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Figure 25: Sensitivity analysis of the temperature in the inner zone for different fuels at A=S and EGR=20%. A positive sensitivity coefficient stands for a reaction increasing reactivity.

Figure 25 Sensitivity analysis of the temperature in the inner zone for different fuels at A=S and EGR=20%. A positive sensitivity coefficient stands for a reaction increasing reactivity.

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Table 1: Properties of alcohols, gasolines and diesel fuels. Adapted from Sarathy et al. [5] and Kalghatgi [16]. Correctly assessing the reactivity of a new fuel or a new gasoline or diesel formulation is largely a chemical kinetics problem. A better understanding of a specific chemical compound’s effects on combustion performances (flame speed, auto-ignition etc.) and emission allow the design of a fuel or fuel blend for an existing technology, the tuning of an engine for an assigned fuel or the concerted development of fuels and engines [17]. Typical surrogate mixtures such as primary reference fuels (PRFs) and toluene reference fuels (TRFs) have started to include different alcohols to investigate their possible impact on commercial fuels [18]. From a chemical kinetic perspective, an accurate description of the combustion chemistry of every surrogate component (n-heptane, iso-octane, toluene, ethanol, butanol etc.) is imperative. The recent review of Sarathy et al. [5] discussed the state of the art understanding of alcohols combustion chemistry, synoptically pointing out many improvement margins and open questions. Following the inputs of Sarathy et al. [5], a joint research effort between the CRECK group at Politecnico di Milano (POLIMD) and the C3 group at National University of Ireland, Galway (NUIG) investigated the auto-ignition propensity of n-C2-Cs alcohols in a twin-piston Rapid Compression Machine (RCM), with the aim of systematically revising alcohols low temperature chemistry.
Table 1: Properties of alcohols, gasolines and diesel fuels. Adapted from Sarathy et al. [5] and Kalghatgi [16]. Correctly assessing the reactivity of a new fuel or a new gasoline or diesel formulation is largely a chemical kinetics problem. A better understanding of a specific chemical compound’s effects on combustion performances (flame speed, auto-ignition etc.) and emission allow the design of a fuel or fuel blend for an existing technology, the tuning of an engine for an assigned fuel or the concerted development of fuels and engines [17]. Typical surrogate mixtures such as primary reference fuels (PRFs) and toluene reference fuels (TRFs) have started to include different alcohols to investigate their possible impact on commercial fuels [18]. From a chemical kinetic perspective, an accurate description of the combustion chemistry of every surrogate component (n-heptane, iso-octane, toluene, ethanol, butanol etc.) is imperative. The recent review of Sarathy et al. [5] discussed the state of the art understanding of alcohols combustion chemistry, synoptically pointing out many improvement margins and open questions. Following the inputs of Sarathy et al. [5], a joint research effort between the CRECK group at Politecnico di Milano (POLIMD) and the C3 group at National University of Ireland, Galway (NUIG) investigated the auto-ignition propensity of n-C2-Cs alcohols in a twin-piston Rapid Compression Machine (RCM), with the aim of systematically revising alcohols low temperature chemistry.
Table 2: O=1.0 alcohols / “air” mixture compositions (% mole fraction) teste in this study.
Table 2: O=1.0 alcohols / “air” mixture compositions (% mole fraction) teste in this study.
Figure 2: A) Calculated pressure history for n-butanol and n-pentanol stoichiometric mixtures at T.~830-840 K and p.~10 bar. B) Comparison between n-butanol and n-pentanol ignition delay measurements at P.~10 bar. Figure 2a shows simulated pressure profiles for n-butanol and n- pentanol at T.=830 K and 838 K, respectively. Experimental ignition delay times of the two alcohols at p=10 bar are compared in Figure 2b, highlighting the higher low temperature reactivity of n-pentanol, and a converging ignition propensity for T>~950 K. The experimental data thus obtained agree well with previous literature measurements in RCM and Shock Tubes (ST). Deviations between measurements at similar conditions have to be attributed to differences in the facility design and in diluent compositions (N2/Ar/COz) in data from different authors. Deviations from ideal behavior caused by heat loss and complex fluid dynamics were taken into account for RCM simulations. Volume histories necessary for the correct simulation of the new data presented in this study are reported in the Supporting Information.
