Coherence function in the evaluation of combustion engine Technical condition

TOMASZ KAŁACZYēSKI

University of Technology and Life Sciences

Summary

The actual tendency to develop methods and techniques to diagnose vehicles is arousing a growing interest and demand for technical condition analysis in explora-tion condiexplora-tions. The problem with the design and exploitaexplora-tion of modern machines and technical devices is to recognize the technical condition in operational condi-tions. To define the correct dynamic model and choose the best research method is the question of the industry field, and this paper aims to explore this topic.

Keywords: coherence function, vibration process, symptoms, combustion engine 1. Introduction

This research relates to vehicle diagnostics and focuses around the following investigative prob-lems in the peculiarity of piston combustion engines in the vibration process [5]:

– the analysis of vibration processes for sorted temporary sections connected with ignition period for the control of the process of burning in the engine;

– the investigation of the influence of changing factors on the value of diagnostic parameters of the vibration process – the conditions of investigation, exploitation and construction fac-tors relating to investigation vibrodiagnostics of the engine,

– the temporary and ghostly selection of vibration in the process to control the condition of mechanical elements of the engine;

2. Properties of the coherence function

The function of coherence defined as follows is the measure of cohesion between two vibrodi-agnostic processes x(t) and y(t) of previously outlined properties [2]:

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f

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f

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f

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f

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Counting ghostly thicknesses for the example above, in the function of the thickness the source process at(t) and a well-known transfer function estimation H1(f), H2(f), we receive:

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f

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f

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f

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H

f

H

f

=

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And now, independently of the transfer function estimation, the coherence function always accepts value one if only the signals x(t) and y(t) come from the same sources. For a larger number of sources than one or with the moment of the appearance of any damage, the coherence function

is non-negative, that is always smaller than the unity. Special attention is also drawn to the sensi-bility of coherence on the location of damages.

One can affirm that the coherence function as a measure of cohesion between two signals has good diagnostic properties because the appearance of a signal of new damage violates cohesion; that is why the coherence function diminishes the previous one.

This function can be used to the inference about rooms in new rolling bearings on the basis of the coefficient of coherence between extorted strength and vibrations received on the external track. The ability of the coherence function to discriminate the rooms can be completely sufficient to the aims of the diagnostics [4].

The use of the coherence function in the diagnostics does not restrain to the cases of straight lines from one passage, but it is equally useful is in the occurrence of propagation vibrations or noise.

The coherence function is the local measure of the similarity processes, so the essential diag-nostic properties in reference to the objects will be named linear and stationary in the field of dynamic time. Yet greater possibilities of uses of diagnostic coherence functions can be found in non-stationary or non-linear mechanical arrangements.

The newest example from this last field is detecting the rooms in the steams of kinetic mecha-nisms or machines [3].

3. Research method

Research relates to the changing value of coherence coefition for chosen characteristic fre-quencies and the value of ghostly thicknesses (the correlation of vibration processes) and transfer function estimation (the function of the passage), together with the estimation of the size of the field under curve in the programme ROTOM.. This method includes the realization of measure-ments received in the figure of temporary courses, and then analyses for the aim of the obtainment of exchanged measures with the programme SIBI.

In the analysis it is possible to use tools to enable the causal – consecutive inference and mul-tidimensional glance at the folded object e.g. the combustion engine (regress, OPTIMUM, SVD, ….).

Fig. 1. The research method

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4. Results of research

The combustion engine No. 138C.2.048 with 1.4l. swept capacity, power 55 kW / 75 KM, generally applied in Fiat Uno 75i.e was an investigation object (see Figure 2).

Fig. 2. The object of investigations

On the object of research present in the laboratory – Combustion Engines in the Department of Working Machines and Vehicles, at the University of Technology and Life Sciences in Bydgoszcz – 33 conditions were simulated, which resulted in the damages of candles and injectors on the individual cylinders of engine and a combination of these damages.

