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Fundamental features of hydrocarbon autoignition in a rapid compression machine
C O M B U S T I O N A N D F L A M E 95: 291-306 (1993)
291
Fundamental Features of Hydrocarbon Autoignition in a Rapid Compression Machine J. F. GRIFFITHS*, P. A. HALFORD-MAW, AND D. J. ROSE School of Chemistry, The University, Leeds LS2 9JT, U.K. Results are ?eported for the autoignition characteristics of n-butane, n-pentane, n-hexane, and n-heptane and also of /-butane, /-octane, and toluene in stoichiometric mixtures with air following mechanical compression to gas temperatures in the range 600-950 K and pressures up to 0.9 MPa. Emphasis is placed on the dependence of ignition delay on compressed gas temperature, on the evolution of reaction as portrayed in the pressure-time records and on features of light output associated with single and two-stage ignition. Two-stage ignition is a clear feature of the n-alkane combustion at low compressed gas temperatures. Single-stage ignition is apparent at somewhat higher compressed gas temperatures, but there is evidence of the first stage reaction having occurred during the final stages of compression in some circumstances. Engine-knock related pressure waves are associated with the autoignition of the n-alkanes, to a lesser extent with the branched chain structures, but not at all with toluene under the present experimental conditions. These general features and also the relationship of the measured pressures to average gas temperatures attained during the ignition delay period are discussed. These data are relevant to the validation of numerical models for the autoignition of hydrocarbons.
INTRODUCTION The spontaneous ignition (or autoignition) characteristics of hydrocarbons have been of interest to chemists and engineers for many decades. The current interest relates predominantly to combustion in reciprocating engines, and the occurrence of knock in spark ignition engines in particular, although the potential for autoignition hazards in industrial environments is also of importance. In experimental studies, commonly it is the duration of ignition delay and its dependence on the reaction conditions of temperature, pressure and composition that is studied, and a given experimental system may be used to obtain comparative behavior of different classes of hydrocarbons or of different members within a class. Sometimes this information is then collated to give empirical relationships for the prediction of behavior in other circumstances, or derived in the form of kinetic expressions based on overall Arrhenius parameters and reaction orders with respect to components [1]. There is little evidence that such relationships have ever been
* Corresponding author. Copyright © 1993 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc.
used successfully (or at least in full confidence of the outcome) to predict behavior in circumstances or applications differing appreciably from those of the original investigations. Over the last decade the development of computers and computational methods has fostered the alternative, more fundamental approach to prediction of behavior through the simulation of events from thermokinetic models representing the detailed chemistry and its associated heat release. This is not without its limitations however, since the coupling of detailed kinetic models to complex fluid mechanical models that represent the gas dynamics associated with every type of experimental system in general use has not yet been fully mastered. In fact, a spatial uniformity of temperature and concentration of species is normally assumed in the kinetic analysis in order to restrict the numerical integration to that of ordinary differential equations. Nevertheless, numerical analysists have helped to enhance considerably the understanding of the intricate mechanisms associated with hydrocarbon combustion over wide ranges of temperature and pressure [e.g., 2]. Perhaps the most useful experimental data of all for the purpose of testing kinetic models 0010-2180/93/$6.00
292
J . F . GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE
are detailed chemical histories, though often concentration profiles and elementary rate data are obtained under isothermal conditions, such as from experiments in closed vessels [3-5] or turbulent flow systems [6, 7], and are usually restricted to certain temperature ranges. Kinetic and mechanistic data from different isothermal studies then have to be combined if they are to be used for the construction of models for the quantitative prediction of behavior under the markedly nonisothermal conditions that exist during oscillatory cool flames or spontaneous ignitions. Thus the onus remains on the experimentalist to provide more comprehensive data, but it is so much more difficult to win detailed quantitative chemical information under conditions of fast changing temperatures, and rarely (if ever) are these data obtained under spatially uniform conditions. The stirred flow reactor [8-11], or wellstirred closed vessel [12], are the closest approaches to the model idealization. Flat flame burners [13, 14] or flow tubes [15] offer viable alternatives as sources of chemical information in nonisothermal reactions, but often the move into other types of combustion