Pre-ignition and super-knock in turbocharged spark-ignition engines

Transcription

1 Original Article Pre-ignition an super-knock in turbocharge spark-ignition engines International J of Engine Research () 6 Ó Shell Research Limite Reprints an permissions: sagepub.co.uk/journalspermissions.nav DOI:.77/ jer.sagepub.com Gautam T Kalghatgi * an Derek Braley Abstract Earlier stuies of pre-ignitions at hot surfaces are first reviewe. The concept of a critical raius of a hot pocket of gas, closely relate to the laminar flame thickness, that is necessary to initiate a propagating flame, has been use successfully to preict relative tenencies of ifferent fuel air mixtures to pre-ignite. As the mixture is compresse, the thickness of potential laminar flames ecreases, an when this becomes of the orer of the thermal sheath thickness at the hottest surface, pre-ignition can occur there, creating a propagating flame. Measure engine pre-ignition ratings are shown to correlate well with laminar flame thicknesses. Preictions are mae concerning the effects of changes in intake temperature an pressure on the pre-ignition of ifferent fuels. A growing current concern is occasional gas-phase, autoignitive, pre-ignitions that can occur in turbo-charge engines, giving rise to very severe autoignition an knock. It is conclue from the evience of engine pressure recors an autoignition elay times of the mixtures that such pre-ignitions have not arisen from autoignition of the fuel, but of a mixture with a smaller autognition elay time than stoichiometric n-heptane air. One possibility is that autoignition occurs at hot spots containing some lubricating oil. It is shown that such pre-ignitions, particularly with catalytic enhancement, coul initiate a propagating flame, rather than autoignitive propagation. In the later, much more severe autoignition arising after pre-ignition, autoignitive propagation velocities at a hot spot are estimate from compute values of the ignition elay times an assume reactivity graients in the fuel air mixture at the hot spot. The severity of the associate pressure pulse is epenent upon the ratios j, of the acoustic spee to the localise autoignitive velocity, an e, of the resience time of the acoustic wave in the hot spot to the short excitation time in which most of the chemical energy is release. The regime in which a localise etonation can be generate in the hot spot is efine by a peninsula on a plot of j against e. A locus is plotte on this figure corresponing to the growing measure intensities of the engine knock as the pressure increases. This is base on compute autoignition elays an excitation times, for an appropriate surrogate fuel. These changes are characterise by j tening to unity an e tening to ever-higher values, with increasingly intense, localise, eveloping etonations. Keywors Pre-ignition, turbo-charging, autoignition, critical size, flame thickness, hot spot, knock, pressure waves, etonation Date receive: 3 February ; accepte: 7 October Introuction At present, the evelopment of irect injection sparkignition (SI) engines focuses on ownsizing, with the reuction in engine isplacement an the use of turbocharging to give better control of the air flow over the full power range, as a means of improving efficiency. Pre-ignition becomes more prevalent in such engines, creating a possible limiting factor to ownsizing. It is akin to uncontrolle spark avance an, because of the alreay increase pressure from turbo-charging, it can lea to particularly severe an amaging autoignition an knock. It can be a rare event, with as many as 3, normal cycles between pre-ignitions. Problems with pre-ignition initially arose in the 95s, following improvements in octane numbers (ONs) an increases in compression ratios. Pre-ignitions were shown to be initiate at hot combustion chamber eposits, spark plugs an valves. 3 This earlier start to the combustion further increase the pressure an temperature of the unburne gas ahea of the Shell Global Solutions (UK), UK * Now at Saui-Aramco, Dhahran, Saui Arabia Corresponing author: Derek Braley, Shell Technology Centre, Chester CH 3SH D.Braley@lees.ac.uk

2 International J of Engine Research () avancing flame front, an the tenency of the en gases to autoignite. The increase pressure uring the compression stroke coul also lea to excessive negative work, loss of power an an increase rate of heat transfer to soli surfaces that might further intensify pre-ignition. 4 This coul cause a single-cyliner engine to stop, but with multi-cyliner engines, the other cyliners continue to operate. However, the high pressure an temperature evelope by pre-ignition coul cause the failure of the piston or the connecting ro, as shown in Figure 6. of Ricaro an Hempson. 4 Downs an Pigneguy 5 evelope an engine-base technique to compare the influence of ifferent fuels on surface pre-ignition, using a Ricaro E6 engine with a fixe compression ratio an spark avance, an a controlle electrically heate coil as a pre-igniter. The minimum electrical power to the pre-igniter that cause surface pre-ignition create a ranking orer of the ifferent fuels: a high power inicate a high pre-ignition rating (PR). 6 In aition, the temperature of the coil was measure. The growing use of turbo-charging has focuse attention on pre-ignition as a performance limiting factor. 7,8 In methanol-fuelle engines, noble metal coatings on spark plug electroes have been observe to promote surface pre-ignition, 7 as has a calciumbase oil aitive. 9 A glow plug was use by Hamilton et al. 8 to create surface hot spots at ifferent temperatures to ifferentiate the propensity to surface preignition an knock intensity between ifferent fuel blens. With ethanol an E85 there was a general tenency for pre-ignition to increase with increases in compression ratio up to 5. 8 When engine performance was becoming more limite by surface pre-ignition than by knock, it was foun that pre-ignition correlate better with combustion chamber eposits than with ONs., Further work suggeste the source of such pre-ignition to be mineral eposits on exhaust valves, usually coming from fuel an lubricant aitives. The most harmful were barium an calcium phosphates, originating from etergent aitives. Surfaces are covere by varying amounts of eposits, the properties of which epen on the fuel, lubricant an operating conitions. 