Detailed Chemistry Promotes Understanding of Octane Numbers and

Energy & Fuels 2006, 20, 2391-2398

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Detailed Chemistry Promotes Understanding of Octane Numbers and Gasoline Sensitivity Marco Mehl,† Tiziano Faravelli,† Fulvio Giavazzi,‡ Eliseo Ranzi,*,† Pietro Scorletti,‡ Andrea Tardani,† and Daniele Terna‡ CMIC Department, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20131 Milano, Italy, and EniTecnologie S.p.A., Via Felice Maritano 26, 20097 San Donato, Milanese (MI), Italy ReceiVed July 24, 2006. ReVised Manuscript ReceiVed September 8, 2006

Detailed kinetic models of pyrolysis and combustion of hydrocarbon fuels are now reliable tools which can aid the design of internal combustion engines required to meet the increasingly stringent pollutant formation and engine efficiency standards. The aim of this paper is to discuss and verify the potential of these kinetic models in analyzing the knock related combustion behavior of hydrocarbon fuels with particular regard to octane numbers and octane sensitivity. Detailed chemistry not only helps to explain the different reactivities of alkanes and alkenes but also the combustion behavior of hydrocarbon mixtures. A two-zone model of a spark ignition engine, coupled with the detailed chemistry of combustion processes, was developed and utilized for the predictions of octane numbers. This model explains the effect of various components on the knocking behavior of the fuel under different operating conditions and is thus a useful tool both in formulating new fuels and designing new engines.

1. Introduction The engines of the future will require increasingly strictly controlled combustion environments, which are determined by complex interactions between the combustion chamber design and the chemical composition and physical and combustion properties of fuel. Therefore, kinetic modeling of fuel oxidation and combustion is then crucial to the development or improvement of key emerging engine technologies, such as, for instance, homogeneous charge compression ignition (HCCI) engines. It is generally agreed that reliable databases and chemical kinetic models are required to develop applications of chemical kinetics coupled with computational fluid dynamics capable of simulating combustion processes realistically.1-2 Although gasolines and liquid hydrocarbon fuels in general have to meet specifications defined by bulk properties, they are complex mixtures of large molecules, made up of hundreds of constituents. The same specifications can be met by a large variety of hydrocarbon mixtures, although the relative amount of different species is constrained by the property requirements. For reproducibility reasons, it is useful to select carefully defined mixtures of reference species with fixed chemical compositions (surrogates) that describe the major characteristics of real fuels.3 Despite this complexity, the antiknock quality or propensity of the fuel is indicated very simply by the octane number (ON) * Author to whom correspondence should be adressed. E-mail: [email protected]. Phone: #39 02 2399 3250. Fax: #39 02 706 38173. † Politecnico di Milano. ‡ EniTecnologie S.p.A. (1) Westbrook, C. K.; Mizobuchi, Y.; Poinsot, T. J.; Smith, P. J.; Warnatz, J. Proc. Combust. Inst. 2005, 30, 125-157. (2) Hughes, K. J.; Griffiths, J. F.; Fairweather, M.; Tomlin, A. S. Evaluation of models for the low temperature combustion of alkanes through interpretation of pressure-temperature ignition diagrams. Phys. Chem. Chem. Phys. 2006, 8, 3197-3210. (3) Hudgens, J. W., Ed. Workshop on Combustion Simulation Databases for Real Transportation Fuels. NIST, Gaithersburg, Maryland, September 4-5, 2003.

which significantly affects engine performances.4-5 Blends of the two primary reference fuels (PRFs) n-heptane (ON ) 0) and iso-octane (ON ) 100) define the intermediate points on the scale of research and motor octane numbers (RON and MON). The octane sensitivity S is defined as the difference between RON and MON:

S ) RON - MON

(1)

S is a measure of “octane depreciation” at high speeds, the tendency of gasolines to exhibit antiknock behavior that becomes increasingly worse than that of the PRF corresponding to the RON as the engine speed increases. By definition, PRFs have zero sensitivity, while most practical fuels have S > 0. The higher the octane number, the better the antiknock quality of the fuel and the higher the potential to improve power and acceleration performance.6-7 With the aim of classifying the differences between the two octane numbers, Kalghatgi introduced an empirical parameter K, a weight coefficient applied to S, measuring the “real badness” of S under a given engine and operating condition:

