Octane Blending and Oxidation Chemistry of Ethanol-Hydrocarbon Mixtures

Transcription

1 Octane Blending and Oxidation Chemistry of Ethanol-Hydrocarbon Mixtures Hao Yuan March 218 Submitted in total fulfilment of the requirements of the degree of Doctor of Philosophy Supervised by A/Prof. Yi Yang Co-Supervised by Prof. Michael Brear Department of Mechanical Engineering THE UNIVERSITY OF MELBOURNE Produced on archival quality paper

2 Copyright 218 Hao Yuan All rights reserved. No part of the publication may be reproduced in any form by print, photoprint, microfilm or any other means without written permission from the author.

3 Abstract The strong anti-knock property of ethanol makes it a preferred blending component for gasoline to improve spark ignition (SI) engine performance. Despite its widespread use, understanding several, particular aspects of ethanol s interaction with different components of gasoline is still lacking. This work therefore performs the following three studies to investigate the interactions among ethanol and hydrocarbon fuels. First, a method for correlating octane numbers is developed for toluene reference fuels (TRFs) blended with ethanol. This method combines linear regression and exhaustive (or brute-force) searching for optimal correlations. The proposed correlations reproduce the measured RON and MON with a maximum absolute error smaller than two octane numbers. Despite the empirical nature, the correlations demonstrate the significance of linear by mole blending rules for TRF fuels and provide insights on the chemical interactions between ethanol and different hydrocarbons. The work of the optimal octane number correlations has been published in Fuel [Yuan et al., Fuel, 188 (217), p.48]. Second, a five-component gasoline surrogate is developed to emulate the octane blending behaviours of gasoline and ethanol. The surrogate contains iso-pentane, n-pentane, cyclohexane, 1- hexene, and 1,2,4-trimethylbenzene and is developed using extensive Cooperative Fuel Research (CFR) engine testing. The formulated surrogate captures the synergistic RON blending behavior between the target gasoline and ethanol over the entire blending range, with a hydrocarbon composition similar to the target fuel. Lastly, a Pressurised Flow Reactor (PFR) experimental study is carried out to study oxidation chemistry of a fuel matrix including neat fuels, binaries, gasoline surrogates, and gasoline surrogates/ethanol mixtures. The measured species profiles are simulated with published kinetic models. The result indicates that further investigations on toluene and its interaction chemistries with other compounds are needed for understanding the oxidation of surrogate fuels. iii

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5 Declaration This is to certify that: 1. the thesis comprises only my original work towards the PhD, 2. due acknowledgement has been made in the text to all other material used, 3. the thesis is fewer than 1, words in length, exclusive of tables, maps, bibliographies and appendices. Hao Yuan, March 218 v

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7 Acknowledgements I would like to express my gratitude the following people for their supports during my PhD study. This thesis would not have been possible without them. Yi Yang and Michael Brear (my academic supervisors) Their constructive advice and insightful guidance have been of tremendous help to me during the past four years of my PhD study. Zhongyuan Chen and Zhewen Lu Zhongyuan helped me with the CFR engine experiments and we worked together for the past four years. Zhewen helped to build the PFR and worked together with me on the PFR experiments. Tien Mun Foong and Al Knox Tien Mun offered me great help in starting the CFR engine experiment and modelling at the beginning of my PhD. Al provided technical supports in the CFR engine overhaul. James Anderson and Thomas Leone (research engineers at Ford) James and Thomas provided the data for the engine modelling work and offered valuable suggestions for my PhD research. Monica Pater Thanks Monica for her help with purchasing of research equipment and chemicals. My friends within the Thermodynamics Group Thank all of you for making the past four years such an enjoyable experience. My beloved family and girlfriend Foremost, I wish to extend thanks to my family for the continuous and unquestioning support over the last 3 years. I also would like to thank my girlfriend, Jie Jian, who shared all my sadness, happiness, failure and success during my PhD study and wish her every success in her own PhD project. vii

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9 To my mum ix

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11 Contents 1 Introduction Energy consumption and climate change Biofuels for transportation Increased biofuels production Ethanol as a fuel additive Octane blending of ethanol and hydrocarbons Literature Review Overview of Knock Essence of knock Characteristics of knock Anti-knock Characteristics of Ethanol/Hydrocarbon Blends Octane numbers of ethanol/hydrocarbon blends Charge cooling effect Chemistries of Ethanol and Hydrocarbons Experimental techniques Shock tube Rapid compression machine Well-stirred reactor Pressurised flow reactor Combustion chemistry of hydrocarbons and alcohol Combustion chemistry of alkanes Combustion chemistry of aromatics Combustion chemistry of ethanol Chemical Interactions of Fuel Mixtures Interactions between alkanes Interactions between PRF and toluene xi

12 Cross reactions via large radicals Cross reactions via radical pool Interactions between ethanol and hydrocarbons Summary and research questions Experimental Methods Overview CFR engine Overview The Structure of the CFR engine Methods for standard octane number tests Pressurised flow reactor Overview Reactor structure Mixer Sampling probe Experimental conditions Gas chromatography Overview of the gas chromatography Identification and quantification of species Optimal Octane Number Correlations for Toluene Reference Fuels (TRFs) Blended with Ethanol Introduction Algorithm for correlation development The Scheffé polynomial based correlation Linear regression Data for correlation development and validation Criterion for correlation development Procedures for optimal correlation development and validation Optimal correlations Optimal RON correlation for TRF/ethanol mixtures Development of the optimal correlation Validation of the optimal correlation Optimal MON correlation for TRF/ethanol mixtures xii

13 Development of the optimal correlation Validation of the optimal correlation Summary The Octane Numbers of Binary Mixtures and Gasoline Surrogates Blended with Ethanol Introduction The RONs of binary mixtures Binary mixtures of hydrocarbons Binary mixtures containing ethanol The RONs of gasoline surrogates blended with ethanol Detailed hydrocarbon analysis for the Australian production gasoline Strategy for emulating the octane number of the gasoline Comparison between production gasoline and its surrogates when blended with ethanol Summary Oxidation of Ethanol and Hydrocarbon Mixtures in a Flow Reactor Introduction Kinetic modelling approach Neat fuels Isooctane Ethanol Toluene Test mechanism Sensitivity analysis Updated toluene sub-mechanism Binary mixtures Ethanol and isooctane Toluene and isooctane Ethanol and toluene Gasoline surrogates PRF TRF Gasoline surrogates/ethanol mixtures PRF91 and ethanol xiii

14 6.7.2 TRF91-3 and ethanol Comparison of fuel reactivities Summary Conclusions and Recommendations for Future Research Conclusions Recommendations for future research References 127 A Octane number data used for optimal correlation development 151 B Liquid volume based correlations 155 C Modelling of Trace Knock in a Modern SI Engine Fuelled by Ethanol and Gasoline Blends 157 C.1 Introduction C.2 Numerical methods C.3 Formulation of gasoline surrogates C.4 NO sub-model C.5 GT-Power modelling C.5.1 Full flow model C.5.2 Reverse run model C.6 Two-zone model of autoignition C.7 Modelling of trace knock C.7.1 Raw pressure data C.7.2 Approach for modelling trace knock C.7.3 Example of modelling approach C.8 Modelling results and discussion C.8.1 UFI engine results C Non-kinetic factors C Effect of ethanol content C.8.2 DI engine results C.8.3 The effect of NO C.9 Summary D The kinetic model for the flow reactor 18 xiv

15 List of Figures 1.1 The outlook for (a) energy consumption and (b) oil demand before 235 [1] Global biofuels production in the last ten years [5] Annual U.S. average ethanol content of finished gasoline from 21 to 216 [17] The pressure trace of a knocking cycle and its corresponding non-knocking cycle suppressed by tetraethyl lead [23] The Midgley and Boyd bouncing pin apparatus for knock detection [28] Image series for both non-knocking and knocking engine cycles [32] Measured (a) RONs and (b) MONs for the ethanol and gasoline blends Measured (a) RONs and (b) MONs for ethanol blended with isooctane, n-heptane and toluene Measured RON values for ethanol/gasoline blends under standard and modified conditions [13] CAD model of the combustion chamber [45] The comparison between overall and effective octane numbers [14] Separation of chemical octane and charge cooling effects on knock limit [11] Schematic of a shock tube/rapid compression machine Schematic of a well-stirred reactor [47] Structure of a pressurised flow reactor Simplified scheme for the primary mechanism of oxidation of alkanes at low temperatures [51] Simplified scheme for the oxidations of benzene and toluene [132] Measured MONs of toluene blended with isooctane [187] Comparisons of cool flame (open symbols) and autoignition delay times (filled symbols) of neat isooctane and isooctane/toluene mixture [19] The system of the CFR engine The structure of the CFR engine xv

16 3.3 The piston head (a) before and (b) after overhaul Schematic of the Pressurised Flow Reactor system Structure of the Pressurised Flow Reactor The mixer a) cutaway view and b) orifices distribution CO 2 concentrations at 1 bar and 9 K in the flow reactor with air flow rate of 6.2 g/s and CO 2 flow rate of.71 g/s The sampling probe a) cutaway view b) three thermocouples Reactor temperature profiles for isooctane oxidation at 1 bar and 9 K with equivalence ratio of Gas Chromatography-21ATF plus from Shimadzu Flow chart of Gas Chromatography The temperature program for GC analysis The spectrum of isooctane oxidation at 9 mm under 9 K and 1 bar The spectrum of ethanol oxidation at 5 mm under 9 K and 1 bar The spectrum of toluene oxidation at 7 mm under 93 K and 1 bar The GC calibrations for (a) isooctane, (b) n-heptane, (c) toluene and (d) ethanol Data distribution on simplex lattices with filled circles representing development data and open ones for validation data Residual error between the development data and correlated RON from (a) linear bymole correlation, (b) five terms correlation, (c) six terms correlation and (d) seven terms correlation Variation of a) R 2 and b) MAE with optimal combination of terms in RON correlations of increasing length Residual error between the validation data and a) 7 and b) 8 term RON correlations on a molar basis Residual error between the development data and correlated MON from (a) linear bymole correlation, and (b) seven terms correlation Variation of a) R 2 and b) MAE with optimal combination of terms in MON correlations of increasing length Residual error between the validation data and a) 7 and b) 8 term MON correlations on a molar basis Measured RONs for Australian production gasoline, PRF91, and TRF91s blended with ethanol [9] xvi