systems, such as rapid compression machines [16-19] or shock tubes [20, 21], makes direct access to detailed chemistry much more difficult and there is an inclination to settle for less than the full chemical story from the measurements made. Nevertheless there is scope to validate the overall response of kinetic models from ignition delays and other global measurements in these more "applied" circumstances. In this paper we present data on ignition delays, pressure-time records and light output associated with the spontaneous ignition of the lower molecular weight n-alkanes (n-butane, n-pentane, n-hexane, and n-heptane) and also of/-butane,/-octane, and toluene, over a range of compressed gas temperature in a rapid compression machine (RCM). Stoichiometric fuel + air mixtures were studied. The experimental temperatures covered the range in which an overall negative temperature coefficient of rage was observed, at least for the substances that are capable of exhibiting this phenomenon. The pressure-time records are an extremely important addendum because they reveal strik-
ing differences in the evolution of single and two-stage ignition over the range of compounds selected for study. Our purpose was to obtain data that we could use to test "reduced" kinetic models of alkane combustion designed for application to the computational fluid dynamic interpretation of autoignition following rapid compression. However, features of general interest with regard to alkane combustion were observed, comparisons may be made with results obtained from other experimental systems, and the data may serve also as a basis for validation of modeling studies other than our own.
EXPERIMENTAL METHODS, APPARATUS AND DETERMINATION OF Tc Experimental Aspects Full details of the apparatus and experimental procedures have been described previously [17, 22, 23]. The salient points are that the hydrocarbon gases or vapors were mixed with "air" consisting of 21% oxygen and a 79% composite of nonreactive components comprising argon, nitrogen, or CO 2. The proportions of these three were varied to alter the overall heat capacity of each mixture, and hence the ratio cv/c~ (= T). Thus the range of compressed gas temperatures 600-950 K could be covered at a fixed compression ratio of the machine (CR = 11.00 + 0.15"1). This variation in CR results from very small variations in the final position of the piston as the operating conditions are changed. The hydrocarbons used were of analytical or research grade. Each fuel + "air" mixture was premixed and then transferred, at an initial pressure of either 26.7 or 33.0 kPa, from a storage reservoir to the combustion chamber prior to compression. The gaseous reactants were then compressed by a piston driven by compressed air, the stroke taking 22 ms. The cylinder and combustion chamber temperature were maintained at 345-355 K, with a precision of + 1 K. Thus the compressed gas densities from the respective initial pressures were 102.5 + 1.5 and 131.0 + 1.5 mol m -3. Compressed gas pressures in the respective ranges 0.6-0.75 MPa or 0.75-0.9 MPa over the compressed gas temperature
HYDROCARBON AUTOIGNITION IN A RCM range 600-950 K were obtained as 3' was successively increased in each fuel composition at a constant initial pressure. The respective compositions for each fuel are given in Table 1. Pressure-time data during the compression and throughout the post compression period, 200 ms in total, were measured by Kistler transducer and recorded digitally on a pc at a sampling rate of 200/zs per point, via an A / D converter. The ignition delay (t~), measured from each record, was defined as the time from the end of compression to the maximum rate of pressure rise in ignition. This information could be calculated automatically from the digital data, but, as discussed below, there are circumstances in which some subjective judgment was required. On this account we normally made measurements of t~ from the analogue display on the computer screen of each experimental result via a "zoom" facility in the software, which enabled any part of the picture to be expanded and the time or pressure at any point then to be obtained as a digital output from the cursor location. The accuracy of this measurement for a single experiment was limited to the interval of the digital data collection (0.2 ms). In general the ignition delays were reproducible, over 3-6 experiments at each reactant composition and compressed gas temperature, to +_2.0 ms at long durations and + 1.0 ms at short durations. Pressure records were also obtained via a transient data recorder so that the time resolution could be magnified × 103. This was important in the identification of the conditions at which extremely high rates of pressure rise accompanied ignition and gave rise to an "engine knock" related phenomenon. Light output was also detected in some experiments by use of a photomultiplier (1P28)
293
through a polymethyl methacrylate window that comprised the cylinder head. These signals were also recorded digitally on a second channel of the pc. The purpose of this part of the study was to observe the chemiluminescence (CH20*) associated with the first stage of two-stage ignition and to study how the time of its occurrence was effected by the experimental conditions. Results are reported briefly here.
