Troels Dyhr Pedersen.indd - Solid Mechanics

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- 42 - - The non-carbon radical activity utilizes the H atom from reaction 333 to produce new HO2 and OH radicals, as well as converting the H2O2 intermediate that was stored during the LTR reactions: # 26 : H + O2 → HO2 # 50 : 2HO2 → H 2O2 + O2 # 51: H O → 2OH 2 2 The second step is the final oxidation of CO to CO2. # 40 : CO OH → CO + H + 2 The non-carbon radicals act differently in the second step, since now only OH is in demand. The H from reaction 40 is used by reaction 8 to form OH and O. The O is then used in reaction 10 to produce two more OH. Hence a total of three OH are produced for every H supplied. HO2, which was produced in reaction 46, combines with roughly a third of the OH in a terminating reaction. # 8 : # 10 : # 49 : H + O HO 2 2 → O + OH O + H O → 2OH 2 + OH → H O + O 2 2 The overproduction of OH radicals ensures that the rate of CO conversion is mainly limited by the temperature. Any CO existing after the combustion will therefore be due to quenching or crevice volumes, or simply a low combustion temperature. Only a very small concentration of CO is produced due to dissociation of CO2, since the temperature is not high enough for dissociation to be significant. 10.6 Manipulating low temperature reactions 10.6.1 Effect of methanol on low temperature reactions The main purpose of using methanol is to obtain a higher octane rating of the fuel, obtained mainly by increasing the ignition delay of the fuel. In other cases DME has been used as an ignition improver for methanol or other fuels in DI CI engines. In an HCCI engine both fuels are premixed with the inlet air. The principal role of the methanol is to function as an inhibitor of the LTR heat release. When methanol constitutes about 30 % of the total fuel mass, the LTR heat release is almost eliminated. The inhibition of the LTR reactions is due to the consumption of OH radicals by reactions 78 and 110, which are the major paths of oxidation for methanol: # 78 : OH + OH → CH OH + H O CH3 2 2 CH3OH + OH → CH3O H 2 # 110 : + O
- 43 - - Both of the radicals formed in the reactions above react to form formaldehyde: # 83: CH 2OH + O2 → CH2O + HO2 # 40 : CH O M → CH O + H + M 3 + 2 The OH consumption by reactions 78 and 110 means that less DME reacts through reaction 274, and thus the chain propagating reaction is reduced. The advantage of reducing the LTR is that the HTR is delayed proportionally with the LTR reduction. The amount of methanol can thus be used to control the combustion phasing. The effect of the dual fuel combination with methanol was demonstrated in cooperation with NTSEL in September 2008. Paper II in the appendix describes the outcome of the experiment. It was found that with about 30 % (mass) methanol added to the intake the combustion could be delayed to shortly after TDC, which improved the indicated efficiency and reduced the pressure rise rate. Furthermore, a change in the combustion towards less knocking could be observed from the engine noise. 10.6.2 Effect of EGR The use of exhaust gas recirculation (EGR) in diesel engine combustion was introduced to limit the formation of NO by reducing the combustion temperature. By reusing exhaust gas the specific heat capacity of the gas is increased due to the increase in CO2 and H2O concentrations. This reduces the temperature rise significantly. The specific heat ratio is however reduced, which reduces the amount of piston work. There is a limit to how much EGR may be used in diesel combustion, as the oxygen concentration is decreased with increasing EGR. This reduces the rate of reaction in the diffusion flames and hence slows the combustion. A maximum of approx. 30 % vol. of EGR is normally regarded as the limit for diesel engines. Increasing the amount of EGR will result in excess production of particulate matter and inefficient combustion. In HCCI combustion, EGR may be used to control the heat release. By decreasing the specific heat ratio, the temperature during compression will also be significantly reduced due to the lower specific heat ratio. This results in a delayed LTR reaction, but it does not reduce the amount of heat released or radicals produced. The second effect of the EGR is that the oxygen concentration is reduced. It was demonstrated that the reaction rate during the last part of the HTR phase could be slowed down, when the initial oxygen concentration was near the stoichiometric limit. The benefit of EGR is therefore that it allows late timing of the combustion and a reduction of the LTR combustion. This means that detonation and engine knock may be avoided. It is however on the cost of efficiency, as the IMEP is reduced significantly. The effect of EGR was demonstrated in at NTSEL in September 2008. Paper II in the appendix describes the outcome of the experiments with EGR.
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 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 52 and 53: - 50 - - The SPL of the cylinder pr
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
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Pressure [Mpa] 6 5 4 3 2 1 lambda 4
Page 102 and 103:
At lambda 4, IMEP peaks at a compre
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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
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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
Page 150:
DTU Mechanical Engineering Section
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