⁾. The investigation of fault detection of injectors was conducted by Ahmad on a 4-stroke, 6-cylinder engine⁽²⁾ and the applicability of AES for frequencies lower than 50 kHz was demonstrated. AES signals are also used to indirectly determine the pressure in cylinders or the mechanical condition of structures in large 2-stroke low-speed diesel engines, as well as small 4-stroke diesel engines⁽³⁾. The range of the measured vibration frequency depends on the frequency range of the experimental equipment, for example, 20 kHz⁽¹⁾, 30 kHz⁽³⁾, or 50 kHz⁽²⁾.

In this study, an experiment on a large 2-stroke, 6-cylinder marine diesel engine was conducted to investigate the vibrations associated with a diesel engine for frequencies in the range of 50 kHz to 800 kHz for two operation modes.

2. Experimental Overview

The experimental system consisted of three main blocks; namely, the AES, medium transmitter, and the monitoring and analyzing S/W. This S/W was specially designed to measure high-frequency vibrations. This data was then analyzed and compared with a database to partially determine the state of the diesel engine. The overall block diagram of this system is shown in Fig. 1.

Fig. 1

Schematic diagram for high-frequency vibration measurements

2.1 Acoustic Emission Sensor

The Kistler AES that was used in the experients consisted of two parts. One component is the sensor that detects high-frequency vibrations, and the other is a transmitter box that converts the vibrations detected by the sensor into voltage or current output signals⁽¹²⁾. This AES was able to measure vibrations in the frequency range of 50 kHz to 1 MHz with high sensitivity, and is displayed in Fig. 2.

Fig. 2

Image of Kistler AE sensor

Table 1 shows the specification of the AES. Due to the extremely low current output, this device requires a conveyor to convert the signal from a current to a voltage, or simply to increase the power of the signal⁽¹³⁾. The structure of the transmitter box is illustrated in Fig. 3.

Table 1

Specifications of the kistler AE sensor

Fig. 3

Diagram of transmitter box

This device has 2 inputs and 4 output blocks. A direct current (DC) voltage of +24 V is required at the first input (J7) and the sensor signal connects to the remaining input (J4). The current input signal from the sensor is amplified and converted to a voltage by the RF Amplifier block with an adjustable gain. This gain can be adjusted by changing the position of two jumps (J5, J6) on the transmitter board. A bandpass filter block then regulates the output signal and only vibrations in a specific frequency range are allowed to be transmitted, as shown in Fig. 4. There are four types of output signals that include a raw AES output, the root mean square (RMS) output signal with a voltage, the RMS output with a current from 4 mA to 20 mA integrated zero adjustment element, and the alarm output with a contact type for the warning applications.

Fig. 4

Frequency range of the transmitter box

2.2 Medium Transmitter

The medium transmitter is one of the most important equipment employed to convert an analog signal from a sensor to a digital signal for transmission to a computer via a cable or peripheral component interconnection (PCI) slot. There are numerous approaches available for the collection and conversion of the signals generated from sensors including the use of NI equipment, a PCI card, an analog to digital converters (ADC), etc. The combination of this device and the sensors determines the range as well as the amplitude of the measured frequency.

Fig. 5 shows the interface of the high-sampling rate NI USB 6366 BNC that was used in this application. The device specifications are summarized in the user manual⁽¹⁴⁾ and the functionality includes analog input (AI), analog output (AO), digital input (DI), digital output (DO), general counters, frequency generator, etc. In the experiment, only the AI was used to connect with the Kistler AES to measure high-frequency vibrations. The computer receives high-frequency vibration signals from this device via a USB cable. The characteristics of the AI channel for this device are shown in Table 2.

Fig. 5

NI USB – 6366 BNC

Table 2

Characteristics of AI channels of NI USB 6366 BNC

According to this table, two outstanding issues appear during the experiment. Firstly, the programmer should change the input value depending on the actual value of the input signal because the default range is 1 V. The maximum working voltage is 11 V. If the input signal has a threshold that is higher than this value, the AI channels cannot measure this signal. The second issue is the overvoltage protection of all the AI channels that prevent damage to the equipment.

2.3 Monitoring and Analysis Software

The engine vibration analysis and monitoring system (EVAMOS (Brand name of Dynamic Lab. of Mokpo Maritime University) AEC) S/W with AES using the C# programming language was designed to measure and monitor high-frequency vibrations of the engine. The main function of EVAMOS AEC S/W was to handshake with the NI USB 6366 BNC to receive, monitor, and record vibration signals. In addition, the S/W was flexible in choosing the sampling rate and the number of channels, changing the name of the channels, and analyzing the vibration signal. Based on the aforementioned requirements, the S/W consisted of three setup windows; namely, analyzer window, input channels window, and recording setup window. The real-time monitoring window is the main working interface and the analysis function was performed after the data recording process was completed. The flowchart of the program for measurement and analysis is illustrated in Fig. 6.

Fig. 6

Flowchart of the measurement program

Firstly, the vibration data was measured and displayed on the monitoring interface, while also being recorded to analyze and estimate the state of the engine. Secondly, the data was analyzed using the Fast Fourier transform (FFT) method to determine the amplitude and frequency of each of the high-frequency vibrations. Finally, waterfall and contour plots were created based on the analyzed data.