Determination of the Compressed Gas Temperature The ideal, adiabatic compression temperature (Tad) is given by Tad = T/(CR)V-
1,
where CR is the mechanical compression ratio and y is the ratio of the heat capacities of the reactant mixture at Tad. The heat capacities of each component are expressed as temperature dependent polynomials. Thus Tad for a given mixture is derived by an iterative calculation. The initial temperature (T/) is the measured chamber and cylinder wall temperature. Since departures from ideality may occur during compression as a consequence of heat losses and boundary layer effects, a more natural reference for the adiabatic temperature and nonreactive gases, TEd, may be regarded to be that associated with the core gas within the combustion chamber. T~d is derived from the measured pressures at the start and end of compression, Pi and Pc, such that T~ d = T ~ ( p c / p i ) ~ - 1 ~ / ~ .
When the value of 7 for the reactants is temperature dependent, the response to an ideal compression is encapsulated in the foregoing equations in the form
TABLE 1 Reactant Compositions for Each Fuel at ~ = 1 1.0 n C 4 H l 0 + 6,5 0 2 + 24.5(Ar + N 2 + CO 2) 1.0 iC4H10 + 6.5 0 2 + 24.5(Ar + N 2 + C O 2) 1.0 nCsH12 + 8 , 0 0 2 + 30.1(Ar + N 2 + C O 2) 1.0 nC6H14 + 9.5 0 2 + 35.7(Ar + N 2 + C O 2) 1.0 nC7H16 + 11.0 0 2 + 41.4(Ar + N 2 + C O 2) 1.0 iC8H18 + 1 2 . 5 0 2 + 47.0(At + N 2 + C O 2) 1.0 C 6 H s C H 3 + 9 . 0 0 2 + 33.9(Ar + N 2 + C O 2)
ln(CR) = f7~"~
1
din T
and
ln( pc/Pi) = fTr~a- - d"Yl n T . 7--1 In practice, T~d and T."d may be derived by an iterative procedure in which the compres-
294
J . F . GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE
sion is considered to occur incrementally (> 100 steps) with the value for 3, taken to be that at the average temperature in each step. In the present application this method yielded compressed gas temperatures that were within 0.5 K of the correct value. A comparison of the values for Tad and Tad, related to a particular experiment, gives an indication of the closeness of the approach to an ideal, adiabatic compression. However, the evaluation of T"d is valid only if no reaction occurs during the final stages of compression. As we shall see, this condition is not satisfied in many experiments reported here. Thus in order to maintain consistency throughout this paper the value of Tad is quoted as the reference temperature for the end of compression, based on CR = 11.0. There is an error of + 4 K, which arises from a combination of the uncertainties in the combustion chamber temperature and in the compression ratio. Another parameter that also represents the temperature at the end of compression is the average compression temperature, Tc, which is derived from the ideal ga s law applied to initial and final compression conditions: T~ T/