17 5.2 RONs of isooctane blended with toluene on a a) volume basis and b) mole basis from this study. RONs of isooctane blended with ethylbenzene on a c) volume basis and d) mole basis from [28] RONs of n-heptane blended with toluene on a a) volume basis and b) mole basis from this study. RONs of n-heptane blended with ethylbenzene on a c) volume basis and d) mole basis from [28] RONs of cyclohexane blended with toluene on a a) volume basis and b) mole basis from this study. RONs of cyclopentane blended with ethylbenzene on a c) volume basis and d) mole basis from [28] RONs of 1-hexene blended with toluene on a a) volume basis and b) mole basis from this study. RONs of diisobutylene blended with ethylbenzene on a c) volume basis and d) mole basis from [28] RONs of cyclohexane blended with isooctane on a a) volume basis and b) mole basis from this study. RONs of methylcyclohexane blended with isooctane on a c) volume basis and d) mole basis from [28] RONs of 1-hexene blended with isooctane on a a) volume basis and b) mole basis from this study. RONs of 2-heptene blended with isooctane on a c) volume basis and d) mole basis from [28] RONs of cyclohexane and 1-hexene blended with ethanol on a a) volume basis and b) mole basis The comparisons of the gasoline/ethanol mixture and different gasoline surrogates blended with ethanol The measurements of (a) CO and CO2, and (b) isooctane from the neat isooctane oxidation experiment at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines), Andrae (dashed lines) and Atef et al. (dotted lines) using the corrected temperature profile from the three-thermocouple method (c) The measured intermediate species profiles from the neat isooctane oxidation experiment at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines), Andrae (dashed lines) and Atef et al. (dotted lines) The reaction pathways for IC4H8, XC7H14, and YC7H14 from the isooctane experiment at 9mm The reaction pathway for IC3H5CHO from the isooctane experiment at 9mm xvii

18 6.5 The measurements of (a) CO and CO2, and (b) ethanol from the neat ethanol oxidation at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines), Mittal et al. (dotted lines), Marinov (dashdot lines) and Andrae (dashed lines) using the corrected temperature profile from the three-thermocouple method (c) The measured intermediate species profiles from the neat ethanol oxidation at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines), Mittal et al. (dotted lines), Marinov (dashdot lines) and Andrae (dashed lines) The reaction pathway for CH3CHO from the ethanol experiment at 5mm The measurements of CO and toluene (a) from the neat toluene oxidation at 93 K and 1 bar, and the modelling results from Mehl et al. (solid lines), Yuan et al. (dashdot lines), Metcalfe et al. (dotted lines), Andrae (dashed lines), Zhang et al. (large dashed lines), and Pelucchi et al. (large dashdot lines) using the corrected temperature profile from the three-thermocouple method (b) The measured benzene profile from the neat toluene oxidation at 93 K and 1 bar, and the modelling results from Mehl et al. (solid line), Yuan et al. (dashdot line), Metcalfe et al. (dotted line), Andrae (dashed line), Zhang et al. (large dashed lines), and Pelucchi et al. (large dashdot lines) The brute-force sensitivity analysis of CO for the toluene oxidation at 93 K and 1 bar The measurements of CO and toluene from the neat toluene oxidation at 93 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dashed lines) The measured CO profiles of different binary mixtures (a-c) of isooctane and ethanol at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dashed lines, overlapping with the solid lines) using the corrected temperature profile from the three-thermocouple method (d) The measured CO profiles of different binary mixtures (a-c) of isooctane and toluene at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dashed lines) using the corrected temperature profile from the three-thermocouple method (d) The measured CO profiles of different binary mixtures (a-c) of ethanol and toluene at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dashed lines) using the corrected temperature profile from the three-thermocouple method (d) xviii

19 6.15 The measurements of (a) CO and CO2, and (b) isooctane and n-heptane from the PRF91 oxidation at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines), TestMech (dashed lines) and TestMech without chemical interactions between parent fuels or fuel-like species (dotted lines) using the corrected temperature profile from the three-thermocouple method (c) The measured intermediate species profiles from the PRF91 oxidation experiment at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dashed lines) The measured species profiles: (a) CO, CO2, and toluene, (b) isooctane and n-heptane from the oxidation of TRF91-3 at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines), TestMech (dashed lines) and TestMech without chemical interactions between parent fuels or fuel-like species (dotted lines) using the corrected temperature profile from the three-thermocouple method (c) The measured intermediate species profiles from the TRF91 oxidation experiment at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and Test- Mech(dashed lines) The measured species profiles: (a) CO, CO2 and ethanol, (b) isooctane and n-heptane from the oxidation of PRF91 blended with 73.7% ethanol by mole (5% by volume) at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dashed lines) using the corrected temperature profile from the three-thermocouple method (c) The measured intermediate species profiles from the oxidation of PRF91 blended with 73.7% ethanol by mole (5% by volume) at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dotted lines) The measured species profiles: (a) CO, isooctane, and n-heptane, (b) CO2, toluene, and ethanol from the oxidation of TRF91-3 blended with 87.7% ethanol by mole (75% by volume) at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dashed lines) using the corrected temperature profile from the threethermocouple method (c) The measured intermediate species profiles the oxidation of TRF91-3 blended with 87.7% ethanol by mole (75% by volume) at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dotted lines) xix

20 6.23 The measured species profiles: (a) CO, isooctane, and n-heptane, (b) CO2, toluene, and ethanol from the oxidation of TRF91-3 blended with 7.5% ethanol by mole (5% by volume) at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dashed lines) using the corrected temperature profile from the threethermocouple method (c) The measured intermediate species profiles the oxidation of TRF91-3 blended with 7.5% ethanol by mole (5% by volume) at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dotted lines) The measured species profiles: (a) CO, isooctane, and n-heptane, (b) CO2, ethanol, and toluene from the oxidation of TRF91-3 blended with 44.3% ethanol by mole (25% by volume) at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dashed lines) using the corrected temperature profile from the threethermocouple method (c) The measured intermediate species profiles the oxidation of TRF91-3 blended with 44.3% ethanol by mole (25% by volume) at 9 K and 1 bar, and the modelling results from Mehl et al. (solid lines) and TestMech (dotted lines) The CO and corrected temperature comparisons among isooctane and ethanol The CO and corrected temperature comparisons for two binary mixtures: ethanol plus isooctane and ethanol plus toluene The measured CO profiles of PRF91 and TRF91-3: (a) without ethanol and (c) with ethanol. The corrected temperature profiles of PRF91 and TRF91-3: (b) without ethanol and (d) with ethanol B.1 Residual error between the development data and correlated RON from (a) linear byvolume correlation, (b) seven terms correlation B.2 Residual error between the development data and correlated MON from (a) linear byvolume correlation, (b) seven terms correlation C.1 Comparison of the simulated ignition delay of the formulated gasoline surrogate (Table C.1) using the original LLNL model and the extended model containing NO in a constant volume reactor without NO present initially. Equivalence ratio = 1, 3bar, 7-12K161 C.2 Experimental CA5 vs. NMEP for ethanol/gasoline blends at 1:1 CR and 15 rpm with DI [11] C.3 The full flow GT-Power model for the single cylinder engine in [11] xx

21 C.4 The sensitivity analysis for the convection multiplier, to the cylinder wall temperature, T wall. Dashed lines represent the minimal RMSE at each wall temperature. RMSE values ( 1 4 ) are indicated by the numbers on the contours C.5 Unburned gas temperature profiles at different wall temperatures from the GT-Power reverse run C.6 Measured and simulated pressure traces from the GT-Power reverse run C.7 Raw and band pass filtered pressure traces (left), and power spectra from Fast Fourier Transform (FFT) analysis (right) for the most advanced pressure traces under standard knocking for isooctane in a CFR engine (a and b), and under trace knocking for E, UFI, NMEP=42kPa (c and d) and E5, UFI, NMEP=1324kPa (e and f) in a single-cylinder engine from the experimental study [11] C.8 Modelled results for E5 and UFI at NMEP=1324kPa with the MFB profile being swept 171 C.9 Comparison of measured and modelled spark timing for trace knock using different representative traces for E, E2 and E5. All cases are with UFI fueling C.1 Variation of modelled (using the 95th percentile advanced trace) and measured spark timing for trace knock for E, E2 and E C.11 MFB at autoignition for spark timings that are one degree earlier than the spark timing for trace knock C.12 Comparison of modelled and experimental spark timing for trace knock with DI and UFI for E5. Modelling results are from 95th percentile most advanced pressure traces. 177 C.13 Modelled spark timings for trace knock without residual NO compared to equivalent results with residual NO for different fuel mixtures and injection methods D.1 The comparison between the modelled results of the neat isooctane oxidation at 9 K and 1 bar in the PFR using Chemkin and the model developed in this study xxi

22 List of Tables 3.1 Operating conditions for the RON and MON measurements [25, 26] The composition of the dilute TEL [25, 26] Experimental conditions for the PFR study Response factors for gaseous fuels Response factors for intermediate species in liquid phase Terms of the Scheffé polynomial with four variables Coefficients of first order terms in the Scheffé polynomial RONs of cyclohexane and 1-hexene from different studies [2, 27] Interactions of binary mixtures on a mole basis Volume fractions of hydrocarbon groups in the Australian production gasoline Top ten most abundant species in iso-, n- and cyclo-paraffins Top ten most abundant species in aromatics and olefins Formulated gasoline surrogates Equivalence ratios of gasoline/ethanol and GS11/ethanol at standard knocking conditions The physical properties of the gasoline and the gasoline surrogates Test fuels and reaction mechanisms for modelling Experimental conditions for the PFR study Reaction changes to LLNL s toluene sub-mechanism A.1 Octane number data used for developing the optimal correlations for TRF/ethanol mixtures A.2 Octane number data used for validating the optimal correlations for TRF/ethanol mixtures C.1 Gasoline and surrogate fuel compositions (%vol) xxii