Figure 2: A) Calculated pressure history for n-butanol and n-pentanol stoichiometric mixtures at T.~830-840 K and p.~10 bar. B) Comparison between n-butanol and n-pentanol ignition delay measurements at P.~10 bar. Figure 2a shows simulated pressure profiles for n-butanol and n- pentanol at T.=830 K and 838 K, respectively. Experimental ignition delay times of the two alcohols at p=10 bar are compared in Figure 2b, highlighting the higher low temperature reactivity of n-pentanol, and a converging ignition propensity for T>~950 K. The experimental data thus obtained agree well with previous literature measurements in RCM and Shock Tubes (ST). Deviations between measurements at similar conditions have to be attributed to differences in the facility design and in diluent compositions (N2/Ar/COz) in data from different authors. Deviations from ideal behavior caused by heat loss and complex fluid dynamics were taken into account for RCM simulations. Volume histories necessary for the correct simulation of the new data presented in this study are reported in the Supporting Information.
Figure 3: C-H (black) and C—C (red) bond dissociation energies (kcal mol”). Adapted from [37]. Prior to the systematic revision of n-butanol and n-pentanol kinetics, the thermodynamic properties of the fuels, fuel radicals (alkyl, peroxy, hydroperoxy alkyl etc.) and stable species (enols, ketohydroperoxides etc.) have been obtained based on the revised group contributions by Burke et al. [47]. Properties of the lumped low temperature species have been obtained as a selectivity based average of the different possible isomers. It has to be noted that, according to the lumping procedure proposed by Ranzi et al. [44, 45] both the forward and the reverse rate constant in the low temperature pathways are explicitly assigned, nullifying the effect of updated thermochemistry of typical ow temperature species. Figure 3 shows bond dissociation energies (BDE) for n-butanol C-H and C-C bonds, highlighting the weakening effect of the hydroxyl functional group on vicinal bonds. As summarized in Figure 1 such effect vanishes after the B-positions. Fuel consumption mostly occurs through H-abstraction to form fuel radicals. OH and HO are the dominant abstracting radical over the whole temperature range of interest for HCCI combustion. The observed bond strength hierarchy directly impacts relative selectivity to the different H-abstraction channels. Rate constants for H- abstraction by OH, HO2 and CH3 on alcohols-specific positions have been adopted from Zhou et al. [48-50]. Alkane-like positions are treated according to the systematic approach of Ranzi et al. [51], as already reported in previous studies [34-36].
Figure 3: C-H (black) and C—C (red) bond dissociation energies (kcal mol”). Adapted from [37]. Prior to the systematic revision of n-butanol and n-pentanol kinetics, the thermodynamic properties of the fuels, fuel radicals (alkyl, peroxy, hydroperoxy alkyl etc.) and stable species (enols, ketohydroperoxides etc.) have been obtained based on the revised group contributions by Burke et al. [47]. Properties of the lumped low temperature species have been obtained as a selectivity based average of the different possible isomers. It has to be noted that, according to the lumping procedure proposed by Ranzi et al. [44, 45] both the forward and the reverse rate constant in the low temperature pathways are explicitly assigned, nullifying the effect of updated thermochemistry of typical ow temperature species. Figure 3 shows bond dissociation energies (BDE) for n-butanol C-H and C-C bonds, highlighting the weakening effect of the hydroxyl functional group on vicinal bonds. As summarized in Figure 1 such effect vanishes after the B-positions. Fuel consumption mostly occurs through H-abstraction to form fuel radicals. OH and HO are the dominant abstracting radical over the whole temperature range of interest for HCCI combustion. The observed bond strength hierarchy directly impacts relative selectivity to the different H-abstraction channels. Rate constants for H- abstraction by OH, HO2 and CH3 on alcohols-specific positions have been adopted from Zhou et al. [48-50]. Alkane-like positions are treated according to the systematic approach of Ranzi et al. [51], as already reported in previous studies [34-36].