3. Experimental Results

3.1 Implementation of the System

This investigation was conducted on cylinder no. 6 of the 158 k Deadweight Tonnage (DWT) oil tanker shown in Fig. 7. As displayed in Fig. 8, cylinder no. 6 is the nearest one to the turning wheel that is tested under the misfiring conditions during the sea trial process.

Fig. 7

Image of the 168 k oil tanker

Fig. 8

Image of the main engine of the 158 k oil tanker

The vibration of the main engine was caused by the pressure of the combustion gases in addition to the reciprocating inertial force of the engine and the hydrodynamic force of the propeller. The specifications of the propulsion system of this vessel are summarized in Table 3.

Table 3

Specification of the propulsion system of the oil tanker

In this implementation, the Kistler AES was used to measure the high-frequency vibration in the horizontal direction of the cylinder, and a tachometer was utilized to measure the speed of the diesel engine. The arrangement of the sensors is shown in Fig. 9 and the real position is shown in Fig. 10.

Fig. 9

Arrangement of the sensors for vibration measurements

Fig. 10

Position of acoustic emission sensor

3.2 Vibration Results

Measurements were performed using the same RPM under the normal firing and misfiring conditions of cylinder no. 6. The chosen sampling rate of the NI USB 6366 BNC was 2.00 MS/s, and the first channel was the default connection to the tachometer signal to determine the engine speed. Although the maximum continuous rating of the diesel engine was 73 r/min, the experiment could not be conducted at such a high revolution rate because the limit of engine speed under the misfiring conditions is only 60 r/min.

Comparing the data between the normal firing conditions and the misfiring conditions facilitate the determination of the different outstanding characteristics. Based on the measurements, it is also possible to determine the difference between the TB on and TB off conditions. A database can then be created to estimate the mechanical condition of the diesel engine.

Figures 12, 13 show the experiment results for the misfiring conditions. Figures 14, 15 show the results for the normal firing conditions for TB on, while Figs. 16, 17 show the results for the normal firing conditions for TB off.

Fig. 11

Laptop computer running EVAMOS AEC S/W

Fig. 12

Result for vibration in the transverse direction (waterfall) for the body of cylinder no. 6 under misfiring conditions with TB on

Fig. 13

Result for vibration in the transverse direction (contour) for the body of cylinder no. 6 under misfiring conditions with TB on

Fig. 14

Result for vibration in the transverse direction (waterfall) for the body of cylinder no. 6 in the normal firing conditions with TB on

Fig. 15

Result for vibration in the transverse direction (contour) for the body of cylinder no. 6 in the normal firing conditions with TB on

Fig. 16

Result for vibration in the transverse direction (waterfall) for the body of cylinder no. 6 in the normal firing conditions with TB off

Fig. 17

Result for vibration in the transverse direction (contour) for the body of cylinder no. 6 in the normal firing conditions with TB off

Fig. 18

Result for vibration in the transverse direction (waterfall) for the body of cylinder no. 6 in the misfiring conditions with TB on

From the results shown in Figs. 12 ~ 15, it is evident that the vibrations that occur under the normal firing conditions have a larger amplitude that than the vibrations that occur under the misfiring conditions.

The vibration of the normal firing mode for TB on was similar to the case of TB off in the frequency range of 50 kHz to 300 kHz. However, they exhibited a small different for very high frequencies (approximately 350 kHz and 750 kHz). A different was not observed in the misfiring and normal firing conditions at these frequencies with TB on. The vibrations in the frequency range of 300 kHz and 350 kHz were smaller for the misfiring mode compared to the vibrations associated with the normal firing modes, as shown in Fig. 19.

Fig. 19

Result for vibration in the transverse direction (waterfall) for the body of cylinder no. 6 in the normal firing conditions with TB on

Based on Fig. 19, it is evident that the amplitude of the vibration at 350 kHz for TB on is higher than that for TB off. At approximately 750 kHz, there are two pulses for TB on but only one pulse for TB off, as shown in Fig. 20.

Fig. 20

Result for vibration in the transverse direction (waterfall) for the body of cylinder no. 6 in the normal firing conditions with TB off

4. Conclusion

This study investigated the possibility of using an AE technique for high-frequency vibration measurement and other auxiliary equipment for application in a marine engine under the real-working conditions of a ship. The results revealed that the Kistler AES was consistent in terms of measuring vibrations in the high-frequency range from 50 kHz to 800 kHz. The equipment used functioned as expected under the real-working conditions of the investigated marine vessel. They were not affected by the ambient conditions of the ship. Although the temperature of the cylinder body was almost 110 °C, the AES still can still operate normally over a long period. The vibration measurements were conducted at 2 MS/s via 8 channels using EVAMOS AEC S/W. This implies that this system can be used to performed measurements on a marine diesel engine with a maximum of 7 cylinders (1 channel for the Tachometer signal and 7 channels for the 7 cylinders).