Octane Index ) RON - KS ) (1 - K)RON + KMON (2) Detailed kinetics and engine simulations allow a clearer understanding of the physical meaning of these parameters and the octane numbers. They also help clarify the differences and similarities between different hydrocarbon fuels. The natural result of this kinetic analysis should be a better understanding (4) Edgar, G. Ind. Eng. Chem. 1927, 19, 145. (5) Leppard, W. R. A Comparison Of Olefin And Paraffin Autoignition Chemistry: A Motored Engine Study; Paper #892081, SAE: Baltimore, MD, 1989. (6) Kalghatgi, G. Fuel Anti-Knock Quality; Paper No. 01-3584, SAE: Warrendale, PA, 2001. (7) Kalghatgi, G. Fuel Autoignition quality of practical fuels and implications for fuel requirements of future SI and HCCI engines; Paper No. 01-0239, SAE: Warrendale, PA, 2005.

10.1021/ef060339s CCC: $33.50 © 2006 American Chemical Society Published on Web 10/18/2006

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of the effect of specific components and a greater ability to match fuel formulation with engine octane requirements. 2. Detailed Kinetic Models: Low and High Temperature Mechanisms Detailed kinetic schemes of pyrolysis and oxidation of hydrocarbon fuels were developed by taking advantage of their very useful hierarchical modularity characteristic.8 The core of the model consists of a detailed submechanism for C1-C4 species, while the modular structure allows the progressive extension of the scheme to new species and/or the study of pollutant formation simply by introducing new reactions and submechanisms. On the basis of analogy rules for similar reactions, only a few fundamental kinetic parameters are needed to extend the scheme to heavier species. These intrinsic rate parameters define the classes of primary propagation reactions, appropriate to the different temperature ranges. The main peculiarity of these detailed kinetic schemes is that they cover a wide range of operating conditions of hydrocarbon oxidation, particularly the high and low temperature regions.9 At temperatures above 1000 K, the decomposition reactions of hydrocarbon fuels and alkyl radicals (R•) largely prevail on the different reaction paths. The transition between low and high temperature mechanisms is determined by the equilibrium reaction:

R• + O2 T ROO• At temperatures below 850 K, the direct addition of O2 to the alkyl radical is favored and the resulting peroxy radical undergoes a branching reaction path that ultimately results in the formation of ketohydroperoxydes. These very unstable intermediates rapidly form two radicals with unimolecular decomposition reactions. The propagation reactions of fuel consumption occur via H-atom abstraction primarily by OH• below 700 K, while at higher temperatures HO2• radicals also play a significant role. The shift in equilibrium is the reason for the existence of an NTC region (negative temperature coefficient, i.e., the region where conversion decreases as the temperature increases). This reduction in reactivity is due both to the reduced importance of the branching reactions of peroxy radicals and, also, to the greater formation of HO2• radicals which are considerably less reactive than OH•. The same mechanism applies to normal and branched alkanes. The different reactivity of n-heptane and iso-octane can be explained quite simply on the basis of their molecular structures. If the corresponding peroxy radicals are compared, it is clear that the isomerization reactions of linear peroxy radicals are faster, when compared to the branched structure of peroxy radicals derived from iso-octane. This detailed kinetic scheme has been widely tested over many years using a large amount of experimental data referring to ideal reactors such as rapid compression machines (RCM), shock tubes (ST), and batch and flow reactors,10-11 and it is (8) Westbrook, C. K.; Dryer, F. L. Proc. Comb. Inst. 1980, 18, 749767. (9) Ranzi, E.; Dente, M.; Goldaniga, A.; Bozzano, G.; Faravelli, T. Prog. Energy Combust. Sci. 2001, 27, 99. (10) Ranzi, E.; Gaffuri, P.; Faravelli, T.; Dagaut, P. A Wide Range Modeling Study of n-Heptane Oxidation. Combust. Flame 1995, 103, 91106. (11) Ranzi, E.; Faravelli, T.; Gaffuri, P.; Sogaro, A.; D’Anna, A.; Ciajolo, A. A Wide-Range Modeling Study of Iso-Octane Oxidation. Combust. Flame 1997, 108, 24-42.