Figure 5: Schematic representation of alcohols low temperature oxidation mechanism. Thicker arrows represent new or particularly relevant pathways it alcohols oxidation with respect to alkanes. Similarly to alkanes, the fuel radical can isomerize or decompose via B-scission reactions [52]. At lower temperatures alkyl radicals interact with oxygen forming peroxy radicals, activating the typical low temperature reaction pathways [5, 44, 53]. A schematic representation of the low temperature oxidation mechanism of alcohols is given in Figure 5, highlighting pathways that are particularly relevant or new pathways with respect to alkanes (thicker arrows).
Figure 5: Schematic representation of alcohols low temperature oxidation mechanism. Thicker arrows represent new or particularly relevant pathways it alcohols oxidation with respect to alkanes. Similarly to alkanes, the fuel radical can isomerize or decompose via B-scission reactions [52]. At lower temperatures alkyl radicals interact with oxygen forming peroxy radicals, activating the typical low temperature reaction pathways [5, 44, 53]. A schematic representation of the low temperature oxidation mechanism of alcohols is given in Figure 5, highlighting pathways that are particularly relevant or new pathways with respect to alkanes (thicker arrows).
Figure 4: H-abstraction selectivity as a function of temperature from Rate of Production Analysis at T=800 K and p=30 bar.
Figure 4: H-abstraction selectivity as a function of temperature from Rate of Production Analysis at T=800 K and p=30 bar.
Figure 6: R--CH-OH+0O2 <> R-(C=O)-H+HO> high pressure limit rate constant, k = 4.5 1012exp(—3500/RT). Units are cm’, mol, s, cal. have been theoretically investigated by Zador et al. [54] and by da Silva et al. [55], clearly highlighting in the potential energy surface analysis the low lying pathway leading to the formation of HOz and acetaldehyde. Despite a more systematic theoretical evaluation of R- *CH-OH + Oz for a series of alcohols would be necessary in order to extrapolate meaningful rate rules, the high pressure limit rate constant of da Silva et al. [55], already adopted by Sarathy et al. [5] and Heufer et al. [38], has been corrected accounting for the extra ~4 kcal/mol reported by Zador [54] and co-workers at a higher level of theory. Figure 6 compares such rate constants.
Figure 6: R--CH-OH+0O2 <> R-(C=O)-H+HO> high pressure limit rate constant, k = 4.5 1012exp(—3500/RT). Units are cm’, mol, s, cal. have been theoretically investigated by Zador et al. [54] and by da Silva et al. [55], clearly highlighting in the potential energy surface analysis the low lying pathway leading to the formation of HOz and acetaldehyde. Despite a more systematic theoretical evaluation of R- *CH-OH + Oz for a series of alcohols would be necessary in order to extrapolate meaningful rate rules, the high pressure limit rate constant of da Silva et al. [55], already adopted by Sarathy et al. [5] and Heufer et al. [38], has been corrected accounting for the extra ~4 kcal/mol reported by Zador [54] and co-workers at a higher level of theory. Figure 6 compares such rate constants.
Figure 7: formation of a-hydroxy-hydroperoxyalkyl radical and its interactions with O2 to form a carbonyl-hydroperoxide (e.g. hydroperoxy pentanal) and HO2. The formation of a carbonyl compound and HO: is also considered in successive steps when internal isomerization reactions (RO2~QOOH) lead to the formation of a a-hydroxy-hydroperoxyalky] radical, and its successive interactions with Os undergo similar pathways, as summarized in Figure 7.