23 C.2 Gasoline and surrogate fuel properties C.3 Specifications for the single cylinder SI engine [11] C.4 Experimental conditions for modelled trace knocking cases C.5 Vibration mode frequencies from [266] C.6 Comparison between peak frequencies from the FFT result and the prediction C.7 Inputs to the two-zone modelling obtained from GT-Power for the UFI cases in Table C.4 and their corresponding 95 th percentile raw pressure traces C.8 Inputs to the two-zone modelling obtained from GT-Power for the DI cases in Table C.4 and the 95th percentile raw pressure traces xxiii

24 Chapter 1 Introduction 1.1 Energy consumption and climate change With the growth of the world economy, more energy is required in the future. Based on the estimations of the BP Energy Outlook in 217 [1], the growth of the total energy consumption in the next 2 years is over 3% with the world economy to double in this period, as shown in Fig.1.1. Half of the growth is expected to come from renewables, nuclear, and hydroelectric power, but the fossil energy sources, such as coal, gas, and oil, still provide over three-quarters of total energy supplies. Among all these conventional energies, the oil consumption is predicted to be the largest. Meanwhile, more than half of the oil demands come from transportations, as shown in Fig.1.1(b). In the foreseeable future, it is expected that Internal Combustion Engines (ICEs), which predominantly rely on oil, will serve as the main propulsion systems for transportations [2] owing to low cost, high reliability, long durability, and fast refuelling. The deep understanding and accurate control of combustion process have enabled the emergence of novel engine technologies for higher efficiencies and lower emissions, such as direct injection, turbocharging, and downsizing. New types of engine utilising advanced combustion modes, such as spark assisted homogeneous charge compression ignition technology, are emerging [3]. The predominant use of fossil fuels in ICEs produces a significant amount of carbon dioxide (CO2) which accounts for approximately 25% of global greenhouse gas emissions responsible for global warming [4] and other pollutants such as nitrogen oxides (NOx), carbon monoxide (CO), particulate matter (PM), and soot. Compared with the fossil fuels, biofuels produced from biomass are renewable and produce less CO2, soot, and unburned hydrocarbon (HC) emissions, which have been widely used as alternative neat fuels or fuel additives. 1

25 Billion toe 18 Renewables 16 Hydro 14 Nuclear Coal 12 Gas 1 Oil 8 Mb/d Power Buildings Industry Non-combusted Ships, trains & planes (a) 4 Trucks Transport 2 Cars (b) Figure 1.1: The outlook for (a) energy consumption and (b) oil demand before 235 [1] 1.2 Biofuels for transportation Increased biofuels production To reduce the GHG emissions, biofuels have been used as, in most cases, fuel blending components in nowadays transportations due to their cleaner emissions compared with the conventional fuels. Fig.1.2 shows the global biofuels production in the past ten years [5]. The overall amount is twice of the value from ten years ago, indicating increasing importance of biofuels. The increased production enables higher levels of biofuels blending in the fossil fuels Ethanol as a fuel additive Among all biofuels productions, ethanol is the predominant compound and has been extensively used as a transportation fuel. Generally, renewable ethanol fuel can be sustainably produces in many countries [6, 7]. Besides, when blended with gasoline, ethanol reduces the emissions of CO and unburned hydrocarbon in exhaust [8]. Finally, ethanol is known to have high octane numbers [9 12] and significant charge cooling effect [9, 11 15], which suppress the knock in spark-ignition (SI) engines and thus improves the engine efficiencies. Ethanol has been widely used as a fuel additive in the gasoline with the blending ratios of 1% or 85% by volume (known as E1 and E85) in most cases. As the largest ethanol production country, U.S. blends ethanol extensively in the gasoline and nearly all their gasoline are sold with ethanol 2

26 S. & Cent. America Europe & Eurasia Figure 1.2: Global biofuels production in the last ten years [5] blended and the amount of ethanol blended into the gasoline is around million gallons per day in 217 [16]. Of note is the breaking through of the so-called blend wall [17] - the point where ethanol occupies 1% in the gasoline. As shown in Fig.1.3, the ethanol concentration in gasoline gradually increased in the past seven year and exceeded 1% last year, which is the consequence of increased production of biofuels. The application of ethanol contained gasoline in Brazil has an even longer history and goes much further compared with the U.S.. The ethanol content in Brazilian gasoline has been mandatorily required to be higher than 25% since 27 [18]. As the world s third largest ethanol producer, China recently planned to roll out ethanol-added gasoline nationally by 22 and significantly improve the ethanol production and related technologies by 225 [19]. Figure 1.3: Annual U.S. average ethanol content of finished gasoline from 21 to 216 [17] 3

27 1.3 Octane blending of ethanol and hydrocarbons With the increasing amount of ethanol blended into the gasoline, a number of experimental studies [9 11, 2] have been performed to investigate the blending behaviours between ethanol and hydrocarbon fuels. Among all these works, Foong et al. [9] shows that ethanol blends synergistically with isooctane and n-heptane, but antagonistically with toluene. Besides, they also found that ethanol blends synergistically with toluene reference fuels. Nevertheless, the causes for these non-linear octane blending behaviours are not well understood. In practical applications, it is essential to understand and utilise these non-linear behaviours, which helps to exploit the benefits from ethanol. More specifically, it is desirable to formulate base gasoline with hydrocarbons blending synergistically with ethanol, which helps to further improve the anti-knock performance of the fuel mixture. However, the gasoline is a complex fuel mixture containing hundreds of different hydrocarbons which, in most cases, are expected to blend non-linearly with ethanol. Besides, the interactions between these hydrocarbons should not be ignored either. It is worthy noting that exploring all aforementioned octane blending behaviours is not realistic, and thus fundamental experiments are required as well to understand the chemical origins of the non-linear blending behaviours, which would provide insights into fuel design. To sum up, despite the wide and increasing use of ethanol for gasoline blending, the optimal use of ethanol with gasoline is not fully understood. To shed light on interactions between ethanol and major components in the gasoline, this study investigates octane blending and oxidation chemistry of ethanol and hydrocarbon mixtures. 4

28 Chapter 2 Literature Review 2.1 Overview of Knock Essence of knock Knock is the sound caused by the extremely rapid energy release in the unburned air-fuel mixture (also known as end gas ) ahead of the propagating turbulent flame [21]. The abnormal combustion in the end gas results in high local pressures whose non-uniform nature causes pressure waves to propagate in the chamber. The oscillations of the pressure waves may cause the entire combustion chamber to resonate at its natural frequency, which leads to a loud metallic pinging noise that defined as knock [22]. A typical in-cylinder pressure trace of a knocking cycle of isooctane together with its corresponding non-knocking trace suppressed by tetraethyl lead are shown in Figure 2.1 [23] where P i and P f represent the initial pressure rise and the later peak-peak pressure oscillation. Their responses for PRFs were investigated by [24] under different compression ratios for both RON and MON conditions. It was found that P i correlates well with the knock intensity defined in the ASTM manual [25,26], while P f relates to the engine vibrations. To quantify the knock intensity, a bouncing pin apparatus, shown in Fig.2.2, was developed by Midgley and Boyd [27] back in When the engine knock occurs, the diaphragm vibrates due to the in-cylinder pressure oscillations, which pushes the bouncing pin to close the electrical contacts. The bouncing-pin fluctuations are measured by the gas evolution from an electrolytic cell filled by sulphuric acid and distilled water. The electrical outputs, affected by the cycle to cycle variations, are averaged to represent the knock intensity. The modern knock sensor used in the CFR engine is an electronic emulation of the original bouncing pin apparatus but in a compact format. The physical and chemical processes of engine knock have gained wide attention since the early 2th century [29 31]. Nowadays, it is generally accepted by the research community that the spon- 5

29 Figure 2.1: The pressure trace of a knocking cycle and its corresponding non-knocking cycle suppressed by tetraethyl lead [23] Figure 2.2: The Midgley and Boyd bouncing pin apparatus for knock detection [28] taneous oxidation with rapid energy release will occur in parts or all of the end gas region when the 6

30 pressure and temperatures of one or several gas pockets in the end gas are adequately high [21]. During the autoignition, the pressure trace first has a rapid increase and then oscillates with decaying amplitude due to the pressure waves generated from the auto-ignited hot spots Characteristics of knock With high-speed imaging, knocking combustion can be observed photographically. Figure 2.3 shows the time series images for non-knocking and knocking cycles [32]. The normal flame front can be clearly observed in the non-knocking cycle and the first image of the knocking cycle. The dark crescent-shaped region ahead of the flame front is the unburned gas zone where autoignition occurs (in frame G). Then the unburned gas zone becomes brighter and hotter with the propagation of the autoignition region. Finally, the end gas gets burned completely in frame J. Figure 2.3: Image series for both non-knocking and knocking engine cycles [32] Autoignition occurs at places with the most favourable conditions for the low temperature oxidations, and reaction propagation depends on the inhomogeneity of temperature and compositions of the unburned gas. Based on the temperature gradients, the end gas autoignition could propagate from the hot gas pocket in three modes [33]: With low temperature and steep temperature gradients, the end gas will produce a weak pressure propagating from the centre and is attenuated. In this phase, combustion undergoes a gradual transition to knock and is regarded as non-knocking combustion. With high temperature and small temperature gradients, the end gas will generate simultaneous chemical reactions following the occurrence of the autoignition. The knock intensity is positively correlated with the propagation speed of the reaction front. Moderate knock occurs under this condition. 7