3 Pre-ignitions are sporaic, necessitating research ata to be either store in a large linear ring memory or through an appropriate event trigger. 4 Present causes of pre-ignition have been attribute to surface ignition, resiual gas istribution, hot particles in the resiual gas mixing with the fresh charge, fuel ecomposition incurre by boiling behaviour of iniviual components an charge heterogeneity. 4 Surface pre-ignition an gas-phase autoignition were rule out by Han et al., 5 an pre-ignition was attribute to iffusive combustion of mixture portions resulting from the previous cycle. Later, Dahnz et al. 6 conclue, in their stuy of a turbo-charge engine, that the possibility of preignitions at hot surfaces coul be virtually exclue. They propose a ifferent mechanism for gas-phase pre-ignition, attributing it to lubricant oil roplets from the oil film on the cyliner liner, with a lower selfignition temperature than the fuel. In summary, recent work on pre-ignition in turbocharge engines, sometimes creating very intense knock, suggests pre-ignition is characterise by: (a) (b) (c) () (e) (f) higher pressures an temperatures that spasmoically give rise to gas-phase pre-ignitions; autoignitions at even higher pressures an temperatures that might cause very severe, amaging, knock; injector sprays striking surrouning surfaces an stripping lubricating oil from them; lubricating oil an fuel mixing with the main hot charge an autoigniting ue to the lower ignition temperature; resiual gas or particles creating hot spots; ifferent pre-ignition rankings of ifferent fuels. The paper first analyses pre-ignition at hot surfaces an the conitions necessary for the establishment of a propagating flame. This is followe by analysis of preignition remote from surfaces. For this to occur, localise autoignition must occur at a suitable gas-phase hot spot an initiate a propagating flame. For autoignitive pre-ignition, the autoignition elay time must be short enough, an the size of the hot spot must excee the critical size for self-heating an be large enough to initiate a propagating flame. Early preignition compresses the mixture to higher temperatures an pressures, at which autoignition an excitation elay times are significantly lower, an it is shown how these can cause super-knock. Pre-ignition at hot surfaces Theoretical approaches Historically, the necessary conition for ignition of a mixture was specifie by the minimum energy necessary for ignition using an electric spark. 7 However, this energy also comprise that transferre to the electroes, electrical losses in the circuit an the energy generate in the shock wave, an was, clearly, not solely a function of the mixture. Furthermore, the energy in a spark ischarge, with electron temperatures of about, K uring the initial generation of a plasma, was well in excess of that require to sustain a spherical explosion flame of minimum size. The concept of the minimum ignition energy then shifte to that of a minimum critical size that must be attaine by a hot kernel for a flame to propagate. The size woul be of the orer of the laminar flame thickness. 7 Such a minimum ignition energy can be compute using etaile chemical kinetics an might be efine as the energy imparte to the mixture to initiate a spherical flame with equilibrate burne gas at the equilibrium temperature. If the imparte energy is insufficient in the initial inuction perio, the

3 Kalghatgi an Braley 3 Table. Pressure an temperature effects on burning velocities an flame thicknesses of ifferent stoichiometric mixtures, n =.35 for q. See equations () an (3). u o (m/s) at. MPa n o 5 (m /s). MPa 3 K p m q Ethanol.5 (358 K).53.3 (358 K).76 (3 K).6 Hyrogen 3,4.3 (3 K).6. (365 K). (3 K). i-octane 5.48 (358 K).5.8 (358 K). (358 K). Methane 6.36 (3 K) (3 K).5 Gasoline 7,8.45 (348 K) (.5) propagation spee of the kernel ecreases an a flame cannot evelop. As the energy is increase, eventually, extensive chain branching occurs an the flame evelops, whilst being subjecte initially to a high flame stretch rate. 8 The minimum ignition energy an the associate critical size are key factors in etermining whether pre-ignition might occur. To avoi lengthy chemical kinetic an stretche flame computations, a relatively simple an convenient approach to the critical size for flame initiation at a hot surface will be aopte. This is base on the work of Zelovich et al. 9 an Longting He an Clavin. The critical raius, r f, of a hot spherical pocket of burne gas to initiate a propagating flame is given by r f = exp b Le ðþ where is the thickness of a laminar flame, here given by the kinematic viscosity ivie by the laminar burning velocity, n=u, Le is the Lewis number an b the Zelovich number, b = ET ð b T u Þ=RT b, where E is the activation energy for the laminar burning velocity an T u an T b are unburne an burne gas temperatures. For given values of Le an b, r f is proportional to an the larger is, the more ifficult it will be to initiate a flame. Clearly, pre-ignition in SI engines is epenent upon the way in which, for any potential flame, changes uring the compression of the mixture, an, consequently, upon the changes in u an n. These will be ifferent for ifferent fuels. Effects of ifferent fuels The epenence of u on pressure, P, an temperature, T, is usually expresse in general terms by u = u o (T=T o ) m ðp=p o Þ p ðþ where u o is the laminar burning velocity at an initial pressure, P o, an temperature, T o, whilst the exponents m an p epen upon the ranges of T an P an the fuel air mixture. Invoking equation (), with o = n o =u o, an with the kinematic viscosity varying as T 3/ P = o = ðn=n o Þðu o =u Þ= ðt=t o Þ 3= ðp o =PÞðu o =u Þ = ðt=t o Þ 3=m ðp=p o Þ p ð3þ Values of u o, n o, m an p, for stoichiometric values of five contrasting fuels, hyrogen, ethanol, i-octane, a gasoline an methane, are liste in Table. The gasoline ata are those measure for the gasoline by Tian et al. 7 an Jerzembeck et al. 8 Unlike the other four pure fuels, gasoline is a variable