Figure 7: formation of a-hydroxy-hydroperoxyalkyl radical and its interactions with O2 to form a carbonyl-hydroperoxide (e.g. hydroperoxy pentanal) and HO2. The formation of a carbonyl compound and HO: is also considered in successive steps when internal isomerization reactions (RO2~QOOH) lead to the formation of a a-hydroxy-hydroperoxyalky] radical, and its successive interactions with Os undergo similar pathways, as summarized in Figure 7.
Table 3: Kinetic parameters of the lumped oxidation reactions of n-butanol and n-pentanol (units are mol, cm, s, cal). Beyond the kinetic details of Table 3 it is possible to explain the higher reactivity of n-pentanol (Figure 2) through some rather simple observations. Beside the higher rate of consumption due to H- abstraction reactions by OH, the longer alkane-like carbon chain Page 6 of 17
Table 3: Kinetic parameters of the lumped oxidation reactions of n-butanol and n-pentanol (units are mol, cm, s, cal). Beyond the kinetic details of Table 3 it is possible to explain the higher reactivity of n-pentanol (Figure 2) through some rather simple observations. Beside the higher rate of consumption due to H- abstraction reactions by OH, the longer alkane-like carbon chain Page 6 of 17
Figure 10: Normalized ketohydroperoxide mole fraction (x/xo,fue) for n-butanol and n-pentanol stoichiometric mixtures at T.~830-840 K and P.~10 bar. This enhanced isomerization step translates into a more effective low temperature branching for n-pentanol, highlighted in terms of normalized ketohydroperoxides mole fraction in Figure 10. The ignition delay time also corresponds to the time at which complete consumption of ketohydroperoxides occurs.
Figure 10: Normalized ketohydroperoxide mole fraction (x/xo,fue) for n-butanol and n-pentanol stoichiometric mixtures at T.~830-840 K and P.~10 bar. This enhanced isomerization step translates into a more effective low temperature branching for n-pentanol, highlighted in terms of normalized ketohydroperoxides mole fraction in Figure 10. The ignition delay time also corresponds to the time at which complete consumption of ketohydroperoxides occurs.
Figure 9: A) RO2z>QOOH (solid lines), QOOH>ROz2 (dashed lines) isomerization rate constants of n-butanol (red) and n-pentanol (blue). B) equilibrium rate constant defined as kro2sq00n/Kooon>ro2- allows for enhanced isomerization steps in the low temperature oranching pathways, as reported in Figure 9. In particular, Figure 9a shows forward (solid lines) and backward (dashed lines) RO2+QOOH isomerization lumped rate constants, whose ratio results in a ~20% nigher equilibrium rate constant (Figure 9b) in n-pentanol oxidation compared to n-butanol.
Figure 9: A) RO2z>QOOH (solid lines), QOOH>ROz2 (dashed lines) isomerization rate constants of n-butanol (red) and n-pentanol (blue). B) equilibrium rate constant defined as kro2sq00n/Kooon>ro2- allows for enhanced isomerization steps in the low temperature oranching pathways, as reported in Figure 9. In particular, Figure 9a shows forward (solid lines) and backward (dashed lines) RO2+QOOH isomerization lumped rate constants, whose ratio results in a ~20% nigher equilibrium rate constant (Figure 9b) in n-pentanol oxidation compared to n-butanol.
Figure 12: Experimental and simulated ignition delay times of stoichiometric n-pentanol/O2/Ar and n-butanol/air mixtures. Thin line: Sarathy et al. mech [5]. Figure 11: Experimental and simulated ignition delay times of stoichiometric n-butanol/O2/Ar and n-butanol/air mixtures.
Figure 12: Experimental and simulated ignition delay times of stoichiometric n-pentanol/O2/Ar and n-butanol/air mixtures. Thin line: Sarathy et al. mech [5]. Figure 11: Experimental and simulated ignition delay times of stoichiometric n-butanol/O2/Ar and n-butanol/air mixtures.
