Acoustic emission mapping of diesel engines for spatially located time series—Part II: Spatial reconstitution

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Abstract

In this study the characteristics of AE wave transmission around and through the cylinder head of a small four-stroke fuel injection diesel engine are investigated. Two sets of measurements have been carried out, one with simulated sources at various points on the cylinder head and one with the engine running, using a nine-sensor array. Propagation of AE waves from the simulated sources to the array sensor positions indicates attenuation factors of around 8.5. It is important to be able to estimate source–sensor distance accurately and this has been done using a threshold technique to identify the arrival time of a low frequency high amplitude wave at the sensors, then to deduce the wave speed from that information and hence calculate the source–sensor distance (transmission path length). Engine running tests were carried out and analysis focused on reconstitution of injector and exhaust valve opening events using signal mapping techniques. For injector events, the attenuation factors were similar to those measured from the simulated sources. For exhaust valve-opening events the attenuation factors varied between 5 and 7 with relatively poor correlation. This difference is thought to be due to variation in the actual source position during valve opening because the exhaust valve actuation involves a cam, push rod and rocker design. The results can be used to reconstitute the timing and amplitude of the AE emitted at the source which enables much more focussed monitoring of specific components and processes identified in the signals.

Introduction

Acoustic emission (AE) can be used as a non-intrusive method to monitor the mechanical events, components and processes in diesel engines [1], [2], [3], [4], [5] and can be used to diagnose faults and running conditions. AE has been used successfully to detect exhaust valve leakage [1], fuel injection behaviour [2] and various aspects of the combustion process [3], [4]. One major advantage of using AE is that, at the high carrier frequencies from 0.1 to 1 MHz, there is a very high signal-to-noise ratio. There are a number of AE sources in engine operation such as valve impacts, fluid flow/injection, combustion and sliding and rolling contact, which it is very useful to be able to monitor non-intrusively. However, these sources can be complex, generating a number of wave modes at various positions on the engine. The propagation of the AE waves through an object like an engine is also very complex, with wave attenuation/dispersion, transmission/reflection needing to be considered. To optimise monitoring and diagnostic capabilities it is necessary to be able to identify each source of the AE and extract information specific to that source, a process which is described here as spatial reconstitution of the acquired AE time series. To enable any kind of spatial reconstitution, a sensor array and information about the mechanical operation (timing) of the events is needed. In order to choose a suitable sensor array, studies of AE wave propagation in engines must be carried out.

In Part I of this study [6] a simplified model of AE wave propagation through various objects was developed. The same modelling techniques are applied here where, firstly, simulated sources are used to generate AE at various locations on the cylinder head where actual sources are expected and data is acquired from an array of sensors. The simplified model of AE wave propagation [6] is then applied to the data to find appropriate attenuation coefficients and path lengths. Then, under normal engine operating conditions, AE data are acquired, signals are mapped, events identified and a spatial reconstitution technique is developed. The sensor array is limited to positions around the cylinder head in this work.

In general, AE wave propagation in a real structure attenuates along a transmission path. Other studies [6], [7], [8] have identified internal damping, reflection, refraction, mode conversion and diffraction as factors which affect AE transmission/attenuation. Other work, mostly relating to structural integrity monitoring (e.g. [9], [10]), has identified important wave components. Nivesrangsan et al. [6] have presented a preliminary study of AE wave transmission on relatively simple cast iron objects, including an engine base, a surface table, a narrow strip and a cylinder block. They found that there are two dominating modes of AE wave propagation (as did Holford and Carter [10]); a low amplitude, fast wave, which may itself consist of one or a number of modes, precedes the main signal (slow wave), which carries the peak amplitude of the signal. The AE wave propagates through these cast iron structures mainly in two frequency bands around 100–200 kHz (low frequency) and 300–350 kHz (high frequency). In Part I [6], attenuation of AE energy and the effects of geometry, transmission and reflection along each transmission path was considered and, rather than attempt to describe the wave mathematically (which is probably intractable), a simplified exponential attenuation relationship was proposed. Good correlation was found between this model and AE wave propagation and this successfully confirmed estimates of transmission path length using a time of flight technique [10]. The results of that work together with mapping techniques developed by El-Ghamry et al. [4] are applied here to spatially reconstitute AE source signals associated with fuel injection and exhaust valve opening using a sensor array on the cylinder head. Source location using time of flight and energy based techniques will be considered in Part III of this work [11].

Section snippets

Acoustic emission energy

In order to study the propagation of AE around the cylinder head using the techniques developed in Part I [6], it is necessary to determine the energy in the acquired AE signal. For simulated sources, this is a simple matter of determining the energy in a specific time or frequency window for a single event. For signals acquired from a running engine, it is possible to map the engine cycle onto the AE time series and identify individual events within a cycle. It is then possible to select

Experiment

Two sets of experiments were carried out on the cylinder head of a Perkins 4-stroke, four cylinder, 74 kW diesel engine using a sensor array illustrated in Fig. 1. For both sets of experiment a four-channel PCI-6115, National Instruments data acquisition (DAQ) card was used. For simulated source tests four Physical Acoustics, micro-80D broadband AE sensors with preamplifier (including band-pass filter between 0.1 and 1 MHz) were used with one positioned beside the source and three positioned on

Simulated source results

The data acquired from the simulated sources was used to determine the effective source–sensor distance and the attenuation factors, which could then be compared with results found in Part I [6] and used for the engine running test results in the next section. It is important to be able to estimate reliably the source–sensor distance because this will then be used to study the attenuation of the AE energy with distance and calculate the attenuation factors (using Eq. (4)). A computer generated

Engine running tests

Examples of the AE signals acquired from one cycle of the running engine are shown in Fig. 4 against crank angle for each of the sensor positions P1–P9. The mapping of the mechanical events onto crank angle is shown along the bottom of Fig. 4. As can be seen, the largest AE events are associated with injection (INJ) around top dead centre (TDC) for each cylinder and these are observed at all sensor positions to a greater or lesser extent. Smaller amplitude events can also be seen for exhaust

Crank angle—frequency analysis

Fig. 9 shows an example contour plot of crank angle against frequency for receiver sensor at P1 on the cylinder head for the engine running at 800 rpm with no load. The frequency spectrum has been determined for the time period associated with each degree of crank angle. The maximum value of each spectrum is then used to normalise the data so that spectral energy can be compared across the crank angle domain despite the variations in signal energy with crank angle. It can be seen that there are

Spatial reconstitution

Fig. 12 shows normalised raw AE signals for various sensor positions where the source was emitted from injector 4 at low-speed and no load. Each signal was normalised using its maximum amplitude and, as an example, the results shown for sensors 4, 5 and 6 were acquired for the same engine cycle. The energy content shown for each signal (Ei,i=1,2,,9) was determined using Eq. (2), and the arrival time shown for each sensor (Ti,i=1,2,,9) was determined (referred to zero at the sensor closest to

Conclusion

The propagation of AE signals has been studied using a multiple AE sensor array on the cylinder head of a diesel engine. Two sets of tests were carried out where simulated source data and engine running data were collected. AE events associated with fuel injection showed attenuation factors of around 8–9 and good correlation with the exponential absorption model. The measured attenuation factors of exhaust valve opening events were more variable, between 5 and 7, with slightly poorer

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