Figure 1. (a) Ignition delay times of iso-octane in a RCM at P ) 12-42 bar.14 (b) Ignition delay times of a surrogate gasoline (n-heptane: i-octane:toluene ) 17:63:20% vol) at 20 and 55 bar.15

being continuosly upgraded on the basis of new experimental measurements.12-13 To provide a simple example of comparisons between experimental and predicted results, Figure 1a and b reports some comparisons of typical ignition delay times in shock tube experiments. Figure 1a refers to iso-octane oxidation in a pressure range between about 10 and 40 bar,14 while ignition delay times of a surrogate gasoline (n-heptane:iso-octane:toluene ) 17:63:20% vol) at 20 and 55 bar are reported in Figure 1b.15 The whole kinetic scheme is able to simulate the combustion behavior of real transportation fuels gasolines and jet fuels, with the use of several reference components.16 The complete scheme, including low and high temperature mechanisms, is available in CHEMKIN format on the web [www.chem.polimi.it/creckmodeling], with thermodynamic data and transport properties. (12) Mehl, M.; Lucchini, T.; D’Errico, G.; Onorati, A.; Giavazzi, F.; Scorletti, P.; Terna, D.; Faravelli, T.; Ranzi, E. Experimental and kinetic modeling study of octane number and sensitiVity of hydrocarbon mixtures in CFR engines; Paper 24-077, SAE-NA: Warrendale, PA, 2005; pp 1-10. (13) Mehl, M.; Faravelli, T.; Ranzi, E.; Lucchini, T.; Onorati, A.; Giavazzi, F.; Scorletti, P.; Terna. D. Kinetic Modeling of Knock Properties in Internal Combustion Engines; SAE: Toronto, 2006. (14) Minetti, R.; Carlier, M.; Ribacour, M.; Therssen, E.; Sochet, L. R. Comparison Of Oxidation And Autoigition Of The Two Primary Reference Fuels By Rapid Compression. Proceedings of the Twenty-Sixth Symposium (International) on Combustion, The Combustion Institute: 1996; pp 747753. (15) Gauthier, B. M.; Davidson, D. F.; Hanson, R. K. Shock tube determination of ignition delay times in full-blend and surrogate fuel mixtures Combust. Flame 2004, 139, 300-311. (16) Ranzi, E. A wide range kinetic modeling study of oxidation and combustion of transportation fuels and surrogate mixtures. Energy Fuels 2006, 20, 1024-1032.

Understanding Octane Nos. and Gasoline SensitiVity

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Table 1. OH• Addition Reactions on n-Hexenes and Waddington Mechanism to Form Aldehydes

Gasolines and liquid fuels are commonly obtained as a blend of refinery streams and can include large quantities of alkenes. Moreover, a significant quantity of alkenes is present as intermediates in the combustion of saturated species. The oxidation mechanism of alkenes always involves the initial formation of alkenyl radicals and their successive oxidations with the possible degenerate branching path, resulting in the formation of peroxide radicals and ketohydroperoxide species. Two major differences can be highlighted in respect to the alkane oxidation mechanism. The first is the formation of very stable allyl or allyl-like radicals, while the second relates to the possible addition of propagating radicals, with a scavenging effect due to the presence of double bonds. As a consequence of their relative stability, allyl radicals can reach high concentrations and are available for oxygen addition and/or recombination reactions. Moreover, the O2 addition needs to overcome the radical stability and the parent allyl-peroxy radicals to decompose in a favored way. On this basis, the ceiling temperature shifts toward lower temperatures and the low temperature mechanism becomes less important. Furthermore, it is mostly the OH• addition reactions together with the successive O2 addition that form an OH• radical and two aldehydes, via the Waddington mechanism. To provide an example of this, the different OH• addition reactions on n-hexene isomers are reported in Table 1. Various aldehydes, ranging from formaldehyde to pentanal, are the final and net products of this mechanism. The same products could also be obtained through the four center molecular addition of O2 on the double bond. For instance, the addition reactions on butenes form the three different aldehydes:

Rate parameters for these reactions are k ) 1011 exp(-39 000/ RT) [m3/(kmol s)]. In agreement with previous kinetic analysis,17-19 radical addition reactions prevail over the corresponding H-abstraction (17) Minetti, R.; Roubaud, A.; Therssen, E.; Ribaucour, M.; Sochet, L. R. Combust. Flame 1999, 118, 213-220. (18) Prabhu, S. K.; Bhat, R. K.; Miller, D. L.; Cernansky, N. P. Combust. Flame 1996, 104, 377. (19) Wilk, R. D.; Cernansky, B. P.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 1989, 77, 145-170.