Figure 11 compares experimental ignition delay time [60-63] for stoichiometric butanol/O2/Ar and butanol/air mixtures with kinetic model predictions, over a wide range of temperature and pressures. In a similar way, Figure 12 compares experimental [38, 64, 65] and calculated ignition delay time for n-pentanol. The kinetic mechanism is able to predict temperature and pressure dependence of ignition delay time for both fuels. Maximum deviations for n-butanol are within a factor of ~2 for the RCM data at 10 bar and the shock tube data at 8 bar (Figure 11, blue open squares and black open triangles). n-pentanol predictions systematically deviate from the 9 bar shock tube data [38] (Figure 12 green open diamond). Similar deviations were observed by Heufer et al. [38] and Sarathy et al. [5] (thin green line in Figure 12).
Figure 11 compares experimental ignition delay time [60-63] for stoichiometric butanol/O2/Ar and butanol/air mixtures with kinetic model predictions, over a wide range of temperature and pressures. In a similar way, Figure 12 compares experimental [38, 64, 65] and calculated ignition delay time for n-pentanol. The kinetic mechanism is able to predict temperature and pressure dependence of ignition delay time for both fuels. Maximum deviations for n-butanol are within a factor of ~2 for the RCM data at 10 bar and the shock tube data at 8 bar (Figure 11, blue open squares and black open triangles). n-pentanol predictions systematically deviate from the 9 bar shock tube data [38] (Figure 12 green open diamond). Similar deviations were observed by Heufer et al. [38] and Sarathy et al. [5] (thin green line in Figure 12).
Figure 13: Sensitivity of ignition delay times to rate constants for n-butanol- n-pentanol/air mixtures at T=650 K and p=30 bar. different carbon chain length (i.e. longer alkane-like moiety). The sensitivity coefficients reported in Figure /3 have been scaled assuming a value of -1 for the most inhibiting reaction in both cases (OH+FuelRa+H20). At such temperature the most of the low temperature reactive flux produces ketohydroperoxides (KHYP) whose decomposition provides OH radicals to the H-abstraction reactions. The abstraction on the alkane-like moiety of the molecules largely increases reactivity, allowing the onset of the low temperature branching pathway. To a lesser extent this is also observed for the H- abstraction from the B position. Clearly the H-abstraction from the a- site strongly decreases the reactivity as the only fate of such radical is to produce HO: and the corresponding aldehyde.
Figure 13: Sensitivity of ignition delay times to rate constants for n-butanol- n-pentanol/air mixtures at T=650 K and p=30 bar. different carbon chain length (i.e. longer alkane-like moiety). The sensitivity coefficients reported in Figure /3 have been scaled assuming a value of -1 for the most inhibiting reaction in both cases (OH+FuelRa+H20). At such temperature the most of the low temperature reactive flux produces ketohydroperoxides (KHYP) whose decomposition provides OH radicals to the H-abstraction reactions. The abstraction on the alkane-like moiety of the molecules largely increases reactivity, allowing the onset of the low temperature branching pathway. To a lesser extent this is also observed for the H- abstraction from the B position. Clearly the H-abstraction from the a- site strongly decreases the reactivity as the only fate of such radical is to produce HO: and the corresponding aldehyde.
Figure 14: Sensitivity of ignition delay times to rate constants for n-butanol /air mixtures p=30 bar and varying temperatures. At higher temperature (T=1200 K), the number of fuel specific reactions which are sensitive to ignition delay time determination largely decreases. In fact, the characteristic time of decomposition of fuel radicals (~10° s') does not allow for any interaction with molecular oxygen, resulting in dominant role of the Co-C2 chemistry.