small. The experiments are performed under relatively dilute conditions and, in general, the moles of product approximately equal the moles of reactant during the "slow oxidation" of hydrocarbons. With the precision of the experimental measurement the compressed gas pressure (s.d. + 1%, in general), Tc never fell much below Tad (Table 2). However, in many circumstances the value of Tc exceeded Tad quite considerably, which was attributed to exothermic reaction occurring in the final stages of compression (see Table 2 and later discussion). A study of the series of pressure-time records for each fuel over a range of compressed gas temperature also revealed the conditions at which there were detectable extents of reaction during compression. An illustration of this feature can be seen in Fig. 5 in connection with the combustion of n-heptane in relation to the other fuels at a similar prescribed value of Tad. RESULTS
Ignition Delay as a Function of Compressed Gas Temperature The primary experimental data comprise the pressure-time records. However, it is easier to assimilate these results when they are put in perspective in terms of the overall dependence of tg on Tad for each of the fuels investigated. These are represented in Figs. 1 and 2 for each fuel within the two compressed gas pressure ranges, which correspond to the reactant densities 102.5 and 131.0 mol m -3, respectively. Common to the n-alkanes, at both pressure (or
Pc pi(CR)
"
This is the only relationship that is applicable to reactive as well as nonreactive conditions during the compression stroke. We assume that not only departures from ideal behavior but also the change in the number of moles of reactants as a consequence of reaction are
TABLE 2 Comparisons of Tad and T c for the Results Shown in Figs. 3 - 9 (T,"d is the predicted core temperature for nonreactive gases) Nonreactive ~ d (K) (±4K)
( ± 2 K)
678 708 758 785 813 856 895
681 712 761 787 812 847 881
nC4H m
T~a (K)
~ (K) 676 706 754 782 807 846 886
± ± ± ± ± ± ±
3 3 5 3 3 3 4
Too (K) ( ± 4 K) -717 755 785 815 858 906
nCsH12
T~ (K) -721 ± 751 ± 771 ± 808 ± 850 ± 885 ±
5 4 5 6 7 7
Tad (K) ( ± 4 K) 673 704 746 776 810 864 908
nC6HI4
Tc (K) 707 711 758 801 847 857 895
± ± ± ± ± ± ±
4 6 4 7 13 3 5
Tad (K) ( ± 4 K) 680 711 757 789 819 859 911
nC7H16
T¢ (K) 682 705 763 829 861 891 929
± ± ± ± ± ± ±
4 4 2 3 7 6 5
Tad (K) ( ± 4 K) 672 709 753 802 816 864 910
Tc (K) 714 ± 748 ± 892 ± 878 ± 911 ± 960 ± tign =
4 14 7 15 7 6 0
HYDROCARBON AUTOIGNITION IN A RCM
160
C4H1° i ]C4H10
1
I
60 (/)
~nC4H1o
m~'ll' nCsH,2\
/~ilC~H~° /
,\ \ \\
• •
i 2oJ
o
10C~
r
6O0
7OO
8CO
9O0
T,w'K
Fig. 1. The dependence of overall ignition delay on the ideal adiabatic compression temperature (Tad) for reactant mixtures as given in Table 1. The compressed gas densities throughout the temperature range are 131.0 + 1.5 tool m-3.
density) ranges, are three distinct regions of the ignition delay as Tad is raised. That is, the low- and high-temperature regions exhibiting a positive temperature dependence of t~ as a function of Tad, separated by the intermediate range in which there is a negative temperature coefficient (ntc) of t i. The considerably greater reactivity as the carbon number within the n-alkane series is increased is evident both
~//4
6O
nCsH14
/ .e,.,, E
.IT
°i
nC4H10
i
CH3i ~" i
'~"
i
2O
Fig. 2. The dependence of overall ignition delay on Tad as in Fig. 1, but at the lower compressed gas densities of 102.5 + 1.5 mol m -3.