mixture of components, with consequent variable properties. Values of p were base on pressures between. an at least MPa, an values of m on temperatures between 3 K an 36 K. Unfortunately, there are few ata available for the higher operational values of T an P in engines, an ata have ha to be extrapolate. Of particular importance are the changes in the thickness of any potential flame uring polytropic compression, an the effects of any increases in the temperature or pressure of the intake air. If the gas is compresse polytropically with a polytropic inex of n from initial conitions inicate by the suffix o, equation (), when combine with the polytropic law, T/T o = (P/P o ) (n)/n, yiels u =u o = ðp=p o Þ q ð4þ in which q = [m(n )/n] p. Derive values of q with n =.35, are given in Table. With regar to the changing values of uring compression, it can reaily be shown that substituting the polytropic relationship into the last term in equation (3) yiels = o = ðp=p o Þ :6q ð5þ The exponent (.6 q) is always numerically negative, an hence the potential flame thicknesses always ecrease with increasing pressure. When, for a particular mixture, the potential flame thickness has ecrease to approximately that of the thermal sheath aroun any appropriate hot surface, pre-ignition becomes possible. PRs, measure in engines, have been presente in comparative terms, normalise to values of for an i-octane mixture by Ricaro an Hempson 4 an Downs et al. 5,6 The same atum will be use for the ifferent mixtures, with equation (5) giving the ratios of the flame thickness of the ifferent fuels, (fuel A) to that of i-octane (fuel B) as the pressure increases. The pressure P is measure in atmospheres an P o is equal to atm. In equation (5), the subscripts A an B for, o an q refer to the two fuels, to give A = B = ð oa = ob ÞP (qa + qb) ð6þ

4 4 International J of Engine Research () Normalise flame thickness methane i-octane hyrogen ethanol gasoline Pressure (atm) Figure. Potential laminar flame thicknesses, normalise by those of i-octane, for ifferent stoichiometric mixtures uring polytropic compression. Values of A / B, foun in this way for the five selecte fuels, are plotte against P in Figure. The potential flame thicknesses at the lower pressures woul be too large for pre-ignition. As they approache values that might be comparable to the thickness of potential preigniting thermal sheaths at hot surfaces, the normalise thicknesses feature on the figure tene towars steay values. Relative to stoichiometric i-octane air, the large thickness for methane makes it the most resistant to pre-ignition, while the significantly smaller thickness for hyrogen makes it the least resistant. The i-octane is more resistant than the gasoline, which is more resistant than the ethanol. The experimental erivation of such comparative ratings has been the objective of several engine investigations, 5,,9 which have consistently shown i-octane to have a goo resistance to pre-ignition. Downs an Theobal 6 measure the minimum electrical power for a pre-igniter to cause pre-ignition of ifferent fuels in a Ricaro E.6 single-cyliner engine. Fuels were rate accoring to the measure electrical power to cause pre-ignition: the higher the minimum electrical power for pre-ignition, the higher the PR. The i-octane was assigne a PR value of an cyclohexane one of. When the thermal sheath aroun the pre-igniter attaine a thickness comparable to r f, surface preignition might be expecte to occur. Of the pure hyrocarbons stuie by Downs an Theobal, 6 only the aromatics m-xylene an p-xylene ha ratings higher than, whereas lightly catalytically cracke gasoline ha a lower rating of Menra et al. 7 use the maximum possible spark avance before the onset of surface pre-ignition as a measure of resistance to pre-ignition. The i-octane prove to have the greatest resistance followe, in escening orer, by the gasoline, ethanol an methanol. Ethanol an its blens certainly have higher preignition tenencies than gasoline, which increase with compression ratio. 8 The goo resistance of methane to pre-ignition is well-atteste as is, in contrast, the poor resistance of hyrogen. The latter is usually expresse by its low minimum ignition energy of. mj for a stoichiometric mixture, which contrasts with a value of.3 mj for stoichiometric methane. 3 Such is the proneness of hyrogen engines to pre-ignition, for practical applications, the maximum value of the equivalence ratio is limite to about.6. 3 The ranking orer of normalise flame thickness in Figure therefore correspons well with the measure values of PR an other engine assessments. If hyrogen is exclue, then for the common stoichiometric hyrocarbon air mixtures, the ifferences in Le, b an n in equation (), at a given temperature an pressure, are unlikely to be very significant an r f shoul therefore be close to being inversely proportional to u. The engine measurements by Downs an Theobal 6 were mae over a range of equivalence ratios, close to stoichiometric, where pre-ignitions were most likely, with the normal operating temperature of the surface at which ignition occurre ranging between 8 C an 9 C. The inlet air temperature was 6 C an the compression ratio was 6. For a given fuel, the PR was a minimum when the mixture strength was aroun % richer than stoichiometric, where u woul be close to its maximum value. Hamilton et al. 8 foun that for given experimental conitions, pre-ignition was not possible outsie a range of mixture strengths straling stoichiometry. In so far as the minimum electrical power in the etermination of the PR occurs at the minimum value of r f an the maximum value of u, namely u max, it is appropriate to plot values of PR against the reciprocal of u max. There are few measurements of u over the ranges of temperature an pressure employe in the engine ratings. Consequently, the maximum values of u at.34 MPa an 45 K were taken from the ata bank of Figure 5 in Farrell et al., 3 an these are reprouce in Table. Values of PR are taken from Figure 9 of Ricaro an Hempson 4 an, for ethanol, from Thring. 3 These are plotte against the reciprocal of u max in Figure. Clearly, there is a broa correlation between PR an u max, which can provie an initial ranking of the propensity of fuels to pre-ignition, with equation (6) giving more etaile information. It is to be expecte that increasing exhaust gas recirculation, without any increase in intake temperature, woul increase resistance to pre-ignition by reucing u. However, this valuable metho of ranking fuels, ominate by the effect of a thermal sheath aroun a fixe igniter, provies no information on the effect of ifferent surfaces. Values of research octane number (RON) an motor octane number (MON), also are given in Table, an the associate PR values are plotte against each of these in turn in Figures 3 an 4. In both cases, the correlation is poor. This confirms the finings in Downs an Theobal 6 that, although the PR of all fuels was improve by the aition of lea, there was no general