Figure 14: Sensitivity of ignition delay times to rate constants for n-butanol /air mixtures p=30 bar and varying temperatures. At higher temperature (T=1200 K), the number of fuel specific reactions which are sensitive to ignition delay time determination largely decreases. In fact, the characteristic time of decomposition of fuel radicals (~10° s') does not allow for any interaction with molecular oxygen, resulting in dominant role of the Co-C2 chemistry.
Figure 15: Experimental [33] and simulated ignition delay times for TRF, n- butanol, TRF/ n-butanol (52.5 % iso-octane, 9.1 % n-heptane, 18.4 % toluene, 20.0 % n-butanol in volume), gasoline and gasoline/n-butanol in air mixtures at p=20 bar and g=1.0. Recent experimental and kinetic modelling study focused on PRF and TRF mixtures blended with butanol [33, 40]. As an additional validation target of the proposed n-butanol and n-pentanol low temperature mechanism, comparison of the POLIMI mechanism with these experimental data are reported in Figure 15 and Figure 16.
Figure 15: Experimental [33] and simulated ignition delay times for TRF, n- butanol, TRF/ n-butanol (52.5 % iso-octane, 9.1 % n-heptane, 18.4 % toluene, 20.0 % n-butanol in volume), gasoline and gasoline/n-butanol in air mixtures at p=20 bar and g=1.0. Recent experimental and kinetic modelling study focused on PRF and TRF mixtures blended with butanol [33, 40]. As an additional validation target of the proposed n-butanol and n-pentanol low temperature mechanism, comparison of the POLIMI mechanism with these experimental data are reported in Figure 15 and Figure 16.
Figure 16: Experimental [40] and simulated ignition delay times for n-butanol/ n-heptane blends (40/60 and 80/20 in mol %) in air at p=20 bar and @=0.4. Figure 17: n-butanol/ n-heptane mixture (50:50) at p= 0.3, p = 10 atm and t: 0.7 s. Experimental data (symbols) [66] and model predictions.
Figure 16: Experimental [40] and simulated ignition delay times for n-butanol/ n-heptane blends (40/60 and 80/20 in mol %) in air at p=20 bar and @=0.4. Figure 17: n-butanol/ n-heptane mixture (50:50) at p= 0.3, p = 10 atm and t: 0.7 s. Experimental data (symbols) [66] and model predictions.
Figure 18: Multi-zone model configuration [32]. The multi-zone model of HCCI engine developed and validated by Bissoli et al. [32, 67] is briefly reiterated in this Section. Figure 18 shows the multi-zone configuration, conceived according to the “onion-skin” approach firstly introduced by Komninos et al. [68].
Figure 18: Multi-zone model configuration [32]. The multi-zone model of HCCI engine developed and validated by Bissoli et al. [32, 67] is briefly reiterated in this Section. Figure 18 shows the multi-zone configuration, conceived according to the “onion-skin” approach firstly introduced by Komninos et al. [68].
Table 4: Ricardo E6 [69]. Engine characteristic and specific model parameters. 3.2 Operability and performance maps of HCCI engines The recent work of Bissoli et al. (Energy & Fuels, 2017, Submitted) focused on the experimental measurements obtained in a Ricardo E6 engine at Brunel University [69] for PRF80, PRF100 and ethanol. For the sake of comparison, the simulations performed in this study are carried out in the same engine configuration. Engine characteristics and specific model parameters are reported in Table 4. These parameters were obtained by performing simulations using 15 zones, with a crevice volume equal to 2% of the clearance volume. As already discussed [32], such number of zones constitutes a good compromise between computational efforts and the detailed description of the phenomena involved.
