Troels Dyhr Pedersen.indd - Solid Mechanics

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- 50 - - The SPL of the cylinder pressure, the acceleration level and the acoustic SPL can be calculated in short intervals and presented as in figure 15: Figure 15: Time domain representation of the SPL of the cylinder pressure, the engine acceleration level and the acoustic sound pressure level Here the SPL is expressed in dB. The red line represents the cylinder SPL which is seen to increase by more than 50 dB after combustion. The acceleration level (white line) increase by a similar dB value and is generally coherent with the cylinder pressure SPL. The acoustic signal (green line) also increases but remains at a higher level. As mentioned, the acoustic signal originates from the cylinder block resonance which is not properly measured by the accelerometer, and these vibrations are present much longer than the higher frequencies which tend do be attenuated fast. The time domain representation is useful to show the signal amplitude in time and how it is attenuated. It is however not possible to determine which frequencies are present in the signal. This may instead be visualized in with the frequency domain representation. 11.6.4 Frequency domain representation To evaluate the frequency content in a discretely sampled signal, a DFT algorithm is used. The specific algorithm commonly applied is called Fast Fourier Transform (FFT). In the analysis, the frequency range from 0 Hz to the Nyquist frequency is split up into a number of equally spaced intervals. The number of intervals is equal to the sampling frequency divided by the number of samples, so that the frequency resolution becomes: f s Δ f = N s Usually the number of samples Ns is required to be a power of two (2, 4, 8, 16 etc). It is desired to have a short sample length in which the signal can be considered stationary. The sampling frequency is however very high, which implies that a large number of samples is also required to obtain an acceptable frequency resolution. To limit the sample
- 51 - - time interval while maintaining a high frequency resolution, a short length of samples can be extended with a tail of zero values. This is referred to as zero padding. The zero values do not affect the analysis, so this approach is very useful when a limited sample length must be analyzed. The FFT algorithm calculates the amplitude of the frequencies in the signal, given by the frequency resolution f. The output from the analysis consists of the frequencies and their respective amplitudes, as well as phasing information. A graphic representation is then used to identify the dominating frequencies. Figure 16: FFT representation of cylinder pressure, engine acceleration and acoustic noise The particular set of data used in figure 16 illustrates that although a range of high frequencies are dominating the cylinder pressure signal as well as the accelerometer signal, the majority of the sound pressure is caused by frequencies from 1-3 kHz, which is the engine resonance frequencies. The disadvantage of the frequency domain representation is that the frequency content changes in time. The graphs can therefore represent either a small time step of the time domain accurately, or a larger time step less accurately, based on an average value. The only way to represent the development in frequency content is to use a frequency spectrum analysis. 11.6.5 Frequency spectrum analysis in the time domain A frequency spectrum chart is suitable for studying the frequencies in the time domain. The frequencies can be displayed in the time domain by splitting up the signal into small overlapping intervals and apply the FFT procedure on each interval. The output of the analysis is an intensity chart with three dimensions: time on the x-axis, frequency on the y-axis and amplitude on the z-axis. The plot can be made in three dimensions, but the zvalues are here defined by a color scale to make the plot two-dimensional. To create these charts, a 256 sample length windowed section of the high pass filtered signal is analyzed by FFT. This is done in an interval progressing one CAD with each iteration.
