Characterization of engine's combustion-vibration using diesel and biodiesel fuel blends by time-frequency methods: A case study
Introduction
Biodiesel fuel is derived from vegetable oils, animal fats, and waste edible oils and is produced by transesterification with methanol or ethanol in the presence of a catalyst. In fact, biodiesel is a methyl or ethyl ester [1], [2] and can be blended up to 20% with diesel fuel. In internal-combustion engines, the presence of oxygen in biodiesel fuel reduces the emissions of hydrocarbons, carbon monoxide, and other pollution. However, results obtained from the reported experiments in 2002 showed that the biodiesel had increased 2% of nitrogen oxide (NOx) emissions for B20 [3].
Various reasons such as combustion and knocking are the main causes of engine vibrations and noise. Compared with spark engines, knocking becomes more rigorous with increasing ignition delay in diesel engines [4]. In this case, knocking creates annoying noises and vibrations, burns the piston crown, and damages to engine parts [5].
Progress in combustion of diesel engines depends on the injection features, such as the number of injections and timings, quantity of fuel, and mean injection pressure. Besides, varying the injection features affects the engine block vibrations [6]. Signal transform methods have been employed to identify knocking from the data captured by accelerometers, which are in time domain, frequency domain, TFR domain [7], [8], and wavelet transform [9], [10]. TFR transforms are among the novel and practical methods for the analysis of structural health monitoring in vibrating systems. Signal analysis methods have multiple applications in solving vibration problems and diagnosis in rotating machines [11]. For example, Short Term Fourier Transform (STFT) can be used to detect different sources of vibration in an engine. Moreover, this method has been used for identifying normal and abnormal combustion-related knocking in a cylinder block. Using the Fast Fourier Transform (FFT) and FFT-based methods, such as estimated power spectral density, is unsuitable for identifying non-stationary events, including cross-terms and systems with rapid changes in time and frequency. However, these methods are fast and have the capability of filtering noises [12]. Considering the literature review, it is necessary to explore the updated procedure.
Previous studies have shown the effect of diesel-biodiesel fuel blends on engine combustion, however, engine knocking has not been investigated carefully in internal diesel engines so far. In this paper, cylinder block vibrations of two types of diesel engines were used in terms of combustion characteristics, knocking, and engine faults.
Section snippets
Experimental set-up and data acquisition
Two four-stroke diesel engines, a single-cylinder engine (ND130-DT95A), and a six-cylinder engine (Perkins 1006-6) were used for the experiments (Table 1). In these engines, the rotational speed is controlled and stabilized by a governor. These engines are usually used for agricultural tractors. The cylinder firing order is 4-2-6-3-5-1 for the six-cylinder engine. For both engines, valves timing are, IVO 25° before BDC, EVC 28° after TDC, IVC 55° aBDC, and EVO 120° aTDC. A dynamometer was
Knock and fault detection
Fig. 2a shows the inner cylinder pressure versus crankshaft angle curve for cylinder 2, which recorded simultaneously with the vibration measurement at same run. This cylinder has not any knocking at 1400 rpm (six cylinder engine under full load). With the same conditions, Fig. 2b displays the similar curve for cylinder 4 that is with knocking. Fuel injection and combustion process in these cylinders is as follows: with the start of injection (22° before TDC) there is a delay in the combustion
Conclusions
- i.
In the present study, the time-frequency analysis (STFT and MSC) was used to evaluate the vibration signals derived from combustion of fuels, D100, B20, B40, B60, B80, and B100 for a single-cylinder and six-cylinder engines. Tests conducted for both full-load and no-load modes to recognize the engine knocking and faults for all the fuel blends.
- ii.
Results of this study showed that STFT and Morlet scalogram (MSC) are suitable methods for detecting faults/knocks in an engine. It was found that, in
Acknowledgments
The authors appreciate Tarbiat Modares University, Gorgan University of Agricultural Science and Natural Resources, and Iranian Fuel Conservation Company (IFCO) for their facilities.
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