5 Kalghatgi an Braley 5 Table. Maximum burning velocity u max an pre-ignition rating, PR, for ifferent hyrocarbon fuels: u max is from Figure 5 of Farrell et al., 3 an is the maximum laminar burning velocity measure at a pressure of.34 MPa an a temperature of 45 K. PR is from Figure 4.9 of Ricaro an Hempson, 4 an for ethanol, from Thring. 3 Fuel PR u max, (m/s) RON MON Source of RON/MON -pentene API -hexene API cyclohexane API ethyl benzene rate cumene (isopropylbenzene) API benzene API -methylbutene API cyclopentane rate isopentane API toluene ASTM p-xylene rate isooctane.667 ASTM o-xylene rate ethanol Thring 3 m-xylene rate relationship between PR an ON. There is clear support for this type of surface pre-ignition being a flame initiation, rather than an autoignitive phenomenon. Changes in air intake temperature Consier another set of conitions, in which the temperature of the charge increases at constant pressure from T to T. As a result of this change, will change to. (The prime (9) inicates a value after intake air heating.) Because = n=u, but the pressure oes not change = T. 3=m ð7þ T After this intake heating of the charge, equation (5) is applie to the polytropic compression from P to P 9 an = P :6q P ð8þ Preignition Rating (PR) 5 3 R = RON Figure 3. Pre-ignition rating (PR) versus research octane number (RON) for ifferent hyrocarbons; see Table. Data from Farrell et al. 3 Pre-ignition rating (PR) R = /u lmax (s/m) Preignition Rating (PR) R = MON Figure. Pre-ignition rating (PR) plotte against /u lmax for ifferent hyrocarbons; see Table. PR ata from Downs an Theobal 6 an u lmax ata from Farrell et al. 3 Figure 4. Pre-ignition rating, PR, versus motor octane number (MON) for ifferent hyrocarbons; see Table. Data from Farrell et al. 3

6 6 International J of Engine Research () giving = :6 + q P P ð9þ In the absence of any intake heating an the temperature remaining at T = = ðp =P Þ :6q ðþ From equations (7) an () = ðp =P Þ :6 + q T 3=m T ðþ From equations (9) an () = ðp =P Þ :6 + q T 3=m T P P :6 + q = P P :6 + q T 3=m T ðþ The effects of increases in the intake temperature on surface pre-ignition are compare in terms of whether preignition occurs earlier or later, at a lower or higher, pressure. This involves comparing the two pressures, P an P, with an without intake heating, at which respective flame thicknesses woul become equal, =. For this conition, the value of P P is inicate by P P, an = in equation () gives P + q P :6 = T 3=m T an P P = T (3=m)=(:6 + q) T ð3þ Values of P P from equation (3) are plotte against T T in Figure 5, for the five ifferent fuels in Table (which also gives the values of m an q). When the exponent (3/ m)/ (.6 + q) is positive, as is the case with i-octane, comparable flame thicknesses are only achieve at greater pressures as the intake heating increases. Surface pre-ignition is inhibite by the increase intake temperature. When the exponent is negative, as is the case with ethanol, comparable flame thicknesses are achieve at smaller pressures. Pre-ignition is now more likely as a result of the increase intake temperature. Ethanol has the greatest sensitivity for propensity to pre-ignition followe, in orer, by methane, gasoline, hyrogen an i-octane. From equations () an (3) = P + q P :6 = P :6 + q P ð4þ an for values of P P less than P P,.. The potential flame thickness is increase by the intake heating, with the result that any pre-ignition will be elaye. For values of P P greater than P P, \. The potential flame thickness is ecrease by the intake heating, an surface preignition is more likely. The effects of changes in air intake temperature have not been investigate to the same extent as the effects of ifferent fuels. Up to an inlet temperature ratio of.9, Downs an Theobal 6 foun no measurable effect on pre-ignition, whilst Menra et al. 7 foun susceptibility (P'/P) to pre-ignition to ecrease up to an inlet temperature ratio of.6. In both these stuies, it is not clear what the fuels were. Figure 5 suggests that with a gasoline as the fuel, at small temperature ratios, any effect woul be ifficult to etect with any certainty. In view of the importance of the effects of higher intake temperatures resulting from turbochargers an exhaust gas recirculation, it woul seem that further work is require in this area. The present calculations assume a constant pressure. In practice, this might be increase when the intake temperature increases, in orer to maintain power. 7 Changes in air intake pressure Now consier an increase in the intake pressure from P to P, with the intake temperature remaining unchange. From equation (3), will change at constant T to = P p P ð5þ Equation (5), applie to the polytropic compression from P to P, gives = P P :6q ð6þ The polytropic compression from the original pressure of P to P gives = = ðp =P Þ :6q ð7þ Equations (5) (7) yiel ethanol (T ' /T ) Air Intake Temperature Ratio = P :6q P P p+:6 + q P ð8þ With = in equation (8) i-octane gasoline hyrogen methane Figure 5. Pressure ratio P P for ientical potential flame thicknesses when intake temperature, T, is increase. For a given fuel with P P. P P, pre-ignition becomes more likely; with P P \ P P, it becomes less likely. P P = (q + p:39)=(:6 + q) P P ð9þ