31 With intermediate temperature and temperature gradients, the end gas will create strong shock waves after the initiation of the chemical reactions. Strong pressure waves coupled with very reactive end gas will generate an intensely illuminating flame. In this case, autoignition ends up with a severe and damaging knock. Knock detection techniques can generally be categorised into two types: direct measurements based on in-cylinder parameters and indirect measurements such as sound, pressure or cylinder block vibrations [34 36]. The peak-peak value of the pressure oscillations after band pass filtering was applied to define the knock intensity by [37]. Fast Fourier transform (FFT) and power spectral density (PSD) of raw pressure trace were used to characterise knock in [38, 39]. The third derivative of the pressure trace, which generates a much higher absolute value when knock happens, could also be applied to determine the knock onset point [4, 41]. The occurrence of knock can be determined by engine vibration whose oscillation frequencies depend on the size and shape of the chamber [42]. 2.2 Anti-knock Characteristics of Ethanol/Hydrocarbon Blends Ethanol, as an oxygenated gasoline blending component, has aroused worldwide interests in the last two decades. Beneficial results have been reported in numerous studies related to ethanol fuelled SI engine. Among all these studies, this review summarises ethanol s two most important features: high octane number and charge cooling effect Octane numbers of ethanol/hydrocarbon blends In 1927, Graham Edgar [43] proposed an octane rating scale which defines the knock-limited compression ratios for the blend fraction of the two Primary Reference Fuels (PRFs), namely iso-octane and n-heptane. For instance, a PRF with an octane number of 8 is comprised of 8% iso-octane and 2% n-heptane by volume. Octane rating experiments are conducted in the Cooperative Fuel Research Committee (CFR) engine with standard testing procedures [25, 26]. Research octane number (RON) and motored octane number (MON) are two types of octane numbers associated with different operating conditions. 8

32 The RONs and MONs of different ethanol/gasoline blends on a volume basis are shown in Fig.2.4, based on the studies from Foong [9] and Anderson [44]. The measured RONs show a non-linear relationship with the volume fraction of ethanol. Although both of them exhibit similar trends, the results reported by Anderson [44] are more synergistic (octane number deviates more from the linear blending to the higher octane number side). Fig.2.4(b) shows the synergistic blending behaviours observed in the corresponding MON tests. The differences between these two works are probably caused by the different gasolines used in the experiments RON 1 MON Foong et al.,214 Anderson et al., Ethanol content %(v/v) 82 8 Foong et al.,214 Anderson et al., Ethanol content %(v/v) (a) (b) Figure 2.4: Measured (a) RONs and (b) MONs for the ethanol and gasoline blends The RONs and MONs of isooctane, n-heptane and toluene blended volumetrically with ethanol are shown in Fig.2.5 [9]. With a small amount of ethanol added, the RONs of isooctane are improved significantly, indicating the synergism between ethanol and isooctane. Similarly, the RONs of n-heptane increase non-linearly (constantly above the straight line) with the increased ethanol concentration. Unlike isooctane and n-heptane, toluene blends antagonistically with ethanol. The measured MONs for these three types of blends overall exhibit similar trends to the RONs, although the levels of synergism and antagonism could be different. 9

33 12 11 RON isooctane n-heptane toluene Ethanol content %(v/v) (a) MON isooctane n-heptane toluene Ethanol content %(v/v) (b) Figure 2.5: Measured (a) RONs and (b) MONs for ethanol blended with isooctane, n-heptane and toluene Charge cooling effect The octane numbers of ethanol blended with gasoline and three neat compounds are based on the standard CFR engine tests whose results reflect the autoignition chemistry and the charge cooling effect. The significant latent heat of vaporisation of ethanol enhances the charge cooling effect which decreases the temperature of fuel/air mixture and consequently improves the mixture s anti-knock performance. To quantify the charge cooling effect, the modified RON tests were carried out by Foong et al. [13]. In the standard knock rating experiments, the average fuel/air temperatures of most PRFs are around 36 C which is taken as the reference temperature for the modified RON experiments. After heating the intake air to ensure the temperature of the ethanol-containing fuel and air mixture around 36 C, the charge cooling effect becomes negligible, and the modified RON is a pure reflection of the autoignition chemistry. Fig.2.6 shows that the modified RON is significantly lower than the standard RON with high ethanol concentrations. Nevertheless, the differences between the standard and modified RONs are relatively small when the added ethanol is less than 3% by volume, indicating the autoignition chemistry plays a dominating role for these blends. The charge cooling effect is more distinct in modern engines with direct injection (DI) compared with the standard CFR engine using the carburettor to vaporise the fuel. As shown in Fig.2.7, a single cylinder research engine is equipped with a DI injector and as well as a port fuel injection (PFI) injector to study the charge cooling effect [45]. Unlike the conventional PFI, the widely adopted DI takes advantage of the large latent heat of vaporisation of ethanol. The experimental study carried out by Kasseris et al. [14] investigated the ethanol s charge cooling effect. In their experiments, the engine knock onset timing with the PFI was taken as the reference, and the intake air in the DI mode 1

34 RON Standard RON Modified RON Ethanol content %(v/v) Figure 2.6: Measured RON values for ethanol/gasoline blends under standard and modified conditions [13] was heated until the same autoignition onset timing was observed, which provides a method to quantify the charge cooling effect. Following this experimental study, the effective and evaporative octane numbers were proposed to represent the fuel s anti-knock performances from the autoignition chemistry and the charge cooling effect respectively [15]. Fig.2.8 shows that the charge cooling effect, indicated by the difference between the overall and the effective octane numbers, exhibits a growing trend with the increased ethanol content. Another experimental study conducted by Stein et al. [11] compared the DI with the upstream fuel injection (UFI) to quantify the charge cooling effect. As shown in Fig.2.9, the increase of the net mean effective pressure (NMEP) from E, UFI to E5, UFI results from the high octane number of E5 which is essentially from chemical effect. Meanwhile, the increases from UFI to DI should be attributed to the charge cooling effect which is, not surprisingly, more distinct for E5 than E. The explanation and quantification for the charge cooling effect of ethanol are relatively straightforward, and this phenomenon is well understood from the aforementioned experimental studies. However, the combustion chemistry of the ethanol containing fuel mixtures is more complex and less well known compared with the charge cooling effect. To have a better understanding of the autoignition chemistry, the chemical kinetics of ethanol and surrogate fuels should be thoroughly investigated. 11

35 Figure 2.7: CAD model of the combustion chamber [45] Octane number Effective octane number Overall octane number Ethanol content %(v/v) Figure 2.8: The comparison between overall and effective octane numbers [14] 12

36 35 3 CA5 (deg ATDC) E, UFI E, DI E5, UFI E5, DI NMEP (bar) Figure 2.9: Separation of chemical octane and charge cooling effects on knock limit [11] 2.3 Chemistries of Ethanol and Hydrocarbons The fuel chemistry controls the combustion characteristics and helps to interpret phenomena observed in the engine combustion. Attempts have been made from Foong et al. [46] and the author [12] to investigate knocking combustion with detailed chemistry using kinetic modelling. However, a number of assumptions are inevitable in these studies due to the complicated in-cylinder conditions. Therefore, the investigations of the fundamental combustion chemistry require specially designed combustion reactors where the processes are dominated by the reaction kinetics with well-defined flow conditions Experimental techniques The commonly used combustion reactors include shock tube (ST), rapid compression machine (RCM), well-stirred reactor (WSR), and pressurised flow reactor (PFR) Shock tube Shock tube operates at relatively high temperatures and focuses on the self-ignition of gas mixtures. A mixture of reactants in the shock tube can be compressed instantaneously to a desired temperature and pressure by a plane shock wave, as shown in Fig.2.1. Ignition delay time, defined as the time interval between shock arrival and autoignition, is determined from the pressure trace. The onset of ignition 13

37 can be obtained from the emission/absorption spectra of intermediate combustion species. However, the non-ideal conditions, i.e. the formation of boundary layers in reflected shock tube experiment, limit the observation times to hundred of microseconds. Thus, experiment conditions are constrained to pressure and temperature regimes with short chemical induction times. Figure 2.1: Schematic of a shock tube/rapid compression machine Rapid compression machine With the similar schematic to the shock tube, the rapid compression machine is designed to emulate the combustion process in reciprocating engines, which makes it a relatively complicated system. The movement of a piston compresses premixed mixture in the combustion chamber to a small volume, high pressure, and temperature, which initiates the ignition. The pressure and temperature histories are controlled by the compression ratio, initial pressure and mixture composition. Similar to engines, the physical phenomena inside the rapid compression machine are complicated. Unknown wall heat transfer, blow-by due to piston crevices and large-scale disturbances of reacting mixture caused by piston movement complicate the interpretation of the experimental results of the rapid compression machine. Besides, it is challenging to use extractive sampling method to measure mixture composition. Normally, the data from rapid compression machine are modelled with a homogeneous reaction condition and empirically determined heat loss function Well-stirred reactor The well-stirred, or perfectly-stirred reactor is assumed to have entirely homogeneous mixing inside the control volume, as shown in Fig.2.11 [47]. A common type of well stirred reactor is called jet-stirred reactor which uses high velocity inlet jets to facilitate the mixing process. An essential characteristics of the well-stirred reactor is the perfect mixing assumption which considers the time required for the mixing is much shorter than the mean residence time of the fluid in the reactor. However, the actual residence time in a WSR is less defined, which is supposed to follow a residence time function, not a single value. This has to be assumed in the modelling. The WSR operates with less dilution, short residence times, and higher temperatures. However, the non-ideal conditions 14

38 Figure 2.11: Schematic of a well-stirred reactor [47] in the experiment, such as imperfect mixing and heterogeneous chemistry, make it complicated to fully interpret the experimental results Pressurised flow reactor The pressurised flow reactor is designed to provide a convective-reactive environment, where diffusion along the flow direction is minor or can be neglected. The schematic of a flow reactor is shown in Fig2.12. In the PFR experiment, the vaporised fuel and the heated oxidizer enter the reactor through two separated lines and mix with each other via a mixer. The reaction starts to occur at the same time due to the high temperature of the mixture. When the mixture flows along a long insulated or heated reactor tube, a hot water cooled sampling probe is used to extract the gas mixture continuously. The movement of the gas sampling probe is driven by a program controlled motor, which could be applied to extract gas mixtures at different positions. By measuring the gas temperature which determines the gas velocity, the tube distance can be converted to the residence time. The time histories of the mole fractions of reactants, intermediates and products can be measured by gas chromatography (GC) located downstream of the sampling probe. The ability of producing the species profiles of reactants, intermediates and products is the most significant advantage of flow reactor compared with other reactors. A potential issue of the flow reactor lies in the mixing process where inhomogeneous reactions are occurring. The negative effects from 15