reactions mainly for light alkenes (such as ethylene and propylene), due to the difficulty of isomerization of the allylperoxy radical. Figure 2 shows a detailed reaction flux analysis of 1-pentene oxidation at 650 K with the indication of the relative weights of the major reaction paths. The OH• addition reaction accounts for about 35% of the overall fuel consumption. As a matter of simplifications, successive isomerizations and O2 additions with the possible formation of ketohydroperoxides bearing an alcohol function are not reported. A 1-penten3-yl radical (38%) prevails in the H-abstraction reactions. As a combined result of the successive O2 additions, decomposition, and isomerization reactions, only about 30% of the fuel consumption moves through the branching paths to form ketohydroperoxide species, at this low temperature. Moreover, H• and HO2• addition reactions to form n-pentyl and alkylhydroperoxy radicals are not reported due to their very limited importance under these conditions. The NTC region of alkenes is clearly less intense than that of the alkane oxidation mechanism. 3. Comparison between Alkane and Alkene Oxidation Comparing the reactivity of n-pentane and 1-pentene highlights further interesting features of their relative combustion behavior.20 Figure 3 shows the different reactivity of n-pentane and 1-pentene oxidation, by comparing the typical ignition delay times in a RCM at two different pressures (7 and 37 bar). These model predictions agree quite well with the experimental measurements of Minetti et al.,17 as already discussed in previous papers. In order to complete this analysis, the predicted ignition delay times are also reported for a shock tube (ST) device operated at higher temperatures and at the same pressures. At low temperatures, 1-pentene exhibits a very weak NTC and its autoignition is slower than that of n-pentane. At higher temperatures, on the other hand, alkene reactivity increases and the corresponding autoignition becomes faster. This is due to the weaker CsC bond in the allylic position and to the fact that the unimolecular decomposition of 1-pentene is (20) Faravelli, T.; Gaffuri, P.; Ranzi, E.; Griffiths J. F. Fuel 1998, 77 (3), 147-155. (21) ASTM 1991 Physical Constants of Hydrocarbon and NonHydrocarbon Compouns; ASTM Data Series DS 4B; ASTM: PCN:05004020-12.


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Figure 2. Low temperature oxidation of 1-pentene at Φ ) 1, 650 K, 1 bar, and 25% fuel conversion (the percentages indicate the relative importance of the different reaction paths).

Figure 4. RON (solid lines) and MON (dashed lines) of alkanes and 1-alkenes.21 Figure 3. Ignition delay times of n-pentane and 1-pentene oxidation in an RCM and in an ST device (Φ ) 1, p ) 7-37 bar).

easier than n-pentane decomposition:

1-C5H10 f C3H5• + C2H5• k ) 1 × 1016 exp(-72 000/RT) [s-1] n-C5H12 f 1-C3H7• + C2H5• k ) 1 × 1017 exp(-81 000/RT) [s-1] Finally, at very high temperatures (T > 1500 K), alkenes turn out to be slightly less reactive than alkanes at both the investigated pressures. This behavior at high temperatures is explained quite simply by the H/C ratio of the different fuels which leads to a higher H radical concentration in the system. Therefore, this difference vanishes with heavier fuels. Similarly, the lower reactivity of alkenes at low temperatures is less relevant when the number of C-atoms in the hydrocarbon chain increases. In fact, the scavenging effect of the double bond is more pronounced for relatively short alkenes; when the length of the alkyl chain increases, the relative influence of the radical

additions diminishes. This trend is consistent with the octane number which rapidly decreases for heavier hydrocarbon fuels. The different reactivity and knocking propensities of alkanes and alkenes has already been presented and discussed by Leppard.5,22 As Figure 4 indicates, the octane numbers of 1-pentene and 1-hexene are higher than that of the corresponding alkanes, due to the lower reactivity of alkenes at low temperatures. Moreover, alkenes are characterized by a high octane sensitivity (S ) RON - MON).23 The slow reactivity in the low temperature range reduces the knock propensity of such compounds, particularly under RON conditions, where operating temperatures are lower. This characteristic makes light alkenes interesting candidates as components for new formulated gasolines. The 1-alkenes are clearly octane sensitive, and this sensitivity decreases for larger species. Thus, larger alkenes have similar RON and MON values. (22) Leppard, W. R. The Autoignition Chemistry of Primary Reference Fuels, Olefin/Paraffin Binary Mixtures, and Non Linear Octane Blending; #922325, Detroit, MI, 1992. (23) Pitz, W. J.; Westbrook, C. K.; Leppard, W. R. Paper No. 912315, SAE: Warrendale, PA, 1991.