7 Kalghatgi an Braley 7 (P'/P) δ i-octane ethanol Intake pressure, P ' (atm) gasoline methane hyrogen Figure 6. Pressure ratio P P for ientical potential flame thicknesses when intake pressure, P is increase. For a given fuel with P P. P P, pre-ignition becomes more likely; with P P \ P P, it becomes less likely. Following a similar proceure to that aopte in the previous section for the effects of increase intake temperature, values of P P from equation (9) are plotte against P in Figure 6, for the five ifferent fuels in Table. When the exponent (q + p.39)/ (.6 + q) is positive, as is the case with ethanol, comparable flame thicknesses are only achieve at greater pressures as the intake pressure increases. Surface preignition is inhibite. When the exponent is negative, as in the case of i-octane, comparable flame thicknesses are achieve at smaller pressures, an pre-ignition is more likely. The i-octane has the greatest sensitivity for propensity to autoignition followe, in orer, by hyrogen, gasoline, methane an ethanol. This tren is contrary to that exhibite for increases in intake temperature in Figure 5. From equations (8) an (9) = P + q P :6 = P :6 + q P ðþ as in equation (4). Again, for values of P P less than P P,.. The potential flame thickness is increase by the increase intake pressure, with the result that any pre-ignition will be elaye. For values of P P greater than P P, \. The potential flame thickness is ecrease by the increase intake pressure, an pre-ignition is more likely. The stuies in the sections on changes in air intake temperature an pressure are confine to their irect influences. However, increases in these also lea to hotter engine cycles an hotter potentially pre-igniting surfaces, which will also increase propensities to preignitions. As in the case of stuies of the influence of air intake temperature, the authors are unaware of any comprehensive stuies of the influence of intake pressure for ifferent fuels. The importance of initiating such stuies is clear. Pressure (MPa x) N Crank Angle Degrees Autoignitive pre-ignition Cycle S Normal Cycle N Engine pressure recors an pre-ignition in engines Figures 7 an 8 show pressure recors, kinly provie by Dr Manz of VW an Dr Rothenberger of GM Powertrain. These were measure in their laboratories on two ifferent turbo-charge engines, each with an intake pressure of. MPa. Engine ran at 75 r/min, an Figure 7 shows a normal, non-knocking, cycle, N, with the spark at 7 crank angle egrees after top ea centre (CAD ATDC) an another, S, which preignites, creating super-knock, with a knock intensity (KI) efine as the peak-to-peak pressure fluctuation, of over 3 MPa. Values of RON/MON were 98/89. Engine ran with fuels of RON/MON of 95/85, at r/min an, in Figure 8, N is a normal cycle, in which marginal knock might occur, with the spark at.3 CAD before top ea centre (BTDC), here without pre-ignition. The cycle K is one with fairly heavy knock, KI ;.4 MPa, with pre-ignition at 6. CAD BTDC, while the cycle S pre-ignites at 9. CAD BTDC, creating super-knock, KI ; 4.5 MPa. The pre-ignitions in the super-knock cycles S an S, respectively, are estimate to have occurre at pressures an temperatures of about.9 MPa an 638 K an at about 3.7 MPa an 68 K. 4 The conitions at the initiation of knock are given in Table 3. In contrast to earlier stuies, moern engines, in which pre-ignition has again become a concern, have irect injection of the fuel, combine with significantly higher in-cyliner pressures as a result of turbo-charging. Empirical evience suggests that in such engines, pre-ignition is unlikely to be initiate at internal surfaces, but in the boy of the charge.,4,6 The work by Zaeh et al. an Dahnz et al. 6 also suggests that fuel roplets from the injecting spray impinge on the cyliner wall an mix with the lubricating oil film, changing its surface tension an viscosity. This oil fuel mixture coul collect in the crevice volume of the top lan of the piston, with the possibility of roplets of the mixture being entraine in the main mixture in the cyliner. S Figure 7. Pressure recors from engine. Source: reprouce with permission from VW, 9.

8 8 International J of Engine Research () Start of bar/ - CA/ MFB 65% Superknock.Cycle S Moerate Knock. Cycle K Normal cycle. Cycle N P (MPa x ) 7 Start of btdc Start of 9 bar/ CA/ MFB 6% Spark = -.3 Crank Angle [ CA] Figure 8. Pressure recors from engine. Source: reprouce with permission from GM Powertrain, 9. Table 3. Conitions at the initiation of knock shown in Figures 7 an 8. Cycle Autoignition CAD P a (MPa) T a (K) Mass fraction burne S 3 CAD ATDC N 5 CAD ATDC 7. 8 unknown K CAD ATDC S CAD BTDC Dahnz et al. 6 propose such a mechanism after observing gas-phase autoignitive pre-ignitions, istribute in space at small, well-efine ignition kernels. Flames propagate from these at the relatively low spees normally associate with spark ignition. It will be shown in the next section that autoignition of the main fuel air mixture is unlikely, an in the following section that the autoigniting hot spot must be somewhat more reactive than stoichiometric heptane air. Autoignition elay times of ifferent fuels Autoignitions of primary reference fuels (PRFs) are first briefly consiere, in terms of how autoignition elay times, t i, vary with T an P. Figure 9 shows a plot of values of t i against /T for PRFs with ONs ranging from to, for stoichiometric mixtures with air at 4 MPa, taken from Fieweger et al. 33 For these PRFs, t i is inversely proportional to pressure, varying approximately as p :7. 34 For the pre-ignition pressures an temperatures of both the super-knock cycles, S an S, of the previous section, Figure 9 suggests a value of t i in the region of ms, or higher, for a 95 ON PRF. This woul be too high for general autoignition of the charge. Similarly, the values of t i for the surrogate mixture that simulates the actual engine fuel, see the section on secon autoignition, woul also be too high for autoignition at these engine conitions. Clearly, the engines were not operating in a regime in which autoignition of the main fuel air mixture was to be expecte, yet autoignitive pre-ignition was inee observe to occur. Hence, another explanation of the autoignition must be sought. The authors are unaware of any ata on the autoignition elay times of lubricating oil air mixtures. Because of the variety of lubricating oil aitives, it is probable that there coul be a wie range of elay times. However, the long-chain molecules of the oils are probably inherently associate with shorter autoignition elay times. In lieu of efinitive ata for lubricating oils, ata for long-chain n-heptane will be use to provie some tentative guiance about autoignitive pre-ignition. To cover the important lower temperature range for n-heptane, the compute autoignition ata of Peters et al. 35 are employe. However, the values of t i in Peters et al. 35 woul seem to be still rather high for autoignition to occur at the measure pre-ignition pressures an estimate temperatures. The value woul be higher than ms at the lower pressure of.9 MPa, an about 4 ms at the higher