Table 4: Ricardo E6 [69]. Engine characteristic and specific model parameters. 3.2 Operability and performance maps of HCCI engines The recent work of Bissoli et al. (Energy & Fuels, 2017, Submitted) focused on the experimental measurements obtained in a Ricardo E6 engine at Brunel University [69] for PRF80, PRF100 and ethanol. For the sake of comparison, the simulations performed in this study are carried out in the same engine configuration. Engine characteristics and specific model parameters are reported in Table 4. These parameters were obtained by performing simulations using 15 zones, with a crevice volume equal to 2% of the clearance volume. As already discussed [32], such number of zones constitutes a good compromise between computational efforts and the detailed description of the phenomena involved.
and O;y,p is its standard deviation. Figure 19: Typical A-EGR operative map of a HCCI engine, together with critical limits of different combustion regions. Adapted from Bissoli et al. (Energy & Fuels, 2017, Submitted).
and O;y,p is its standard deviation. Figure 19: Typical A-EGR operative map of a HCCI engine, together with critical limits of different combustion regions. Adapted from Bissoli et al. (Energy & Fuels, 2017, Submitted).
Figure 20: Qualitative visualization of critical conditions limiting the operability of HCCI engines. Adapted from Bissoli et al. (Energy & Fuels, 2017. Submitted).
Figure 20: Qualitative visualization of critical conditions limiting the operability of HCCI engines. Adapted from Bissoli et al. (Energy & Fuels, 2017. Submitted).
Figure 22: Engine load maps reported as IMEP [bar]. Comparison between PRF80 and TRF/butanol mixture. Figure 22 and Figure 23 exemplify the behavior of the different classes of fuels discussed in this paper, by comparing the performance of PRF80 with the TRF/butanol mixture. As expected, Figure 22 shows that engine load increases when air and EGR ratios decrease, since stoichiometric and undiluted conditions are approached. Both fuels have similar trends in the whole range, with also similar peak values of ~2.5 bar. TRF mixture shows a wider stability, allowing to reach stable combustion at lower loads (~1 bar).
Figure 22: Engine load maps reported as IMEP [bar]. Comparison between PRF80 and TRF/butanol mixture. Figure 22 and Figure 23 exemplify the behavior of the different classes of fuels discussed in this paper, by comparing the performance of PRF80 with the TRF/butanol mixture. As expected, Figure 22 shows that engine load increases when air and EGR ratios decrease, since stoichiometric and undiluted conditions are approached. Both fuels have similar trends in the whole range, with also similar peak values of ~2.5 bar. TRF mixture shows a wider stability, allowing to reach stable combustion at lower loads (~1 bar).
Figure 21: Comparison of predicted HCCI operating limits for the different fuels. Ethanol, PRF80 and PRF100 adapted from Bissoli et al. (Energy & Fuels, 2017, Submitted). Symbols are the operating conditions at which the kinetic features of the system are compared between the different fuels. TRF/butanol surrogate showing a behavior in between ethanol and PRFs mixtures.
Figure 21: Comparison of predicted HCCI operating limits for the different fuels. Ethanol, PRF80 and PRF100 adapted from Bissoli et al. (Energy & Fuels, 2017, Submitted). Symbols are the operating conditions at which the kinetic features of the system are compared between the different fuels. TRF/butanol surrogate showing a behavior in between ethanol and PRFs mixtures.
Figure 23: Indicated thermal efficiency maps [%]. Comparison between PRF80 and TRF/butanol mixture.
Figure 23: Indicated thermal efficiency maps [%]. Comparison between PRF80 and TRF/butanol mixture.
Figure 24: Load and EGR effect on ethanol and n-pentanol ignition delay times. Constant volume batch reactor simulations, p=20 bar. The reason behind the large stability region of n-butanol and n- pentanol have to be referred to their ignition propensity. Beside the kinetic features of long chain alcohols, already discussed in Section 2, Figure 24 shows the effect of engine load and EGR on ethanol and n- pentanol constant volume batch reactor ignition delay times.
Figure 24: Load and EGR effect on ethanol and n-pentanol ignition delay times. Constant volume batch reactor simulations, p=20 bar. The reason behind the large stability region of n-butanol and n- pentanol have to be referred to their ignition propensity. Beside the kinetic features of long chain alcohols, already discussed in Section 2, Figure 24 shows the effect of engine load and EGR on ethanol and n- pentanol constant volume batch reactor ignition delay times.