Figure 5. (a) Temperature profiles [K] vs time [s] of different fuels and PRF mixtures in an RCM (Φ ) 1, 775 K, and 7 bar). (b) Total ignition delay times [ms] of different fuels and PRF mixtures vs temperature [K] (RCM at Φ ) 1, 7 bar).

In order to further clarify the differences between alkanes and alkenes and to provide a clearer illustration of the meaning of octane numbers, Figure 5 shows the typical temperature profiles of different fuels in a rapid compression machine at 775 K and 7 bar. The temperature profiles of the stoichiometric oxidation of n-pentane (RON = MON = 62) and 1-pentene (RON ) 90.9; MON ) 77.1) are compared with those of the PRF mixtures. Two different ignition delay times are evident for all the curves. The first one relates to the cool flames or to the low temperature mechanism, while the second one is the total ignition delay time. This first ignition time is lowest for all the PRFs, and this delay is mainly ruled by n-heptane. The less reactive iso-octane slows the overall reactivity of the PRF mixture. The low temperature mechanism of n-pentane is slower, due to the relatively reduced isomerization rate of pentyl-peroxy radicals. As already observed in Figure 3, the reactivity of 1-pentene at low temperatures is inhibited to some extent by the scavenging effect of the double bond inside the carbon chain. Consequently, the cool flame ignition of both pure fuels is delayed as compared to the corresponding PRFs. The total delay time of pure n-pentane, on the other hand, is the shortest one, while the total delay time of 1-pentene is the highest. This is a first evidence of the limits of octane numbers in properly rating the combustion behavior of pure fuels: PRF mixtures are good surrogates only under specific operating conditions. Figure 5b illustrates this fact even more effectively. The predicted ignition


delay times of the different fuels in the RCM display very similar behaviors for alkanes and PRFs across the whole temperature range but differ significantly for 1-pentene. Similar differences were already experimentally observed and discussed by Griffiths et al.24 This inadequacy of analyzing the combustion behavior of pure fuels and PRF mixtures in simple, ideal reactor devices provides the driving force for new research into improving the simulations of the combustion device on the one hand and producing a more complete definition of surrogate mixtures for gasolines and heavier fuels3 on the other. A better gasoline surrogate can be obtained by adding an aromatic (toluene) and/or an alkene (1-pentene) to the PRF mixture to achieve a more accurate determination of the octane numbers.25-27 In fact, certain difficulties associated with the use of the octane numbers have been recognized: octane numbers in gasoline do not obey to a linear blending law.28 In addition to this, as already observed, the octane sensitivity varies from one class of hydrocarbons to another, alkanes having a lower sensitivity than alkenes and aromatics.6 For these reasons too, the primary propagation reactions of pentane, hexane, and heptane isomers were studied using detailed kinetic schemes in order to more accurately characterize their ignition delay time and the combustion behavior of more complex mixtures.1,29 More recently, also light cyclo-alkanes received increasing attention, both from the experimental and from the kinetic modeling point of view.30-32 In parallel and complementary to this research activity, the combustion behavior of pure fuels and surrogate mixtures in engines also requires in-depht investigation plus more accurate simulation of the research engines in which the octane numbers are experimentally determined. Research and motor octane numbers (RON and MON) are obtained in a single cylinder CFR engine, according to ASTM procedures D2699 and D-2700, respectively. 4. Simulation Model of the Research Engines: RON and MON Evaluations Octane numbers can be predicted on the basis of detailed chemistry and simplified two-zone models.33 The engine simulations are performed using the GASDYN simulation code whose main features are discussed elsewhere.34-36 The (24) Griffiths, J. F.; Halford-Maw, P. A.; Mohamed, C. Combust. Flame 1997, 111 (4), 327-337. (25) Cathonnet, M. Advances and challenges in the chemical kinetics of combustion. Proceedings of the European Combustion Meeting, 2003. (26) Edwards, T.; Maurice, L. Q. J. Propul. Power 2001, 17 (2), 461466. (27) Davidson, D. F.; Gauthier, B. M.; Hanson, R. K. Proc. Combust. Inst. 2005, 30, 1175-1182. (28) Bradley, D.; Morley, C. Low temperature combustion and ignition. In ComprehensiVe chemical kinetics; Compton, G.G., Hancock, G., Pilling, M.J., Eds.; Elsevier: Amsterdam, 1997; Vol. 35, 661-749. (29) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K.; Leppard, W. R. Twenty-sixth Symposium (International) on Combustion, The Combustion Institute: Pittsburgh, PA, 1996; p 2669. (30) El Bakali, A.; Braun-Unkhoff, M.; Dagaut, P.; Frank, P.; Cathonnet, M. Proc. Combust. Inst. 2000, 28, 1631-1638. (31) Granata, S.; Faravelli, T.; Ranzi, E. A wide range kinetic modeling study of the pyrolysis and combustion of naphthenes. Combust. Flame 2003, 132 (3), 533-544. (32) Cavallotti, C.; Rota, R.; Faravelli, T.; Ranzi, E. Ab initio quantum chemistry evaluation of primary cyclohexane oxidation reaction rates. Presented at the Symposium (International) on Combustion, 2006, in press. (33) Hajireza, S.; Mauss, F.; Sunden, B. Hot-spot auto-ignition in spark ignition engines. Proc. Combust. Inst. 2000, 28, 1169-1175. (34) Onorati, A.; D’Errico, G.; Ferrari, G. 1D Fluid Dynamic Modeling of Unsteady Reacting Flows in the Exhaust System with Catalytic Converter for S.I. Engines. SAE Trans., J. Fuel Lubricants 2001; no. 2000-01-0210.