9 Kalghatgi an Braley 9 τ i ( ms) pressure of 3.7 MPa for the autoignitive pre-ignitions observe in the two super-knock cycles, S an S, respectively. The corresponing values of t i for a lubricating oil woul probably be smaller. Whatever the etaile autoignitive mechanism for pre-ignition might be, it was observe to see a normal propagating flame. Overall, three conitions have emerge as necessary for this type of pre-ignition: (a) (b) (c).. T (K) t i must be small enough for autoignition to occur at a hot spot on a sufficiently short time scale; the size of the hot spot must not be less than the critical raius for autoignition, r c ; for the autoignite hot spot to initiate a flame, its raius must not be less than the critical raius for flame propagation, r f, in equation (). These possibilities are examine through a more etaile reference to autoignition elay times an the etermination of r c an r f. Conitions (a) an (b) are iscusse sequentially in the next two sections. Duration of elay times an critical size for autoignition (iso-octane) (n -heptane) /T Figure 9. Autoignition elay times, t i, for stoichiometric PRFs from Fieweger et al. 33 at 4 MPa. With regar to conition (a), t i being small enough, even at the higher pressure conition of 3.7 MPa an 68 K, as alreay mentione, t i is rather too high. With regar to conition (b), the critical raius for gas-phase autoignition, r c, is the smallest raius at which the heat lost by conuction from the hot spot is balance by the chemical heat release, to sustain a steay state. Hot spots with a raius less than this critical value will ecay; those with a greater raius will exploe. The more reactive the hot spot, the smaller the critical raius. For a spherically symmetric pre-ignition centre, a simplifie quasi-steay-state energy equation with an Arrhenius reaction rate of factor A an activation energy E is 36 k r r T r r + AQ expðe=rtþ= ðþ in which k is the thermal conuctivity, T the temperature, r the raius an Q the mass of fuel per unit volume multiplie by the enthalpy of reaction. Thermal explosion theory presents solutions for the critical raius, r c,intermsoftheimensionless critical Frank Kamenetskii parameter, c.solutionof equation () is iscusse by Takeno, 37 who also gives values of c as a function of E n =(E/RT a ), where T a is the ambient temperature. Values of r c are given by c kt = a r c = ðþ AQE n expðe n Þ Although c ecreases with E n, the rapi ecrease in E n expðe n Þ in equation () is much more significant, an low values of r c occur at low values of E n. At temperatures below about 7 K, a stoichiometric n-heptane air mixture has a value of E/R of 5,7 K. 35 With T a = 68 K, estimate as a mean value for the gas-phase autoignitive pre-ignition at the higher pressure of 3.7 MPa, E n =.5 an c = 3.43 for a sphere. Measure values of A erive from autoignition heat release rates in engines were approximately 3 9 s. 38 With a mass fraction of n-heptane an a mixture ensity of 3.8 kg/m 3, Q becomes MJ/m 3. Substitution of these ifferent values in equation () gives a value of r c =.54 mm. This coul be relatively large an, with a value of t i = 4 ms, autoignition woul be unlikely. However, the ata in Peters et al. 35 show that if the mixture temperature were to be above about 7 K, not only t i, but also E/R, woul ecrease. As a result at, say, 763 K, well above the apparent temperature of 68 K, but still at a pressure of 3.7 MPa, t i becomes ms an, on that count, gas-phase autoignition woul become more probable. In aition, E/R woul ecrease to,4 K an E n to 4.9. With c = 3.5 an Q = 3.75 MJ/m 3, r c is reuce to 5.6 mm, a minimum size that can reaily be exceee by a hot spot. Autoignition is now much more probable an, after it, the volume of the original hot spot woul increase about seven fol, giving a minimal raius of mm. Critical hot spot size for flame initiation With regars to conition (c), for a propagating flame to be initiate in the main mixture after gas-phase autoignition, the size of the hot spot must excee the critical raius, r f, of a hot pocket of burne gas, given by equation (). Any potential flame with a value of r f that is less than the raius of the hot spot woul be able to propagate. It is sufficient to approximate the mixture to stoichiometric i-octane air, for which values of u an

10 International J of Engine Research () b are given at up to. MPa an 45 K in Braley et al. 5 It is also sufficiently accurate to extrapolate these values to 3.7 MPa an 763 K using the given expression an ata. 5 These yiel u =.37 m/s an b =7. With Le =.95 an n = m /s, from equation () r f = 4. mm. The minimal raius of the autoignite volume of hot gas of mm, calculate in the previous section is.7 times greater than this, an hence it is possible for a flame to propagate from the hot spot, completing the gas-phase pre-ignition mechanism. At its inception, because of the small size of the autoignite hot spot, the flame woul be laminar, but woul become turbulent as it propagate an its surface became wrinkle ue to the increasing scales of the turbulence. Because of the larger values of t i an t i = T outsie the hot spot, normal flame propagation woul ominate over autoignitive propagation (see the section on the violence of super-knock ). The attainment of all the conitions for an autoignitive gas-phase pre-ignition that creates a normal flame at 3.7 MPa is only possible for a stoichiometric n-heptane mixture hot spot, embee in a stoichiometric i-octane main mixture, by assuming a temperature of 763 K, rather higher than the mean value of 68 K. Autoignition regimes were compute by Dahnz et al. 6 for n-heptane roplets in stoichiometric i- octane air, a surrogate for gasoline, at 4 MPa. It was foun that homogeneous autoignition was only possible at temperatures above 78 K. There are a number of ways in which this autoignitive shortfall might be provie, in aition to lubricating oil with a lower t i. It is establishe that catalytic reactions can enhance pre-ignitions at surfaces, with an without eposits. 