295
from the minimum adiabatic compressed gas temperature at which spontaneous ignition first occurs and from the minimum and maximum duration of the ignition delay within the ntc range. The negative temperature dependence of t i is especially prominent for n-C4H10. There appears to be a trend towards a lower compressed gas temperature at which the minimum point of the ntc exists, but the overall range of Tad within which the ntc region of t i exists does not differ greatly for n-C4H10 to n-C6H12. The behavior of n-heptane is qt~ite different and the negative temperature dependence of t i is hardly perceptible at the higher reactant pressure range (Fig. 1). The ignition delays of i-C4H lO mixtures were considerably greater than those of n-C4Hlo at compressed gas temperatures below about 850 K. An inflexion was observed in the dependence of t i on Tad for i-C4H10 at 800 < Tad < 850 K and t i = 120 ms, which is outside the range displayed in Fig. 1. There is a striking similarity in behavior between the C4H10 isomers at Tad > 870 K, as also reported previously [23]. At Pc = 0.9 MPa (Fig. 1), the measured ignition delays for i-C8HI8 were also rather similar to those of n-C4H10 and i-C4Ht0 at T.dd> 900 K, but this may be fortuitous since there was less close accord between iCsHI8 and n-C4Hi0 at the lower compressed gas pressure of 0.75 MPa (Fig. 2). At the relatively low compressed gas pressures reported here, autoignition of stoichiometric mixtures of i-C8Hls occurred only at compressed gas temperatures greater than 930 K. Autoignition occurred at Tad > 750 K and a negative temperature dependence of t i was observed in the intermediate temperature range when the compressed gas pressure was raised to 1.3-1.5 MPa (molar density > 200 mol m-3). The shape of the curve representing the dependence of the ignition delay of toluene on the adiabatic compressed gas temperature in our rapid compression machine differs appreciably from that of the alkanes (Figs. 1 and 2). The shortest delays were less than 2 ms at the highest accessible compressed gas temperatures. There was a very high sensitivity of ti to compressed gas temperature at the threshold of ignition of stoichiometric mixtures of
296
J . F . GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE
toluene, Tad > 850 K. The longest measured duration did not exceed 40 ms, by contrast to the ability of alkanes to sustain ignition delays of up to 150 ms in the RCM at their minimum compressed gas temperatures for autoignition. The response of toluene is reminiscent of autoignition which is strongly dominated by thermal feedback [24], chain branching playing only a limited part in the range 850 K < Tad < 900 K. The response is similar to that of methanol [25]. The reproducibility in the behavior of toluene was not as good as that of the alkanes.
32]
"!
2, ~. ~ '~
=rage I
mR 2
A5
o
;0 t/ms
Two-Stage Ignition as a Feature of the Pressure Records
Pressure-time records for each of the n-alkanes are shown in Fig. 3-9 at a given values of Tad over the range 670-910 K. Because the compositions of the reactant mixtures varied considerably for each fuel, it was not possible to match Tad exactly in each of the figures. The composition and initial temperature for compression of the equivalent nonreactive mixture is given in each figure legend. The proportion of these diluting gases in each of the reactive compositions was varied to accommodate the effect on the overall heat capacity of the hydrocarbon present and of the oxygen which displaced part of the nitrogen. The correspond-
Fig. 4. Pressure-time profiles showing the compression stroke and the subsequent two-stage ignition of (a) nC7HI6 at Tad = 709 K, (b) n-C6H14 at 7~d = 711 K, (c) n-CsHt2 at Too = 704 K, and (d) n-C4Hj0 at Tad = 717 K. The nonreactive mixture 0.47 CO 2 + 0.53 N 2 at Tad = 708 K (Ti = 351 K) is represented by curve e. Stages 1 and 2 of ignition are identified on the pressure record for n-CsHlz.
ing values of Tad and Tc are given in Table 2. The diagrammatic and tabulated results are a limited representation of a considerable number of experiments for each of the alkanes at many more compression temperatures. A pressure record associated with toluene ignition is also shown in Fig. 9.
32[
\
2'1
2.4!
\
b
,b\
1.6
¢
~"
el
16
d
•
",
K
0
°I
10
J
20
30
40
50
t/mS
o
40
20
60
t/ms
Fig. 3. Pressure-time profiles showing the compression stroke and the subsequent two-stage ignition of a) n-C 7H16 at Tad = 672 K, (b) n-C6H14 at 7~a = 680 K, and (c) n-CsHlz at Z~d = 673 K. The nonreactive mixture 0.62 CO 2 + 0.38 N~ at Tad = 678 K ( T i = 351 K)is represented by curve e.