39 Figure 2.12: Structure of a pressurised flow reactor the inhomogeneity could be minimized by using a specially designed mixer providing fast mixing Combustion chemistry of hydrocarbons and alcohol The experimental results from reactors mentioned above are applied to develop detailed combustion chemistry which provide insights into understanding the autoignition phenomena in SI engines. The reviews of comprehensive chemical mechanisms at high temperatures are available in [48 5], while low temperature chemistry of hydrocarbons was reviewed by [51, 52]. The review mainly focuses on the low temperature oxidations which are relevant to the autoignition in the SI engines. The state of the art combustion chemistry of major hydrocarbon groups and ethanol will be introduced briefly Combustion chemistry of alkanes Considering the significance of the low temperature chemistry in the engine knock, the general reaction pathways for the oxidations of alkanes at low temperatures are first reviewed, whose scheme is shown in Fig.2.13 [51]. The abstraction of a hydrogen atom (H-abstraction) from alkane by oxygen or hydroxyl radical ( OH) initiates the reaction to produce alkyl (R ) and hydroperoxy ( OOH) radicals. Since the rate constant of this type of reaction is sensitive to the radical structure, the branched molecules (e.g., isooctane) have lower rate constants compared with those straight-chain ones (e.g., n-heptane) [53]. After H-abstraction, alkyl radicals react with oxygen molecules to generate a variety of products [52]: R + O 2 ROO (2.1) 16

40 R + O 2 alkene + HO 2 (2.2) The products from R + O 2 reaction change with pressure and temperature. Typically, at low temperature and moderate pressures, alkyl peroxy radical ( ROO) is the primary product, as shown in reaction 2.1. RH + O 2 or OH initiation steps H abstractions R + H 2 O 2 RH (2) O 2 HO 2 + alkene R (1) QOOH (3) O 2 ROO O 2 R + alkene HO 2 ROOH + O 2 degenerate branching steps RO + OH R + H 2 O RH OH + cyclic ethers, aldehydes or ketones OOQOOH U(OOH) 2 XO + OH keto-hydroperoxides + OH degenerate branching steps Figure 2.13: Simplified scheme for the primary mechanism of oxidation of alkanes at low temperatures [51] The thermally unstable alkyl peroxy radical may undergo different reaction paths, which results in the varied progresses of autoignition. Firstly, the alkyl peroxy radical can dissociate back to the alkyl radical and oxygen molecule. If at the same time, the temperature increases to favour reaction 2.2, the overall reaction rate will be reduced, which leads to the so-called negative temperature coefficient (NTC) regime. Secondly, with the low energy threshold, the alkyl peroxy radical decomposes to alkene and hydroperoxy radical even at room temperatures [54], as shown in reaction 2.3. Both theoretical [55] and experimental [56] studies showed that this type of reaction is significant for hydroperoxy radical production. However, hydroperoxy radical is not reactive, which slows or effectively terminates the low temperature reactions. ROO alkene + HO 2 (2.3) ROO QOOH 17 (2.4)

41 The most important reaction path for alkyl peroxy radical, which leads to chain-propagating reactions, is the isomerization via internal H-atom abstraction to form hydroperoxy alkyl ( QOOH) radical, as shown in reaction 2.4. This type of reaction undergoes a cyclic transition state, whose activation energies for isomerization comprising the activation energy for H-abstraction and the strain energy of the cyclic transition state. In this case, both the ring strain energy barriers and the type of abstracted H atom affect the rate constants of these reactions. Then, the unstable hydroperoxy alkyl radical decomposes to cyclic ether and highly reactive hydroxyl radical ( OH). The unpaired electron of the carbon atom of hydroperoxy alkyl radical is vulnerable to the attack from oxygen molecule, as shown in reaction 2.5, which is quite similar to the reaction between alkyl radical and oxygen molecule. QOOH + O 2 OOQOOH (2.5) Afterwards, the OOQOOH radical goes through a second internal H-abstractions, similar to ROO, and forms the Q(OOH) 2 radical. The decomposition of this radical gives hydroxyl radical, which is a chain-branching reaction: OOQOOH Q(OOH) 2 ketohydroperoxides + OH OQO + 2 OH (2.6) Another chain-branching reaction pathway in Fig.2.13 related to reaction 2.7 and 2.8. However, the alkyl hydroperoxide is relatively stable, especially at low temperatures, which results in slow chain branching reactions and thus contributes little to the production of hydroxyl radicals. ROO + HO 2 ROOH + O 2 (2.7) ROOH RO + OH (2.8) The low temperature combustion chemistry of the alkanes provides fundamentals for the development of the state of the art chemical mechanisms for various hydrocarbons of interest to practical fuels. Among all these hydrocarbons, isooctane, n-heptane and pentanes are considered as important in the production gasoline, and their chemical mechanisms are critical in terms of understanding the engine knock. As a representative branched alkane, isooctane is widely used as a surrogate gasoline fuel for engine combustion research. The most well known detailed combustion model of isooctane at both low and high temperatures was developed by Curran et al. [57] based on the experimental results from a jet-stirred reactor [58], flow reactors [59 61], shock tubes [62 64] and motored engines [65, 66], which cover the pressure range from 1 atm to 45 atm and temperature from 55 K to 17 K with equivalence ratio from.3 to 1.5. The model shows good agreements when compared with different experimental results. Then, a gasoline surrogate mechanism developed by Mehl et al. [67] speeds up 18

42 the low temperature oxidation processes with updated rate constants and thermal properties, which produces better agreements to experiments in various operating conditions. A very recent update for isooctane mechanism comes from Atef et al. [68], which is motivated by matching the experimental results [69 73] causing problems for previous mechanisms. With the implementations of the recent results from computational studies in isooctane thermochemistry [74 77], low temperature oxidation kinetics of normal and branched alkanes [78 81], and new alternative isomerization pathways, the latest isooctane mechanism produces improved agreements to the existing experiments, especially those at lower equivalence ratios. N-heptane is a representative fuel for normal alkane, whose combustion chemistry is relatively well understood. Numerous experimental studies were performed in shock tubes [62, 82 86], rapid compression machines [87 9], jet-stirred reactors [58, 91 94], flow reactors [6, 95 97], flame experiments [98 16], and engines [17 111] to study the oxidations of n-heptane over a wide range of conditions. A detailed combustion mechanism of n-heptane was proposed by Curran et al. [112], which not only perform well when matching experimental data but also provides a kinetic frame for the mechanism development. This mechanism was modified by Mehl et al. [67] to incorporate the updated decomposition rates of the alkyl and alkoxy radicals [113], the isomerization rates at low temperature oxidation recommended by [114], and the new reaction pathways from [115, 116]. The most recent update [117] for the mechanism includes AramcoMech 2. [118] for the C C 4 species, the latest chemistry for three pentan isomers [ ], and the base n-heptane sub-smechanism [112]. Although isooctane and n-heptane are normally considered as surrogate gasoline fuels, their fractions in the Australia production gasoline are much less than iso-pentane and n-pentane. Several experimental studies were conducted to investigate the oxidations of pentane isomers in rapid compression machines [12, ], shock tubes [12, 126, 127], a well-stirred reactor [128], an annular flow reactor [129], and a CFR engine [13]. The state of the art chemical mechanism of pentane isomers was developed by Bugler et al. [12] and very recently updated based on experimental results from two jet-stirred reactors [131]. The chemical mechanisms for most alkanes have been renewed in recent two years with constantly emerging experimental, theoretical and modelling studies. However, alkanes alone are not sufficient to emulate the production gasoline with a significant amount of aromatics as octane boosters Combustion chemistry of aromatics As important components in petroleum-derived fuels, aromatics typically show much slower oxidation rates than alkanes, particularly at low temperatures. Understanding the detailed chemical kinetics of aromatics is necessary to interpret the combustion characteristics of the production gasoline. 19

43 Toluene, a representative hydrocarbon in the aromatics family, has been used as a surrogate gasoline fuel along with n-heptane and isooctane to emulate the production gasoline. The major reaction pathways and the state of the art chemical mechanisms for toluene will be reviewed. Fig.2.14 presents the main reaction pathways of toluene and benzene oxidations proposed by Brezinsky [132]. As a very important product of toluene oxidation, benzene may form phenyl radical, phenol, and phenoxy radical after being attacked by those small and reactive radicals. Besides, the former two products, phenyl radical and phenol, react with small radicals to produce phenoxy radical which decomposes to CO and cyclopentadienyl radical. The free electron of cyclopentadienyl radical may combine an H atom to form cyclopentadiene or reacts with those oxygenated radicals to generate cyclopentadionyl radical which produces butadienyl radical and CO. Figure 2.14: Simplified scheme for the oxidations of benzene and toluene [132] Toluene oxidation mechanism can be found on the right part of Fig At the beginning, toluene reacts with small radicals to form benzyl radical, cresol, cresoxy radical and benzene. Benzyl radical is most abundant product from the first step oxidation. Although benzyl radical is very stable, especially at low temperatures, it has several reaction pathways at relatively high temperatures. The benzyl radical may combine with itself to generate bibenzyl and react with small radicals to form other stable molecules, such as benzyl alcohol, ethylbenzene and benzaldehyde. Apart from the reaction pathways proposed in [132], a C 7 H 6 molecule was observed from the decomposition of benzyl radical by the re- 2