6,7,9 It is possible that catalysis is also important for fuel oil roplet reactions at hot spots. In Zaeh et al., the use of an oil with few aitives, particularly those base on calcium an barium, seeme to eraicate previous sporaic gas-phase preignitions. Other possible enhancers of autoignition are severe heterogeneity of the charge, an small hot particles that have been transferre with hot gases from the previous cycle. 39 Clearly, further work is require to evelop etaile mechanisms for autoignitive preignitions. Although manifestations of super-knock are ramatic, the key to their occurrence lies in this earlier gasphase autoignitive pre-ignition. Dahnz an Spicher 4 showe that, although such pre-ignitions occurre with a frequency of less than 4 in, cycles, they were concentrate in bursts of no more than about four or five cycles, with alternating regular burn an preigniting cycles. A possible explanation of this regular perioicity is that, following pre-ignition, the very high pressures of super-knock occur much earlier in the cycle. As a result, at the en of the expansion stroke, the burne gas woul be at a lower temperature than with normal combustion. The burne gas remaining in the cyliner for the next cycle woul consequently lower the mean temperature in that cycle, inhibiting autoignitive preignition. At the en of the expansion stroke in this normal cycle, the burne gas woul be at a higher temperature, possibly sufficient for the resiual gas to enhance pre-ignition in the next cycle. Efficient engine combustion on the fringe of super-knock can be nullifie by insufficiently unerstoo rare cyclic excursions into autoignitive pre-ignitions, as a result of the sensitivity of marginal autoignition to comparatively small in-cyliner changes. The secon autoignition: autoignitive initiation of super-knock This section eals with the consequences of the earlier gas-phase, autoignitive pre-ignition, namely a secon much more violent gas-phase autognition arising at another hotter spot in the en gas. Hot spots are characterise by graients of reactivity, manifest as graients of autoignition elay time. These can inuce autoignitive propagation velocities, the control of which is important in autoignitive engines. Here, for illustrative purposes, it is assume that reactivity graients are ue entirely to graients of temperature an not of composition. The reaction front then propagates at a velocity relative to the unburne gas of u a, given by u a = ð r= t i Þ= ð t i = TÞ ð T= rþ ð3þ If it is assume that t i can be expresse locally at constant pressure in Arrhenius form by t i = A9 expðe=rtþ ð4þ where A9 is a numerical constant, then from equations (3) an (4) u a = T ðt i E=RÞ ð T= rþ ð5þ If T= r is assume to be K/mm, this expression yiels values for u a of 3 an 5 m/s within the hot spot for the low- an high-pressure pre-ignition conitions for n-heptane, respectively, iscusse in the previous section. In contrast, at the onset of the super-knock in Figures 7 an 8, the pressures are 3.3 MPa an.8 MPa for cycles S an S, with estimate temperatures of 98 K an 96 K, respectively. It was assume that this secon gas-phase autoignition was initiate where a knee first appeare in the pressure curve, inicative of a suen change in the heat release rate. In this case, the associate autoignition is not occurring in something akin to a rare heptane air fragment, but at a hot spot in the main fuel air mixture, somewhere in the en gas. We first assume that the mixture is a PRF stoichiometric mixture of 95 ON, with values of t i from Figure 9, with ue allowance for the ifferent pressures. When this allowance is mae at 3 MPa an 9 K, t i is.5 ms an with T= r = K/mm, u a from equation

11 Kalghatgi an Braley (3) attains the higher value of 68 m/s. Autoignition can clearly occur within the available time. With regar to the critical size of a hot spot, the value of AQ is greater than MW/m 3, an E/R is less than, K, 34 giving a hot spot critical raius of about 3 mm, 36 which can reaily be exceee by a hot spot. The origin of knock lies in the pressure waves generate by the rapi rate of change of the rate of volume generation, V=t, at the hot spot reaction front. At a sufficient istance,, from the autoigniting hot spot, the pressure pulse approximates that from a monopole soun source. Simple acoustic theory gives the associate instantaneous soun pressure, p(t), above the ambient at time t as 4 pt ðþ= r 4p t V t tta ð6þ where t a is the time for the soun wave to propagate the istance from the source to its measurement point, through a non-reacting gas of ensity r. If r is the hot spot raius, 4pr is the area of the reaction front propagating relative to the unburne gas at a velocity u a,thenthevolumetricrateofconsumption of unburne gas volume is 4pr u a.thecorresponing volumetric rate of prouction of burne gas is greater in the ratio of unburne to burne ensities, s, hence V=t =4pr u a ðs Þ ð7þ The overpressure above the ambient generate by the propagation of the reaction front through the hot spot, is foun by ifferentiating equation (7) with respect to t an substituting in equation (6). Ultimately, this leas to a imensionless expression for the overpressure 4 pt ðþ max p = r ogs =3 ðs Þ sj + s =3 j t tt a ð8þ p in which j = a=u a, the acoustic spee a = ffiffiffiffiffiffiffiffiffiffi gp=r, g is the ratio of specific heats an t = t=t ro, where t ro = r o =a, the time for an acoustic wave to traverse the original hot spot raius. Equation (8) is only vali in the absence of shock waves an etonations an for