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Figure 7. Pressure and temperature curves in a CFR engine, before ignition: RON (solid lines) and MON (dashed lines) conditions.

Figure 6. Experimental (solid line) and calculated (dashed line) critical compression ratio of PRFs in a CFR engine: (a) RON conditions, (b) MON conditions.

in-cylinder thermodynamic processes are described through a quasi-D approach. Computations were carried out applying the conservation of mass and energy over the cylinder volume which was assumed to be a single open system. The combustion process is decribed by using a two-zone model and by assuming the flame front to be an ideal surface which separates unburned and burnt gases. The burning rate is determined by means of a fractal approach, by assuming the minimum flame wrinkling to be proportional to the Kolmogorov scale and the maximum one proportional to the integral length. The flame growth is fully laminar as long as the flame radius is lower than the typical eddy turbulent length scale, while the flame front velocity is entirely governed by turbulence when the flame radius becomes greater than the Taylor microscale of turbulence. A detailed kinetic mechanism describes the low temperature reactions in the fresh mixture during the compression and autoignition phases, while a simple equilibrium approach is adopted in the burnt zone. Needless to say, the heat released affects the temperature of the unburnt gas and the evolution of the in-cylinder pressure. This coupled model was applied to the simulation of a CFR engine to evaluate octane numbers. Figure 6 compares the predicted and experimental curves of the critical compression ratio (i.e., the compression ratio (CR) at knock onset) under RON and MON conditions for the primary reference fuels (PRFs). The predicted values were obtained using the unburned charge fraction at knock onset as a knock index. In fact, the (35) D’Errico, G.; Ferrari, G.; Onorati, A.; Cerri, T. Modeling the Pollutant Emissions from a S.I. Engine. SAE Trans.; no. 2002-01-0006. (36) Ferrari, G.; Onorati, A.; D’Errico, G.; Cerri, T.; Montenegro, G. An Integrated Simulation Model for the Prediction of S.I. Engine Cylinder Emissions and Exhaust After-Treatment System Performance. Proceedings of the ICE 01 SAE Conference, Capri, Italy, Sept 2001.