44 cent experiment study [133]. The C 7 H 6 was later found to be fulvenallene [134] and the corresponding rate constant was theoretically computed by da Silva et al. [135]. Another important reaction is between benzyl and hydroperoxy radicals which mainly generate benzoxyl and hydroxyl radicals above 7K, but below this temperature, the primary product becomes benzylhydroperoxide. These benzyl radical involved reactions all lead to chain-termination. Unlike alkanes above, aromatics, especially those without long chain, generally don t have low temperature chemistry as the cyclic transition state cannot be formed. The experimental studies for toluene oxidations have been performed using flow reactors [ ], jet-stirred reactors [139, 14], shock tubes [ ], rapid compression machines [146] and laminar premix flames measurements [ ]. To model these experimental results, several detailed chemical mechanisms [137, 139, 145, ] have been developed previously. The recent modelling study was conducted by Metcalfe et al. [138], which combines toluene sub-mechanism from [14] and C C 4 sub-mechanism from [ ]. This toluene mechanism [138] was incorporated into the latest n-butylbenzene model by Nakamura et al. [16] which contains C C 4 sub-mechanism from AramcoMech 1.3 [161] and alkyl-aromatics sub-mechanism from [162]. More recently, Yuan et al. [163,164] proposed a kinetic model for toluene based on their experiments performed in flow reactor and jet stirred reactor Combustion chemistry of ethanol Ethanol, as a renewable fuel and an octane booster, has been added to the production gasoline world widely. The oxidations of ethanol were investigated using shock tubes [165 17], flow reactors [ ], jet-stirred reactors [175, 176], rapid compression machine [17] and laminar flames [177 18]. The most well known detailed ethanol mechanism was developed by Marinov [181] which had been validated against all available experimental data at that time. Marinov s mechanism was first improved by Li et al. [172, 173, 182] to model ethanol pyrolysis and oxidation in a pressurised flow reactor by including modified rate parameters for the decomposition reactions. Later, Dagaut and Togbé [183] updated Marinov s mechanism with the kinetic parameters from quantum chemical calculations for H atom abstraction from ethanol molecule. The existing ethanol mechanism continues to be improved by experiments in different reactors [175, 184]. As critical reaction pathways, the rate constants for the four ethanol decomposition reactions (2.9 to 2.12) have aroused great interests in the research community. Based on the experimental and theoretical studies carried out by Li et al., the rate constants for reaction 2.11 and 2.12 are much lower than those of reaction 2.9 and 2.1. Besides, Li et al. also presented that reaction 2.9 is strongly dependent on temperature and is dominant over the temperature range of 3-25 K at 1 atm. 21

45 C 2 H 5 OH C 2 H 4 + H 2 O C 2 H 5 OH CH 3 + CH 2 OH C 2 H 5 OH CH 2 CH 3 + OH C 2 H 5 OH C HCH 3 + H 2 O (2.9) (2.1) (2.11) (2.12) The reactions between ethanol and hydroxyl radical generate different products (2.13 to 2.15) whose relative fractions are determined by the branching ratios. The ratios applied by both Marinov [181] and Li et al. [172, 173, 182] are from empirical approaches. While a more recent study performed by Mittal et al. [184] adopted the rate constants from Sivaramaskrishnan et al. [185] and tuned the branching ratios to match the experimental results. C 2 H 5 OH + OH CH 3 CHOH + H 2 O C 2 H 5 OH + OH CH 2 CH 2 OH + H 2 O C 2 H 5 OH + OH CH 3 CH 2 O + H 2 O (2.13) (2.14) (2.15) Understanding the chemical mechanisms of the neat compounds is the prerequisite to explain the behaviours of production gasoline and gasoline surrogates over a wide range of conditions. However, the interactions among different components in these mixtures do exist and may play a significant role concerning affecting the overall performances. In this regard, there has been an increasing awareness of the necessity to investigate the chemical interactions. 2.4 Chemical Interactions of Fuel Mixtures Although the oxidation kinetics for neat fuel compounds is relatively well understood, it is often challenging to predict the autoignition of fuel mixtures due to chemical interactions. These interactions are typically divided into two types: the first is via large fuel-like radicals, and the second is via small radicals Interactions between alkanes The cross reactions for alkane mixtures have been studied by Andrae et al. [152, 186]. In their earlier publication [186], the cross reactions between fuel-like radicals were incorporated in the mechanism to explain the experimental results that PRF84 ignites much earlier than toluene/n-heptane mixture with similar RON in HCCI engine at high intake pressure and low intake temperature, since the ignition delays of these fuel mixtures are similar at low intake pressure and high intake temperature. They 22

46 argued that with the added cross reactions, the PRF mixture would be more reactive than toluene/nheptane mixture before the NTC regime. While at low intake pressure and high intake temperature, which is within the NTC regime, the reactivity of toluene/n-heptane mixture was less affected by the NTC effects compared with PRFs. It seems that the addition of the cross reactions increases the reactivity of PRF before the NTC regime and thus improves the predictions of autoignition delays in HCCI engine. However, in their later experimental study [152], the rate constants of the cross reactions have been re-evaluated. When validating the TRF mechanism against the shock tube autoignition delays, the rate constants of the cross reactions were too high, which lead to significantly shorter predicted ignition times than the measurements below 1 K. Besides, excluding those cross reactions had little influence to the modelling results, which suggests that the cross reactions may not be significant in the PRF autoignition. The studies from Andrae et al. [152, 186] showed that the cross reactions between fuel-like radicals are not significant at least when predicting the autoignition delays from the shock tube. However, the cross reactions related to the small radicals are supposed to be important, which are more likely to occur and even affect the overall reactivity Interactions between PRF and toluene The measured MONs of isooctane and toluene mixtures are plotted in Fig.2.15, which indicate the fuel interactions do occur as the MON of 75% isooctane and 25% toluene is lower than those of both neat compounds [187]. It is necessary to understand the fuel interactions before interpreting the complex behaviours of fuel mixtures Cross reactions via large radicals The chemical interactions between PRF and toluene have been studied when developing the comprehensive kinetic mechanism for surrogate fuels [67, 152, 186]. At high temperatures, numerous intermediates, like alkenes and benzyl radical, coexist at the beginning of oxidation and tend to react with each other. The cross reactions between large fuel-like radicals are divided into three groups, as proposed by [188]. The first type is H-abstraction reaction, which was incorporated by Andrae et al. [152,186] in their kinetic mechanisms. In reaction 2.16, RH represents toluene, benzene and benzaldehyde, while QH denotes n-heptane, isooctane, C 3 H 6 and ic 4 H 8. The rate constants of this type of reactions are from the studies by Bounaceur et al. [14] and Da Costa et al. [189]. RH + Q R + QH (2.16) 23

47 MON Volume fraction of isooctane (%) Figure 2.15: Measured MONs of toluene blended with isooctane [187] The second type is recombination reaction between large radicals, which was investigated by Vanhove et al. [19] who detected the molecule methylbutenylbenzne from n-heptane and toluene oxidation in RCM at 83 K. This molecule is supposed to be generated by the combination reaction of benzyl and isobutenyl radicals, as shown in reaction Benzyl radical may react with other alkenyl radicals such as C 2 H 3 and C 3 H 5 as well, whose rate constants were estimated from analogy of benzyl radical reaction in the toluene sub-mechanism. C 6 H 5 CH 2 + ic 4 H 7 C 6 H 5 CH 2 CH 2 C(CH 3 ) = CH 2 (2.17) The last type is addition reaction of phenyl radical to alkenes. The displacement reactions of C 2 H 4, C 3 H 6 and ic 4 H 8 with phenyl radicals are shown from reaction 2.18 to 2.23, as presented by Fahr et al. [191]. Rate constants of these addition reactions were estimated by Tsang [192]. Besides, the reactions between phenyl/benzyl radical and allene (ac 3 H 4 ) are also very important in this type of reactions. The rate constants of reaction 2.24 and 2.25 were estimated by Vereecken et al. [193], and Sakai et al. [188] applied these rate constants to reaction 2.26 and C 6 H 5 + C 2 H 4 styrene + H (2.18) C 6 H 5 + C 3 H 6 C 6 H 5 C(CH 3 ) = CH 2 + H (2.19) C 6 H 5 + C 3 H 6 styrene + CH 3 (2.2) C 6 H 5 + C 3 H 6 C 6 H 5 CH 2 CH = CH 2 + H 24 (2.21)

48 C 6 H 5 + ic 4 H 8 C 6 H 5 C(CH 3 ) = CH 2 + CH 3 (2.22) C 6 H 5 + ic 4 H 8 C 6 H 5 CH 2 C(CH 3 ) = CH 2 + H (2.23) C 6 H 5 + ac 3 H 4 C 6 H 5 CH 2 + C 2 H 2 (2.24) C 6 H 5 + ac 3 H 4 C 9 H 8 + H (2.25) C 6 H 5 CH 2 + ac 3 H 4 C 6 H 5 CH 2 CH 2 + C 2 H 2 (2.26) C 6 H 5 CH 2 + ac 3 H 4 C 1 H 1 + H (2.27) Although the fuel interactions on the large radical level have been observed in multiple studies, their impacts on the fuel mixture performances could be limited as the associated elementary reactions normally have very small rate constants. Note that the chemical interactions related to parent fuels and parent fuel-like radicals were incorporated in the gasoline surrogate mechanisms developed by Mehl et al. [67] and Andrae [187] respectively Cross reactions via radical pool According to Vanhove et al. [193] and Andrae et al. [152], the cross reactions via radical pool may have greater significance than those between large molecules and/or radicals. The ignition delays of neat isooctane and isooctane/toluene mixtures are shown in Fig Both the cool flame and the main ignition delay times increase when toluene is added. Besides, the autoignition delay times of fuel mixture decreases sharply above 83 K, suggesting the promoting effect of toluene on the reactivity at high temperatures. The interactions between toluene and isooctane are of great significance compared with those between toluene and n-heptane because both aromatics and iso-paraffins account for significant fractions of gasoline. Based on the species analysis, Vanhove et al. [19] concluded that toluene is unlikely to change the reaction pathways of isooctane oxidation, but may react with active radicals from isooctane and produce stable benzyl radical to deactivate the reaction pool. The investigation of fuel interactions via the radical pool is challenging, as it requires rigorous species analysis to interpret how the elementary reactions related to the radical pool affect the mixture s performance. To predict the fuel mixture behaviours over a wide range of conditions, more fundamental experimental and computational studies are required to provide accurate rate constants and species profiles to better calibrate the existing chemical mechanisms. 25