Fig. 5. Pressure-time profiles showing the compression stroke and the subsequent single ignition of (a) n - C 7 n l 6 at Tad = 753 K, and the two-stage ignition of (b) n-C6Hl4 at Tad = 7 5 7 K, (c) n-CsH12 at Tad = 746 K, and (d) n-Call10 at T~a = 755 K. The nonreactive mixture 0.27 C O : + 0 . 7 3 N: at Tad=758 K (T~=348.5 K) is represented by curve e. A typical condition at which the average gas temperatures given in Table 3 were calculated is identified by point A.
H Y D R O C A R B O N A U T O I G N I T I O N IN A RCM
32]
3.2
\
2,: I1.
297
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J
0.8
e 0
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17.5
7o
s2.s
t/ms 0
10
20
30
40
50
t/ms
Fig. 6. P r e s s u r e - t i m e profiles showing the compression stroke and the subsequent single ignition of (a) n-C7H16 at Tad = 802 K and (b) n-C6H14 at Zdd = 789 K, and the two-stage ignition of (c) n-CsH]2 at Z~d = 776 K and (d) n-Call10 at Tad = 785 K. The nonreactive mixture 0.19 CO 2 + 0 . 8 1 N 2 at Zdd = 7 8 5 K ( ~ = 3 4 8 . 5 K) is represented by curve e.
The distinction between a two-stage and a single stage ignition of n-CsH12 is exemplified in Figs. 7 and 8. The transition occurs at Tad ~ 850 K. From the more extensive set of results available two-stage ignition of n - C 6 H 1 4 r e -
3.2!
\
\
Fig. 8. P r e s s u r e - t i m e profiles showing the compression stroke and the subsequent single ignition of (a) n - C 7 n l 6 at Tad = 864 K, (b) n-CaHI4 at Tad = 859 K and (c) n - C s H j 2 at Tad = 864 K, and the two-stage ignition of (d) n-CaHlo at Ta0 = 858 K. The nonreactive mixture 0.95 N 2 + 0.05 Ar at Tad = 856 K ( ~ = 336 K) is represented by curve e.
mains distinguishable in the postcompression period only to Tao = 775 K (i.e. at conditions between Figs. 5 and 6). The "limiting" Tad for two-stage ignition is less than the adiabatic compressed gas temperature at the upper end
32~
1
2.4~ 1
?
24,
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a.
b
c c
O8~
~ e
0
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30
40
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60
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Fig. 7. P r e s s u r e - t i m e profiles showing the compression stroke and the subsequent single ignition of (a) n - f 7 H 1 6 at Tad = 816 K and (b) n-C6HI4 at Z~d = 819 K, and the two-stage ignition of (c) n-CsH12 at T~d = 810 K and (d) n-C4H10 at Tad = 815 K. The nonreactive mixture 0.125 C O 2 + 0 . 8 7 5 N 2 at Zad = 8 1 3 K (T, = 3 5 0 K) is represented by curve e.
0
• 0
5
lO
15
~
25
~ms
Fig. 9. P r e s s u r e - t i m e profiles showing the compression stroke and the subsequent single ignition of (a) n-C7H16 at Tad = 910 K, (b) n-C6HI4 at Tad = 911 K, and (c) n-CsH12 at Tad = 908 K, (d) n-CnHx0 at Tad = 906 K, and (f) C 6 H s C H 3 at Ta0 = 900 K. The nonreactive mixture 0.95 N 2 = 0.05 Ar at Tdd = 895 K (T/ = 354 K) is represented by curve e.