Figure 26: Ignition delay times of the different fuels at A~5 and EGR=20 %, p=20 bar. The addition of butanol to the TRF mixture investigated by Agbro et al. [33] is responsible for the extension of the stable region toward lower loads. Ignition delay times at A~S and EGR=20% have been calculated for such mixture and are compared with those of ethanol, n- butanol and n-pentanol in Figure 26.
Figure 26: Ignition delay times of the different fuels at A~5 and EGR=20 %, p=20 bar. The addition of butanol to the TRF mixture investigated by Agbro et al. [33] is responsible for the extension of the stable region toward lower loads. Ignition delay times at A~S and EGR=20% have been calculated for such mixture and are compared with those of ethanol, n- butanol and n-pentanol in Figure 26.
Figure 27: Sensitivity analysis of the ignition delay times to reaction rate constants at p=20 bar. Constant volume batch reactor simulation.
Figure 27: Sensitivity analysis of the ignition delay times to reaction rate constants at p=20 bar. Constant volume batch reactor simulation.
Figure 28: Pressure traces calculated at varying ) and EGR dilution for n- butanol, n-pentanol and the TRF/n-butanol mixture.
Figure 28: Pressure traces calculated at varying ) and EGR dilution for n- butanol, n-pentanol and the TRF/n-butanol mixture.
Figure 29: Sensitivity analysis at CAD=-30.0, for the EGR20 A=3 and EGRS0O A=5 cases.
Figure 29: Sensitivity analysis at CAD=-30.0, for the EGR20 A=3 and EGRS0O A=5 cases.
Figure 31: Maps of CH20 and CO exhaust emissions [g/kWh]. Comparison between PRF80 and TRF/butanol mixture. Figure 37 compares predicted formaldehyde (CH2O) and CO emissions for PRF80 and the TRF/butanol surrogates. CH2O and CO are typical pollutants largely released when incomplete combustion phenomena occur. PRF80 produces higher emissions of formaldehyde, found to be a factor of ~2-5 higher considering the same load and EGR conditions. Also CO emissions are higher of a factor of ~2-3 in the case of PRF80. In general formaldehyde and CO emissions increase for decreasing load and increasing EGR dilution. This phenomena is largely motivated by the decreasing thermal efficiency (Figure 23), typically observed when moving toward leaner and more diluted conditions.
Figure 31: Maps of CH20 and CO exhaust emissions [g/kWh]. Comparison between PRF80 and TRF/butanol mixture. Figure 37 compares predicted formaldehyde (CH2O) and CO emissions for PRF80 and the TRF/butanol surrogates. CH2O and CO are typical pollutants largely released when incomplete combustion phenomena occur. PRF80 produces higher emissions of formaldehyde, found to be a factor of ~2-5 higher considering the same load and EGR conditions. Also CO emissions are higher of a factor of ~2-3 in the case of PRF80. In general formaldehyde and CO emissions increase for decreasing load and increasing EGR dilution. This phenomena is largely motivated by the decreasing thermal efficiency (Figure 23), typically observed when moving toward leaner and more diluted conditions.
Figure 30: Sensitivity analysis at CAD=-4.1, for the EGR20 A=3 and EGR50 =5 cases.
Figure 30: Sensitivity analysis at CAD=-4.1, for the EGR20 A=3 and EGR50 =5 cases.
Matteo Pelucchi: matteo.pelucchi@polimi.it, +39 02 02 2399 3243, Department of Chemistry, Materials and Chemical Engineering “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133, Milan, Italy.
Matteo Pelucchi: matteo.pelucchi@polimi.it, +39 02 02 2399 3243, Department of Chemistry, Materials and Chemical Engineering “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133, Milan, Italy.
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