Page 1: Homogeneous Charge Compression Igni
Page 4 and 5: - 2 - - Table of contents 1 FOREWOR
Page 6 and 7: - 4 - - 11.5.3 Filtering of sample
Page 8 and 9: - 6 - - 1 Foreword The work describ
Page 10 and 11: 3 Résumé - 8 - - This thesis is b
Page 12 and 13: 4 Resumé (dansk) - 10 - - Denne af
Page 14 and 15: 5 Introduction - 12 - - 5.1 About H
Page 16 and 17: - 14 - - 5.3 Scope of investigation
Page 18 and 19: 6 The basics of HCCI combustion - 1
Page 20 and 21: - 18 - - the cylinder. In HCCI comb
Page 22 and 23: 7 Combustion of DME - 20 - - 7.1 Co
Page 24 and 25: - 22 - - 8 Description of experimen
Page 26 and 27: - 24 - - 8.1.3 Piston modifications
Page 28 and 29: - 26 - - combustion event is often
Page 30 and 31: - 28 - - however not possible, sinc
Page 32 and 33: - 30 - - (excess air ratio 3) of ai
Page 34 and 35: dQ dt = h ⋅ A ⋅ - 32 - - ( T
Page 36 and 37: - 34 - - In addition, many reaction
Page 38 and 39: - 36 - - Arrows in the scheme indic
Page 40 and 41: - 38 - - formaldehyde (CH2O) and fo
Page 42 and 43: Figure 12: Termination of the low t
Page 44 and 45: - 42 - - The non-carbon radical act
Page 46 and 47: 10.7 List of species Radicals of hy
Page 48 and 49: - 46 - - If a frequency analysis of
Page 50 and 51: - 48 - - 11.5.3 Filtering of sample
Page 54 and 55: Figure 17: Intensity chart for cyli
Page 56 and 57: - 54 - - 11.7.3 Axial modes Axial w
Page 58 and 59: - 56 - - A case was set up to make
Page 60 and 61: 12 Simulation of detonation phenome
Page 62 and 63: - 60 - - provided more ideal condit
Page 64 and 65: - 62 - - positive in the x- directi
Page 66 and 67: 3,0 x 10 6 2,5 x 10 6 2,0 x 10 6 1,
Page 68 and 69: - 66 - - A detailed drawing of the
Page 70 and 71: - 68 - - In the expressions above,
Page 72 and 73: ( 21) ( 22) - 70 - - The mach numbe
Page 74 and 75: ( 25) - 72 - - Δt v = u ⋅ Δx wh
Page 76 and 77: - 74 - - Figure 29: Development in
Page 78 and 79: - 76 - - 12.5 Observations of deton
Page 80 and 81: - 78 - - On figure 34, the individu
Page 82 and 83: - 80 - - • The engine load obtain
Page 84 and 85: - 82 - - 15. Takayugi Tsuchiya, Yos
Page 86 and 87: - 84 - - 15 Appendix A: Reduced sch
Page 88 and 89: 16 Appendix B: Detonation model in
Page 90 and 91: - 88 - - 17 Paper I: The Effect of
Page 92 and 93: - 90 - - 19 Paper III: Reducing HCC
Page 94 and 95: DESCRIPTION OF THE EXPERIMENT OBJEC
Page 96 and 97: Engine modification To obtain a low
Page 98 and 99: Pressure [Mpa] 6 5 4 3 2 1 lambda 2
Page 100 and 101: Pressure [Mpa] 6 5 4 3 2 1 lambda 4
Page 102 and 103:
At lambda 4, IMEP peaks at a compre
Page 104:
CONCLUSION • It was possible to a
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close to the lean limit. HC is comp
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Reactions do however not stop entir
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METHANOL TEST PROCEDURE In the firs
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Page 8 of 22 0 -5 -10 -15 -20 -25
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With EGR there was a notable change
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Page 12 of 22 9 8 7 6 5 4 3 2 MEOH
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Page 14 of 22 250 200 150 100 50 40
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EMISSIONS The emissions in table 6
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Page 18 of 22 15 12 9 6 3 0 1000 RP
Page 125 and 126:
REFERENCES 1. Mingfa Yao: An Invest
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PM: Particulate matter THC: Total H
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Coupling of pressure waves and reac
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chambers which often have this flat
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The two piston crowns with chambers
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Page 8 of 21 Figure 10: Steel plate
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Page 10 of 21 16.7 kW @ 3000 rpm 17
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v v, ref T T ref with T and Tref be
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dB higher than the 8 cylindrical ch
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Page 16 of 21 Flat chamber 7 BTDC 3
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Page 18 of 21 diesel type chamber 3
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CONCLUSION HCCI combustion generate
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DTU Mechanical Engineering Section
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