298
J. F. GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE
of the ntc region of the ignition delay in nC5H12 and n-C6H14 combustion (Fig. 1). The ignition of n-CTH16 is clearly resolved in two stages at Tad in the range 610-670 K, similar to that for n-f6Hl4 in Fig. 3. However, the exceptional reactivity of n-C7H16 at higher compressed gas temperatures tends to create distinctions of its behavior from that of the other alkanes, as shown in Figs. 3-9. The two stage ignition is not apparent at Tad > 750 K. In common with previous work the first stage is interpreted as the interval from the end of compression to the point of in flexion in the pressure rise, as marked in Fig. 4 [16, 23]. From the sequence of events displayed in Figs. 3-9 it is clear that, for the n-alkanes of C > 4, "single stage ignition" is observed when the duration of the first stage becomes short compared with the duration of the final stages of compression. This does not necessarily mean that no "first stage chemistry" is able to occur. In fact, the excess pressure attained at the end of compression of the n-fTH16 mixture compared with that from the other alkanes, shown in Fig. 5, signifies considerable reactivity during the final stage of compression. Similar remarks apply to n-C6HI4 at Tad = 789 K (Fig. 6). There seems to be no significant contribution to the gas temperature at the end of compression when single stage ignition of nC5H12 occurs (Table 2), suggesting that oxidation in the first stage is not sufficiently vigorous at these reactant densities for there to be much heat release within the timescale of the final stages of the compression stroke. The behavior of n-C4H10 is in marked contrast to that of n-CsH~2, n-f6Hl4 , or n-C7H16. The development of the first stage of two-stage ignition is appreciably slower and the overall ignition delay during n-C4H10 combustion is extremely long. At the present composition and reactant density, the first stage of reaction develops only after the end of compression. Even so, two-stage ignition degenerates to a single-stage ignition at higher compressed gas temperatures: the first stage is no longer apparent when Tad exceeds 850 K (e.g., Fig. 9). In this case the adiabatic compressed gas temperature at which the maximum ignition delay in the ntc region is measured corresponds to that at the disappearance of two-stage ignition.
Light Output in Two-Stage Ignition The two-stage temperature and pressure rise during ignition are accompanied by light output during the ignition delay. This is exemplified in Fig. 10a for n-CTH16 combustion at Taj = 720 K. The first peak in the light output occurred close to the maximum rate of pressure rise in the first stage of ignition. It was very weak compared with the emission which accompanies the second stage. Undoubtedly the first stage of light emission corresponds to that of "cool flames," that is the deactivation of CH20*. At higher compressed gas temperatures, where single-stage ignition of n-C7HI6 is observed in the RCM, the initial light pulse was associated with the final stage of the compression stroke (Fig. 10b). Qualitatively similar behavior was observed in the light-output from n-C6HI4 as Tad was raised. No cool flame emission was detected in the final stage of compression of n-CsH12 when single-stage ignition was observed. There were two peaks in the emission accompanying n-Call10 combustion at Tc < 860 K (Fig. lla), although the intensity of the first peak was appreciably lower than that from n-CTH16 and its rise and fall was much slower. However, at T~ > 860 K, at which single-stage ignition was observed, although there is feeble, continuous light emission after the end of compression there was no significant peak in the light output prior to the luminescence that accompanies the hot stage of ignition (Fig. llb). The light output accompanying i-C8H18 ignition in the present study was similar to that from n-C4H10 shown in Fig. llb. Rates of Pressure Rise at Ignition and Pressure Instabilities As the compressed gas temperature was raised an instability evolved at the peak pressures in ignition of n-CsH12, n-C6H14, or r/-C7H16 (e.g. Figs. 5-9). These examples do not constitute a quantitative record of the phenomenon, but data obtained by use of a transient data recorder showed these to be oscillatory pressure fluctuations (e.g., Fig. 12). The increasing intensity of the oscillatory pressure instability accompanying n-C7H16 appeared to be associ-
HYDROCARBON AUTOIGNITION IN A RCM
299
,,
1.80-
3.00-
ii
1.35-
2.25 -
m
5
--.... eL
0.90 -
0.45 -
1.50-
0.75 -
i UGHT
INTENSITY
o
/
Ia
0
0
]
I
I
10
20
30
0
0
I
I
I
10
20
30
t/ms
(a)
(b)
Fig. 10. The development of pressure (broken lines) and light intensity (solid lines) during n-CTH]~ combustion at (a) T~d = 720 K and (b) T~ = 850 K. The compressed gas density is 102.5 + 1.5 tool m -3.