During the experimental research with the aim of the obtainment, the following guidelines in the exploitation engine were applied:

- 830 r.p.m was executed for the rotatory speed of the engine

- for neutral gear – temperature of the trunk of the engine was 71 degrees - the dynamic state of the engine is described as 30 measuring files

- investigations - were executed with the utilization of two measuring channels, with the fulfilment of the condition of Fourier transformation.

The measuring track consisted of:

- two sensors ICP Accelerometers, model HTM

- two cables of standard series 002 among sensors in the entry of the card In 4, In 1.

The following signals in the figure were measured and processed: TIME – the time response of the signal VIBDAQ.

However, taking into consideration the large quantity of the data and the format of the publica-tion, only an introduction of the possibilities of the programme SIBI will be presented (see figure 3):

 importing UNV format (see figure 4),

 building a matrix of symptoms (see figure 5) used to get an estimate of the vibration proc-ess,

 singular value decomposition,  optimum procedure,

 Input – output relationship functions (see figure 6) is the most important part of this pro-gram. In this application we get data which we need for coherence function analysis.

Fig. 4. Importing UNV format window

Fig. 6. Input – Output relationship functions window Fig. 5. Building a matrix of symptoms window

We use SIBI to get a visualization of:  time response (see figure 7),  spectrum of signal (see figure 8),  coherence function (see figure 9),  correlation (see figure 10),

 transfer function estimation (see figure 11)

The verification of value changes. The measurement of the vibration process in dependence on changeability is possible, which gives the basis for the inference thanks to the programme SIBI. It is possible to do an evaluation of combustion engine technical condition with the use of diagnos-tic property coherence function.

Fig. 7. Time response of two signals

5. Conclusion

Presented methods of diagnosing the condition of the folded object combustion engines are based on the measurements of value steering currents. The vibrodiagnostics of combustion engines uses analysis vibration processes generated in combustion engines as an alternative method. Vibrodiagnostics enables the opinion of the engine condition, using the sensibility estimation of the vibration process.

The estimation of the combustion engine condition with sparkle ignition for the help of the re-search of vibration measures is possible thanks to the utilization to the properties of the coherence function.

Fig. 9. Coherence function for a chosen area

Fig. 10. Correlation between two signals for a chosen area

The implementation of software for the following needs: the acquisition of vibration proc-esses, their processing, statistical inference and visualization facilitates investigations and result analysis.

Bibliography

1. Bartelmus W., Zastosowanie niektórych estymatorów statystycznych sygnału drganiowego jako kryterium oceny stanu zazĊbienia, Politechnika ĝląska Gliwice 1979.

2. Cempel C.: Podstawy wibroakustycznej diagnostyki maszyn, WNT Warszawa 1982. 3. Pawłow B.W.: Badania diagnostyczne w technice. WNT, Warszawa 1976.

4. ĩółtowski B.: Elementy dynamiki maszyn, Bydgoszcz 2002 5. ĩółtowski B.: Badania dynamiki maszyn, Bydgoszcz 2002.

FUNKCJA KOHERENCJI W OCENIE STANU SILNIKA SPALANIA Streszczenie

Trendy rozwojowe nauki i techniki stwarzają zapotrzebowanie na Ğrodki diagno-styczne umoĪliwiające nieinwazyjne monitorowanie procesów eksploatacyjnych ma-szyn. Konstruowanie i eksploatacja współczesnych maszyn wymaga rozpoznania problematyki identyfikacji stanu dynamicznego obiektu poprzez opis modelowy oraz metod badawczych tych modeli.

Poszukiwania nieinwazyjnego monitorowania procesów eksploatacyjnych ma-szyn wytyczają kierunki rozwoju metod diagnostycznych, które nie ingerują w strukturĊ i pracĊ maszyny. Jedną z tych metod jest diagnostyka drganiowa wyko-rzystująca drgania, jako Ĩródło informacji.

Słowa kluczowe: funkcja koherencji, proces drganiowy, symptomy, silnik spalinowy

*This paper is a part of WND-POIG.01.03.01-00-212/09 project. Tomasz KałaczyĔski

University of Technology and Life Sciences Faculty of Mechanical Engineering