Figure 8. Reactivity maps of n-pentane and 1-pentene (panels a and b): air combustion in an RCM at Φ ) 1. The values in the map indicate the total ignition delay times [s].

procedure for the empirical estimation of octane number is based on the knock meter intensity, which is difficult to reproduce using a quasi zero-D two-zone model. In this case, 30% of the residual charge at autoignition is assumed to be heavy knock. The MON and RON curves match the trend of an ASTM critical compression ratio quite well, the general agreement is satisfactory, and the model correctly predicts the knock propensity of PRFs in engine conditions. Typical pressure and temperature curves in CFR engines, just before ignition, are reported in Figure 7. The CFR engine operates at higher temperatures and lower pressures under MON conditions than under RON conditions. As already reported and discussed in Figure 2, n-pentane and 1-pentene differ significantly mainly in the low temperature and NTC region. The reactivity of 1-pentene increases more regularly with a temperature and pressure increase. Figure 8 shows the reactivity maps, in terms of the typical induction times



Figure 9. Reactivity maps of the two surrogate mixtures of Table 2: air combustion in an RCM at Φ ) 1. The values in the map indicate the total ignition delay times [s]. Table 2. Main Properties, Boiling Range, and Composition of Surrogate Mixtures density (15 °C) reid vapour pressure RON MON IBP 10% 50% 90% FBP n-heptane iso-octane toluene MTBE di-isobutylene

Kg/m3 kPa °C °C °C °C °C vol % vol % vol % vol % vol %

surrogate 1

surrogate 2

.7572 21.0 97.3 89.2 66 89 99 102 108 13 42 32 13

.7504 19.5 97.3 86.6 76 89 99 102 109 19 24 26 13 18

Figure 10. (a) Predicted octane requirement (OR) vs engine speed [rpm] (Fiat Lancia 1200, 16 v). (b) BON curves of the two surrogates compared to the OR curve (Fiat Lancia 1200, 16 v).

for different pressures and temperatures, both for n-pentane and for 1-pentene. A similar analysis of knock propensity of different fuels on the basis of ignition delay maps was also presented by Yates et al.37 These total ignition delay times are evaluated under the typical rapid compression machine conditions for stoichiometric air combustion by using the detailed kinetic scheme. According to the experimental measurements, the NTC region of n-pentane moves from about 800-900 K at 10 bar to about 850-950 K at 40 bar. The ignition delay of 1-pentene decreases more regularly from 10 to 1 ms when temperature and pressure increase. The operating pressure-temperature curves of RON and MON conditions are also reported in the same figure. The MON curve clearly moves inside the NTC region at higher temperatures, and thus, the temperature increase does not correspond to an increase of system reactivity. The situation is quite different, however, for 1-pentene reactivity where the higher temperatures associated with MON conditions significantly reduce the ignition delay times. This makes the meaning of the differing sensitivities of the two fuels clearer. RON and MON remain very similar for alkanes, due to the influence of the NTC region and to the combustion behavior similar to that of PRFs. The sensitivity of alkenes is explained simply by the

Figure 11. Operating p-T curves of the Fiat Lancia and of a turbocharged engine in the reactivity maps of the two surrogates: air combustion in an RCM at Φ ) 1. The values in the map indicate the total ignition delay times [s].

(37) Yates, A. D. B.; Swarts, A.; Viljoen, C. Correlating auto-ignition and knock limited spark adVance data for differet types of fuel; Paper No. 01-2083, SAE: Warrendale, PA, 2005.

shorter ignition delay corresponding to the higher temperatures of MON conditions.