these conitions, the final term in j t can be relatively small. The maximum pressure will occur at the maximum raius of the reacting hot spot. The multiplying terms for j in the ominant first term are often of orer unity in engines, so p(t) max =p is of orer j. The smaller the value of j, the greater the strength of the pressure pulses. Uner the observe conitions at the initiation of the superknock, a = 576 m/s, for T= r = K/mm, j = 576/ 68 =.95, an p(t) max =p is of orer unity, for a PRF of 95 ON. This is approximately in accor with the pressure recors in Figures 7 an 8, where the knock intensity is of the same orer as the mean pressure. However, for non-prf mixtures, the values of t i for the PRF with the same ON are insufficiently accurate at high pressure. This is because of a general tenency for t i values of PRF mixtures to ecrease more as the pressure increases than those of non-prf mixtures with the same ON. 34 This is the basis of the ecrease with increasing pressure of a linear weighting factor, K, to the point where it becomes negative, in the relationship for the octane inex (OI) 43 OI = ð KÞRON + K MON ð9þ As a result of the higher values of t i for the non-prf fuels employe in engines an, the OIs were 7 an 5, higher than their RON values. To obtain values of t i commensurate with the non-prfs, values of t i were compute for a surrogate fuel. This comprise 6% i- octane, 9% toluene an 9% n-heptane, by volume. The chemical kinetic computations for the autoignition of this surrogate were reporte by Anrae et al. 44 The compute value of t i at the initiation of super-knock was.3 ms, about twice the value for the 95 ON PRF, with an associate approximate oubling of j. Values of t i for the surrogate fuel at ifferent temperatures an pressures of 4, 8 an MPa are shown in Figure. Where it is clear from the values of t i that autoignition will occur, in orer to estimate its violence, it is necessary to know the pressure an temperature at its onset. This instant is that at which the Livengoo Wu integral attains a value of unity uring the time t9. 45 The integral is efine by I = ð t9 t t i ðt, PÞ ð3þ Values of the autoignition elay time, t i ðt, PÞ, were foun using the etaile chemical kinetics for the surrogate fuel, for the changing conitions uring the compression, an I was evaluate from equation (3). Further etails are given in Kalghatgi et al. Figure shows the integral, I, plotte against CAD for the three knocking cycles. For Cycles S, S an K, I reaches unity at.9 CAD ATDC,.6 CAD BTDC an.4 CAD ATDC, respectively, in remarkably close autoignition elay (µs) MPa /T 4 MPa 8 MPa Figure. Ignition elay times, t i, for stoichiometric mixtures of surrogate fuel. 44

12 International J of Engine Research () Livengoo Wu Integral Crank Angle Degrees agreement with the observe onset of autoignition in both engines, recore in Figures 7 an 8 at 3. CAD ATDC,. CAD BTDC an. CAD ATDC. The violence of super-knock S S K Figure. Livengoo Wu integral for cycles S, S an K. Equation (8) is only vali in the absence of shock waves an etonations. The situation is quite ifferent when there are large changes in ensity at the pressure front, an when chemical energy can be fe into the eveloping strong pressure wave at a hot spot. Uner these conitions, the principal energy release occurs uring an excitation time, t e, which is orers of magnitue less than t i. The rapiity of this release influences the strength of the pressure pulse. 46 A measure of this energy input to the pressure pulse is the resience time of the acoustic wave within the hot spot, t ro ivie by t e, an inicate by e. This is the number of excitation times passing into the pressure pulse uring its time in the hot spot, so that e = ðr o =aþ=t e. When the autoignition front moves into the unburne mixture at approximately the acoustic spee, a, thefrontofthepressurewavegeneratebythe rate of heat release can couple with the autoignition reaction front. The fronts are mutually reinforce to create a amaging pressure spike propagating at high velocity within the hot spot in a eveloping etonation. 47 Clearly with this egree of resonance between the chemical an acoustic waves, when u a = a, thenj = a=u a,isunity. The evelopment of a etonation oes not epen solely upon this resonance, but also upon e. Because of the complexity of this coupling, a full unerstaning of a eveloping etonation requires a irect numerical simulation of the emerging front. Such simulations were mae by Braley et al. 48 an Gu et al. 49 with CO/ H air mixtures uner ifferent conitions. These fuels were selecte because the associate reaction kinetics were relatively well unerstoo. It was foun that the results of the computations coul be usefully generalise by plotting regime bounary values of j against e, ξ B N ξ u K DEVELOPING DETONATION 5 S E ξ l P Figure. Conitions for the occurrence of eveloping etonations in terms of j an e. Supersonic an subsonic autoignitive eflagrations occur in the regions marke P an B, respectively. Data from Braley et al. 48 extene. excitation time (µs).5 as in Figure. Importantly, this shows a peninsula, with an upper limit j u an a lower limit j l, within which, etonations can begin to evelop within the hot spot. Outsie the peninsula are regimes of supersonic, P, an subsonic, B, autoignitive propagation. With regars to the values of these two imensionless groups, those of t i, t e an other parameters compute for the surrogate fuel 44 at the onset of knock are given in Table 4. Very small time increments were necessary to compute t e, an these values for the surrogate fuel at ifferent temperatures an pressures of 4, 8 an MPa are shown in Figure 3. As in Lutz et al., 46 t i varies significantly with T, an relatively less with P (Figure ), whereas t e is a rather stronger function of P than of T (Figure 3). The graient of autoignitive reactivity, T= r in the present case was, not unusually, unknown in the various engine tests. Unfortunately it has, so far, prove impossible to achieve the necessary spatial resolution, or accuracy in temperature measurements, to measure ε 4 MPa 8 MPa MPa /T Figure 3. Excitation time, t e, for a stoichiometric mixture of surrogate fuel. 49