49 isooctane/toluene isooctane isooctane/toluene isooctane Figure 2.16: Comparisons of cool flame (open symbols) and autoignition delay times (filled symbols) of neat isooctane and isooctane/toluene mixture [19] 26

50 2.4.3 Interactions between ethanol and hydrocarbons The interactions between ethanol and hydrocarbons are known to be significant in the SI engines from the experimental study performed by Foong et al. [9]. Meanwhile, several kinetic experimental studies [86, 183, ] were conducted to investigate the interactions between alcohols and hydrocarbons. All these studies focused on the radical pool level competitions between alcohols and hydrocarbons. Ethanol and n-heptane have very different reactivities, and their competition for small radicals have been recently investigated by many groups. At low temperatures, both ethanol and n-heptane undergo H abstraction to generate α-hydroxyethyl and heptyl respectively. The calculation from da Silva et al. [199] suggested that, due to the influence of the OH group, the reaction of α-hydroxyethyl and oxygen molecule proceeds almost exclusively to acetaldehyde and hydroperoxyl radical. As the dominant product from H abstraction, α-hydroxyethyl prohibits ethanol s chain-branching reaction, which results in less OH radicals. At the beginning of the oxidation, n-heptane produces much more OH radicals than ethanol. Later, the two fuels compete for the limited OH radicals at the same time. Consequently, the consumption of n-heptane is decreased during this stage due to less OH radicals available comparing with neat n-heptane oxidation; while ethanol gets relatively more OH radicals and is consumed more rapidly than neat ethanol oxidation. After NTC regime, the decomposition of hydrogen peroxide produces a significant amount of OH radicals consuming remaining fuels. Generally, for ethanol/hydrocarbon mixtures, ethanol acts as OH radical scavenger and therefore suppresses the overall oxidation process. As two common alcohol compounds, ethanol and n-butanol have different lengths of the carbon chain. According to HCCI engine experiment by Saisirirat et al. [196], both ethanol and n-butanol retard the start of combustion, but ethanol has a more pronounced effect regarding suppressing combustion. The oxidations of ethanol/gasoline surrogates were carried out by Dagaut and Togbé [183] and Cancino et al. [2]. The kinetic models proposed by both groups can reproduce the experimental results from the jet-stirred reactor and shock tube respectively. The chemical interactions involved in these studies are still at the radical pool level. Although the studies above all successfully reproduced their own experimental results by the blended mechanisms, these kinetic models haven t been validated for the complex fuel mixtures containing practical gasoline surrogates and ethanol. Further experimental and modelling studies focusing on the oxidations of ethanol and hydrocarbon mixtures are necessary to understand the fuel interactions. 27

51 2.5 Summary and research questions As an abnormal combustion phenomenon in the engine, knock is a consequence of end gas autoignition. To suppress knock, ethanol is often added to production gasoline as an octane enhancer. Numerous experimental studies show that ethanol increases both RON and MON of gasoline, and thus improves anti-knock performance. However, the interactions between ethanol and production gasoline in the CFR engine are complicated. Foong et al. [9] carried out the initial experimental study to understand the complex interactions. They formulated three TRF-based gasoline surrogates with different amounts of toluene added to emulate the blending behaviours between ethanol and the production gasoline. The results showed that the TRF-based gasoline surrogates blend more synergistically with ethanol compared with the production gasoline at a similar RON and aromatic content.. Therefore, more engine tests are required to fully understand the interactions between ethanol and the production gasoline. Ethanol s anti-knock behaviour has been extensively investigated mainly from two aspects: charge cooling effect and chemical kinetics. The high latent heat of vaporisation of ethanol improves the charge cooling effect which increases the knock onset limits and thus the engine efficiency. The chemical effect of ethanol needs to be further clarified, especially when interacting with hydrocarbon fuels. Although the interactions among larger species have been incorporated into the widely used gasoline surrogate mechanisms developed by Mehl et al. [67] and Andrae [187], their impacts on the overall behaviours of fuel mixtures might not be significant due to the small rate constants of these elementary reactions. Therefore, fuel interactions are expected to be more likely to occur with the involvement of small reactive radicals. To predict the fuel interaction, the kinetic model should therefore have accurate rate constants for these reactions involving small radicals, and more kinetic experiments for the fuel mixtures are needed to calibrate the existing models. This study, therefore, aims to investigate the fuel interactions in a CFR engine and combustion chemistry of ethanol containing gasoline surrogates in a PFR. The following research questions are proposed. 1. How should the non-linear octane blending of ethanol and toluene reference fuels (TRFs) be represented? Ethanol is known to blend non-linearly with surrogate fuels under standard knocking conditions in the CFR engine [9]. This study proposes a statistical model to quantify these non-linearities and predict the octane numbers of fuel mixtures containing ethanol and TRFs. 2. What is the gasoline surrogate that best emulates the anti-knock behaviours of production gasoline when blended with ethanol? 28

52 As shown in the prior standard octane number test [9], ethanol blends more synergistically with the TRF-based gasoline surrogates than with gasoline, suggesting that TRFs are not good enough to emulate gasoline. Besides, the octane number blending behaviours between ethanol and the hydrocarbon fuels other than TRF components are not known. Therefore, this study formulates new gasoline surrogates to better emulate the knocking behaviours of the gasoline when blended with ethanol. 3. How does ethanol interact with gasoline surrogates under engine representative conditions in the PFR, and do existing mechanisms reproduce the measured species profiles? Numerous kinetic experiments have been carried out by different groups to study the combustion chemistry for neat fuels and fuel mixtures. However, no systematic experimental study has been carried out to investigate how ethanol interacts with surrogate fuels and more importantly, gasoline surrogates in flow reactors. Also, state of the art gasoline surrogate mechanisms haven t been fully calibrated to predict the behaviours of the fuel mixtures containing ethanol and gasoline surrogates. Therefore, this study performs PFR experiments to study the impacts of ethanol on the reactivities of gasoline surrogates and validates the state of the art chemical mechanisms using these measurements. 29

53 Chapter 3 Experimental Methods 3.1 Overview This chapter first presents the experimental methods for the CFR engine and the PFR which are applied in this study to investigate the fuel interactions. Besides, the applications of the gas chromatography (GC) in the PFR experiment for the identification and quantitative analysis of intermediate species are elaborated. 3.2 CFR engine Overview The engine experiments in this study were carried out in a 1933 Waukesha CFR F1/F2 octane rating engine, as shown in Fig.3.1. The CFR engine is driven by a dynamometer at a constant speed of 6 rpm for RON and 9 rpm for MON. In the experiment, liquid fuels, stored in the fuel bowls, are vaporised by the hot air in the carburettor before entering the engine cylinder. Before the air is heated, it goes through the dehumidifier to get rid of water vapours, as the introduction of water vapours increases the overall reactivity of fuel and air mixture by providing hydroxyl radicals. In the standard engine knock experiments, the compression ratio (CR) is estimated and adjusted based on the ASTM manuals [25,26], while the fuel flow rate is tuned by raising or lowering the fuel bowl height to obtain the standard knocking conditions. The lambda and knock meters, which are housed in a separate electrical cabinet, show the fuel/air ratio and knock intensity respectively The Structure of the CFR engine Fig.3.2 shows the detailed structure of the CFR engine. In Fig.3.2(a), the in-cylinder pressure oscillations during the standard knock rating tests are converted to voltages by the knock sensor mounted 3

54 Figure 3.1: The system of the CFR engine on the top of the cylinder, and the dial indicator is applied to adjust the compression ratios specified in [25, 26]. The condenser on the right upper part of the engine body uses the pressurised tap water to dissipate heat away from the coolant flowing in the engine jacket. The CFR engine body is shown in Fig.3.2(b), (c) and (d) from three different views with all auxiliary parts removed. The cylinder inlet and outlet locate on the left and right side of the engine body respectively. The condenser inlet, sitting on the right upper corner of the exhaust side, guides the engine coolant to the condenser and get cooled. Before starting the standard octane number measurements in this study, the CFR engine went through a top overhaul to clean deposits on the cylinder wall and piston head. The comparison of piston head before and after deposits cleaning is shown in Fig.3.3. The deposits act as an insulation layer reducing the overall heat loss from the gas mixtures to the cylinder wall and piston head, which makes test fuels more prone to knock and thus results in a lower octane number. In this study, all knock rating tests are carried out right after the top overhaul when the CFR engine is in the good condition. 31

55 (a) CFR engine (b) Left view (c) Front view (d) Right view Figure 3.2: The structure of the CFR engine 32

56 (a) (b) Figure 3.3: The piston head (a) before and (b) after overhaul Methods for standard octane number tests The CFR engine test methods for the standard RON and MON have been specified in [25] and [26] respectively, and their test conditions are listed in Table 3.1. Both methods determine the octane numbers of the sample fuel by comparing its standard knock intensity with those of two PRFs whose octane numbers are known by definition. To obtain the standard knock intensity for the sample fuel during the knock rating tests, the cylinder height representing the compression ratio needs to be first estimated and gradually tuned based on the [25, 26]. Although it is desirable to measure both RON and MON of the sample fuels to have comprehensive understandings of their knocking behaviours, the measurements conducted in this study are mostly for RON, since MON is not as important as RON in modern engines, especially under high loads. To ensure the engine s compliance with the ASTM standards [25, 26], the so-called Fit-for-Use tests were conducted using the toluene standardisation fuels with known octane numbers. If the difference between the known and measured octane numbers is within the allowed tolerance, the engine is considered fit for knock ratings in a certain octane number range. Note that the PRFs are not capable of rating any sample fuels with RON larger than 1, and different amounts of the dilute tetraethyl lead (TEL) are blended into isooctane as bracket fuels for RON tests above 1. The compositions of the dilute TEL are listed in Table