Commercial gasolines have RON > MON, i.e., they display a positive octane sensitivity. Sensitivity is usually higher in gasolines containing alkenes. The presence of unsaturated components inside the hydrocarbon mixtures decreases the NTC effect and makes the fuel more sensitive to engine conditions. Table 2 reports some relevant characteristics of two surrogate gasolines having the same RON ) 97.3. Both fuels contain n-heptane, isooctane, toluene, and methyl-tertiary-butyl-ether (MTBE), and only surrogate 2 contains 18 vol % of diisobutylene:2,4,4-trimethyl-1-(and 2-)pentene. These surrogate mixtures were formulated and tested in standard CFR engines in Enitecnologie laboratories. Surrogate 2 shows a higher sensitivity (10.7 vs 8.1), due to the alkene presence, as expected. Once again, this fact can be explained using the reactivity maps, which are calculated from detailed chemistry. Figure 9 confirms that the NTC region, which is more evident for surrogate 1, becomes less significant due to the presence of di-isobutylene in surrogate 2. 5. Assessment of the Bench Octane Numbers (BON) of Surrogate Mixtures in a Multicylinder Automotive Engine. The scope of our analysis has been extended to the assessment of the antiknock behavior of the two surrogate mixtures in a commercial automotive engine (Fiat Lancia 1200, 16 v) under full load conditions.13 Essentially the same coupled model (engine and detailed chemistry) was used. However, in this case, the engine part (GASDYN) of the model was configured to simulate a Lancia engine rather than the CFR. Figure 10a reports the predicted octane requirement (OR) of the engine at different engine speeds, obtained with default engine specifications. The OR was obtained by identifying the PRF which had the maximum ON still knocking. The typical shape of this curve shows a well-defined slope variation at about 4000 rpm, where the maximum volumetric efficiency is achieved. At high rpms, the OR decreases, due to the shorter and shorter residence times. The OR curve is then compared with the corresponding curves of the bench octane numbers (BON) of the two surrogates, as shown in Figure 10b. These curves are obtained (in much the same way as in octane rating using the CFR engine) by identifying the PRF mixture that shows the same knock propensity (as measured by the percent residual charge at knock onset) of the surrogate mixture, under each operating condition. Like Kalghatgi’s “octane index”, BON is a measure of the “real” antiknock value of a fuel under the relevant operating conditions rather than in a standard RON or MON procedure. The general slope of the BON curve of the alkene containing surrogate 2 is higher than that of surrogate 1. Significantly, the BON curve of surrogate 2 matches the engine OR better than that of surrogate 1 due to the higher BON at low speed. In fact, the higher knock propensity at high speed is not a limiting factor (in spite of the “octane depreciation” undergone by both fuels) due to the very low octane requirement of the engine. In other words, at high speeds the fuel containing olefins retains “enough

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octane” to satisfy the engine’s octane requirement. In any case, a marginal octane unbalance at high speed in modern engines equipped with knock sensors is probably not a problem. To complete this analysis, typical p-T curves of the Lancia engine at 2000 and 5000 rpm are reported in the reactivity map of the two surrogates in Figure 9. At 5000 rpm, the octane requirement clearly decreases and the operating curve moves toward more severe conditions, due to the very short cycle times. As a final example, Figure 11 compares the operating p-T curve obtained by slightly supercharging the engine at 2000 rpm with the previous p-T curves. In this case, the operating conditions move to lower temperatures and higher pressures. The presence of a weak NTC region makes surrogate 2 a better option than surrogate 1. According to Kalghatgi,7 in modern engines, at low speed and especially with supercharged engines, the value of K can become very low (even negative, as can be seen in eq 2) for gasolines containing alkenes: a low K value means that the RON is dominating the antiknock behavior of the fuel. As shown in Figure 11, the operating curve of the supercharged engine is very close to (actually “beyond”) the RON one and the MON value does not affect significantly the actual knock propensity of the fuel under these conditions. 6. Conclusions This work presents a general purpose model for knocking prediction in SI engines which couples a quasi zero-D thermofluidynamic engine model (GASDYN code) with a general detailed kinetic scheme for the prediction of autoignition behavior of hydrocarbon fuels. The resulting mechanistic model is reliable and capable of correctly predicting the octane numbers, both the RON and the MON, of simple hydrocarbons mixtures, by simulating their behavior in the CFR engine. Furthermore, the coupled model is able to reproduce the qualitative antiknock behavior of hydrocarbon mixtures by surrogating real gasolines in a modern engine and, in particular, the following: • the decrease in octane requirement with engine speed and the typical shape of the octane requirement curve becoming steeper beyond maximum torque speed; • octane depreciation with engine speed of gasolines, which is more indicative of the real behavior than conventional sensitivity; • improved antiknock behavior at low speeds of olefins, related to a low or even negative value of Kalghatgi’s K, measuring the real badness of conventional sensitivity. This model does explain the effect of different components on the knocking behavior of the fuel under different operating conditions and may prove to be a useful tool in both formulating new fuels and designing new engines. Acknowledgment. The authors are indebted to Prof. A. Onorati and T. Lucchini for providing the GASDYN model for the engine evaluations. The authors also acknowledge the financial support of Enitecnologie. EF060339S

Detailed Chemistry Promotes Understanding of Octane Numbers and

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