The vibration characteristics for 1/80 octave analysis differed for every operation mode and mechanical condition of the engine. According to the measured results, the difference between the vibration characteristics of the conditions was not similar for the complete range of investigated frequencies. In the case of the operation modes, only the range of frequencies lower than 350 kHz was useful for the detection of abnormalities, whereas higher frequencies were not useful. In contrast, for the mechanical modes, only frequencies at approximately 350 kHz and 750 kHz were useful, while the remaining data were not useful. Choosing a suitable range of frequencies in specific cases will further improve the speed of the processing stage. The output results obtained based on the Octave band frequency method have a smaller size compared to the raw data. These results proved that the AE technique is suitable for the investigation of high-frequency vibration characteristics and wave propagation in marine diesel engines for the real-working conditions of a ship.

A comparison of the vibrations associated with the instance value and stored data can assist in the detection of the firing state of a cylinder as well as the mechanical conditions (e.g. TB on-off) of the engine. These vibration characteristics can be used to build a database to facilitate the detection and diagnosis of abnormal operation in diesel engines by applying advanced methods such as machine learning.

Acknowledgments

This study was funded by the Ministry of Trade, Industry, and Energy of KOREA. In addition, it was performed under a government project entitled “Establishment of authorization test and certification center for medium/high-speed marine engines and related equipment”.

References

• Lin, T. R., 2011, Condition Monitoring and Diagnosis of Injector Faults in a Diesel Engine Using In-cylinder Pressure and Acoustic Emission Techniques, Proceeding of 14^th Asia Pacific Vibration Conference, pp. 454~463.
• Ahmad, T. A., 2019, Fault Detection of Injectors in Diesel Engines Using Vibration Time-frequency Analysis, Applied Acoustics, Vol. 143, pp. 48~58. [https://doi.org/10.1016/j.apacoust.2018.09.002]
• El-Ghamry, M., Steel, J. A., Reuben, R. L. and Fog, T. L., 2005, Indirect Measurement of Cylinder Pressure from Diesel Engines Using Acoustic Emission, Mechanical Systems and Signal Processing, Vol. 19, No. 4, pp. 751~765. [https://doi.org/10.1016/j.ymssp.2004.09.004]
• Elamin, F., Fan, Y., Gu, F. and Ball, A, 2010, Diesel Engine Valve Clearance Detection Using Acoustic Emission, Advances in Mechanical Engineering, Vol. 2010, No. 495741, p. 7. [https://doi.org/10.1155/2010/495741]
• Douglas, R. M., Steel, J. A. and Reuben, R. L., 2006, A Study of the Tribological Behavior of Piston Ring/cylinder Liner Interaction in Diesel Engines Using Acoustic Emission, Tribology International, Vol. 39, Vol. 12, pp. 1634~1642. [https://doi.org/10.1016/j.triboint.2006.01.005]
• Loutas, T. H., Sotiriades, G., Kalaitzoglou, I. and Kostopoulos, V., 2009, Condition Monitoring of a Single-stage Gearbox with Artificially Induced Gear Cracks Utilizing On-line Vibration and Acoustic Emission Measurements, Applied Acoustics, Vol. 70, pp. 1148~1159. [https://doi.org/10.1016/j.apacoust.2009.04.007]
• Elasha, F., Greaves, M., Mba, D. and Addali, A., 2015, Application of Acoustic Emission in Diagnostic of Bearing Faults within a Helicopter Gearbox, Proceedings of The Fourth International Conference on Through-life Engineering Services, pp. 30~36. [https://doi.org/10.1016/j.procir.2015.08.042]
• Tan, C. K., Irving, P. and Mba, D., 2007, A Comparative Experimental Study on the Diagnostic and Prognostic Capabilities of Acoustics Emission, Vibration and Spectrometric Oil Analysis for Spur Gears, Mechanical Systems and Signal Processing, Vol. 21, No. 1, pp. 208~233. [https://doi.org/10.1016/j.ymssp.2005.09.015]
• Sharma, R. B. and Parey, A., 2017, Condition Monitoring of Gearbox Using Experimental Investigation of Acoustic Emission Technique, Procedia Engineering, Vol. 173, pp. 1575~1579. [https://doi.org/10.1016/j.proeng.2016.12.250]
• Elforjani, M. and Mba, D., 2010, Accelerated Natural Fault Diagnosis in Low Speed Bearings with Acoustic Emission, Engineering Fracture Mechanics, Vol. 77, No. 1, pp. 112~127. [https://doi.org/10.1016/j.engfracmech.2009.09.016]
• Elasha, F., Greaves, M., Mba, D. and Fang, D., 2017, A Comparative Study of the Effectiveness of Vibration and Acoustic Emission in Diagnosing a Defective Bearing in a Planetry Gearbox, Applied Acoustics, Vol. 115, pp. 181~195. [https://doi.org/10.1016/j.apacoust.2016.07.026]
• Kistler, Acoustic Emission Sensor – for High Temperature & Hazardous, Type 8152.
• Kistler, Acoustic Emission – Piezotron Coupler, Type 5125C.
• National Equipment, Device Specifications NI 6366, Series Data Acquisition 2 MS/s/ch, 8AI, 24 DIO, 2AO.