57 Table 3.1: Operating conditions for the RON and MON measurements [25, 26] Operating parameters RON MON Engine speed 6±6 rpm 9±9 rpm Intake air temperature 52±1. C a 38±2.8 C Mixture intake temperature a N/A 149±1. C Intake air pressure Barometric Barometric Intake air humidity 25-5g H 2 O/kg dry air 25-5g H 2 O/kg dry air Coolant temperature 1±1.5 C 1±1.5 C Oil pressure kpa(g) kpa(g) Oil temperature 57±8. C 57±8. C Spark timing 13 BTDC BTDC c a varied with barometric pressure b temperature measured right before engine inlet c varied with the compression ratio Table 3.2: The composition of the dilute TEL [25, 26] Component TEL Ethylene dibromide Xylene N-heptane Other Mass fraction (%) Pressurised flow reactor Overview The schematic drawing of the PFR system is shown in Fig.3.4. The air comes from an oil-free compressor and goes into the flow reactor after heated to a specified temperature. The air flow rate is controlled by a needle valve and measured by a flow meter. A balanced air stream goes to the gap between reaction tube and wall heater to equalise pressures inside and outside of the reaction tube. The nitrogen is divided into two streams: one is to pressurise liquid fuel out of the cylindrical tank, and another one is used to vaporise the pressurised liquid fuel. These two lines, together with the subsequently merged line are all wrapped with tube heaters to ensure that the liquid fuel is fully vaporised before entering the flow reactor. A strain gauge is used to measure the fuel tank weight to derive the fuel flow rate which is confirmed by the Coriolis flow meter. The gas mixture in the flow reactor is collected by a sampling probe and analysed by a Gas Chromatography, and the remaining mixture is purged into the exhaust system. The reactor pressure is controlled by a back-pressure valve. More detailed information are available in [21]. Although the reactor is designed to run at 5 bar and 1 K, low pressure of 1 bar is used in this 34

58 Figure 3.4: Schematic of the Pressurised Flow Reactor system study due to the limitation from the current air compressor. The air enters the flow reactor with a flow rate of 6 g/s. The pressures of the nitrogen and fuel lines are around 2 bar and 22 bar respectively to achieve choked flow in the reactor. The fuel flow rate is controlled to have equivalence ratio around throughout this study to restrain the heat release Reactor structure As shown in Fig.3.5, the flow reactor is placed vertically to minimise inhomogeneity introduced by gravity which is considered to be significant at high pressures. The air and fuel/nitrogen flow into the reactor via two separated lines and meet at the exit of the specially designed mixer before entering the reaction tube located on top of the mixer. The reaction tube is made of quartz to minimise surface reactions, especially at high temperatures. The quartz tube with a constant 25mm diameter, 4mm thickness and 1mm length is wrapped by three cylindrical ceramic fibre wall heaters to compensate the heat loss from the reacting gases. A water-cooled sampling probe with three thermocouples mounted at the tip is moving inside the quartz tube to collect gas mixtures for Gas Chromatography analysis and measure gas temperatures. 35

59 Figure 3.5: Structure of the Pressurised Flow Reactor 36

60 3.3.3 Mixer In the reactor, the fuel, carried and heated by the nitrogen, starts to react with the preheated air once they meet at the exit of the mixer, where both mixing and reaction occur at the same time. This complicated process at the start of the reactions, which is also common for other kinetic experiments, including stirred reactors, static reactors, leads to the so-called initiation problem in kinetic modelling, where the compositions of gas mixtures are difficult to determine. Although this process is unavoidable, a minimised mixing length does help to moderate the initiation problem. Fig.3.6 shows the specially designed orifice-plate mixer which accelerates the mixing of fuel and air. The air flows into the reactor through 21 evenly located orifices with the diameter of 1.75mm, while the fuel/nitrogen mixture flows along four parallel through-channels which are in a direction perpendicular to the air flow path and is injected via twelve small nozzles with the throat diameter of.18mm located in the centers of the squares outlined by four large orifices. To further improve the mixing process, the fuel/nitrogen flow is choked at small nozzles, which produces the same injected mass at each nozzle, regardless of its position and pressure variations. In the experiments, the fuel/nitrogen pressure is always kept at least twice of the reactor pressure to ensure the occurrence of the choked flow. Figure 3.6: The mixer a) cutaway view and b) orifices distribution To examine the mixing length of the specially designed mixer, a validation experiment of CO 2 /air mixing was carried out [21]. In this experiment, the flow reactor was heated to 9 K and pressurised to 1 bar with 6.2 g/s air flow coming from the large orifices, while CO 2 was injected into the reactor through small nozzles with the flow rate of.71 g/s. The mole fraction of CO 2 was measured along the centreline by a non-dispersive infrared (NDIR) analyser with a resolution of 2 PPM in Horiba emission bench. As shown in Fig.3.7, the mole fraction of CO 2 fluctuates at the start due to a huge concentration difference, but rapidly reaches the equilibrium value around 1 mm downstream of 37

61 the mixer, which is significantly shorter than the typical mixing length, 25 mm, cited for PFRs of similar design [22, 23]. Mole fraction of CO Distance (mm) Figure 3.7: CO 2 concentrations at 1 bar and 9 K in the flow reactor with air flow rate of 6.2 g/s and CO 2 flow rate of.71 g/s Sampling probe In the flow reactor, the sampling probe, driven by a linear actuator shown in Fig.3.5, is to extract reacting gases along the centreline of the quartz tube. The cutaway view of the sampling probe is shown in Fig.3.8(a). The sampling probe quenches reactions in the sampled gas with recirculating hot water. Since the probe connects to the Gas Chromatography which runs at atmospheric pressure, the sampled gas is choked at the probe tip as long as the reactor pressure is larger than 2 bar. The calculated choked flow rate of sampled gas accounts for.2% of the total flow rate in the reactor, indicating the impact of gas sampling on the bulk flow is minimum. Figure 3.8: The sampling probe a) cutaway view b) three thermocouples 38

62 The sampling probe also measures gas temperatures which are critical for kinetic investigation. To get accurate temperatures in combustion, heat radiation has to be handled carefully and rigorously. Three K-type thermocouples with different junction sizes (.27,.8 and.94 mm) and measurement uncertainties of ±.25% are mounted axis-symmetrically on the probe tip, as shown in Fig.3.8(b). Based on the three-thermocouple method, the corrected gas temperature is calculated using the Eqn.3.1 and the uncertainty of the corrected temperature is estimated to be ±5 K at 9 K (smaller at lower temperatures). The detailed description for the three-thermocouple method is available in [21]. The application of this method is illustrated with an example of isooctane oxidation at 1 bar and 9 K shown in Fig.3.9 where the corrected temperatures are compared with the measured ones from three thermocouples. The thermocouple with smaller junction size is less affected by the complicated heat radiations inside the reactor, and therefore has higher temperatures than those with larger junction sizes, indicating the real gas temperatures should be measured using a thermocouple with zero junction size. Although it is not practical and possible to have such a thermocouple, the three thermocouple method provides a good estimation for the real gas temperatures which are, not surprisingly, higher than measured temperatures. Note that this approach for the estimations of real gas temperatures is proven to be theoretically rigorous, but the measured temperatures are known to fluctuate in a certain range especially at lower temperatures, which might lead to obviously unreasonable corrected temperatures in rare circumstances. To handle this issue, the problematic corrected temperatures are interpolated using adjacent good results. T gas = T 1 d 1 d 2 1 d 1 d 2 2/5 T 2 2/5 T 1 1 d 1 d 3 d 1 d 3 2/5 T 3 T T 2 T T 3 2/5 T T 2 T T 3 (3.1) Experimental conditions The general operating conditions of PFR experiments in this work are listed in Table 3.3. To reach these conditions, the PFR is first heated using hot air flow at 11 K and 1 bar plus three wall heaters with tunable power outputs. During the warming up, the metal temperatures of flanges are monitored and used as an indication of the thermal equilibrium. When the metal temperatures are stable, nitrogen flow at 5 K and 2 bar is introduced to the reactor, and the temperature of air flow and power output of wall heater are tuned to reach a new equilibrium in the reactor tube. Once the temperatures at the mixer exit, the outer surface of reactor wall, and the probe tip locating at the end of the tube are all stabilised to the set value, e.g., 9 K, a new equilibrium is reached, and then the fuel will be 39

63 Temperature (K) T 1 with junction size of.27mm T 2 with junction size of.8mm T 3 with junction size of.94mm Corrected Temperature Distance (mm) Figure 3.9: Reactor temperature profiles for isooctane oxidation at 1 bar and 9 K with equivalence ratio of.58 injected into hot nitrogen flow at a slightly higher pressure around 23 bar. The fuel rate is measured by the strain gauge and the Coriolis flow meter. The difference between these two measurements are generally below 5%. Table 3.3: Experimental conditions for the PFR study PFR parameter Set value Reactor pressure (bar) 1 Reactor nominal temperature (K) 9-93 Equivalence ratio Air flow rate (g/s) 6 Nitrogen flow rate (g/s).32 Reynolds number in the reactor tube 8 Fuel/nitrogen pressure (bar) 2-21 Fuel/nitrogen temperature (K) 5 4

64 3.4 Gas chromatography Overview of the gas chromatography The reacting gases collected by the sampling probe go to the gas loops of the GC for quantitative species analysis. Fig.3.1 shows the picture of GC used in this study, which contains GC itself and an auxiliary sampling system installed on the left plate. The inlet and outlet of the sampling system locate on top of the insulation container which accommodates rotator 94 and 93. On the upper left of the container locates a manual controller for rotator 92 which connects ten sampling loops and is placed in another insulation container mounted on the back of the plate (not shown in Fig.3.1). Each rotator has a motor to drive it, and the motor for rotator 92 is installed on the lower middle part of the front, while the other two motors for rotator 94 and 93 are attached on the back. The positions of these rotators are shown on the indicators and controller. Besides the loop based sampling system, two injectors are installed on the top of the GC for the typical manual injections. Figure 3.1: Gas Chromatography-21ATF plus from Shimadzu The connection between the GC and the aforementioned auxiliary system is shown in Fig During the PFR experiments, gas samples are introduced into the GC via a sampling loop system, which involves two working modes: sampling and analysis. In the sampling process, the rotator 94 stays in the current position, specified as position A, which guides the gas sample flow from point 1 to 6, and then to the inlet of the rotator 92. As shown in Fig.3.11, the rotator 92 has both inner and outer 41