ated with an increase in the rate of pressure rise in the early stages of ignition (Fig. 12). There was no evidence of this oscillatory instability in the combustion of n-C4H~0 at the gas densities studied in the present work, but similar features were reported from n-CaHt0 corn-
bustion in the RCM at higher gas densities [26]. The effect can also be seen in the pressure records reported in more recent studies of the autoignition of n-C4Hl0 [27] and of nC5H12 [28] in rapid compression machines. Although these events are not identical to "en-
2.0
1.5 1.50.
t~ n
t~ a.
~= 1.o
=E
D.
I1. ........................
0.75
;'/
05
LIGHT
~s
o
/
INTENSITY
/ s"
;o
'
4'o
T
io l/ms
t/ms (a)
(b)
Fig. 11. The development of pressure (broken lines) and light intensity (solid lines) during n-C4H i(i combustion at (a) Tad = 750 K and (b) Tad = 880 K. The compressed gas density is 131.0 + 1.5 m o l m -3.
300
J . F . GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE 3.2-
3.2-
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I
I
1
2
3
4
5
t/ms
t/ms
Fig. 12. Pressure-time profile during n-C7H16 combustion at a compressed gas density of 131.0 + 1.5 mol m -3 and Tad = 662, 805, and 896 K, respectively. The corresponding pressure records obtained by a transient data recorder over a 5-ms time interval show the increasing amplitude of the oscillatory pressure instabilities that occur as T~o is raised.
gine knock," because there is no corresponding spark ignition in the present experiments, there is a similarity insofar that autoignition from reaction centers that develop at different rates in different locations of the chamber are believed to be the cause [26]. Oscillatory pressure fluctuations were virtually absent from i-C4H10 oxidation and entirely absent from the combustion of C6HsCH 3 throughout the entire range of compressed gas temperatures and reactant densities investigated (Fig. 13). This phenomenon was apparent in the combustion of i-C8H18 only a t Tad ~" 930 K (Fig. 13).
resulting from the piston motion [29]. As has been discussed previously, the heat loss rates are highest just after compression ceases because the residual gas motion as a result of the rapid compression is greatest at this stage [23]. Since there is very little change in numbers of moles of material, especially during the ignition delay, an average gas temperature in the closed, constant volume combustion chamber can be interpreted from the measured pressure (p) during the postcompression period. Thus, for ideal gases, the average gas temperature (T) is given by the relationship T
DISCUSSION
Assessment of Gas Temperature and' Average Temperatures Reached in the First Stage of Ignition Spatial temperature variations exist within the combustion chamber during the postcompression period, despite the residual gas motion
T/
p pi(CR) '
where subscript i signifies initial conditions and CR is the compression ratio ( = 11.00 _+ 0.15). Of particular interest is the average gas temperature that is attained during the transition from the first to the second stage of twostage ignition. Since most modeling is based on
HYDROCARBON AUTOIGNITION IN A RCM an assumed uniform temperature, when either full or reduced kinetic schemes are implemented [30-32], this information is relevant to the quantitative validation of numerical modelling of the two-stage autoignition of alkanes. At conditions in which the first stage of two-stage ignition of the n-alkanes is well resolved (e.g. Figs. 3 and 4), a pressure fall occurs after the piston stops, and the initial pressure record from each of the reactive systems follows very closely the decay exhibited in the corresponding record of a nonreactive mixture. Thus in the initial stage the rate of heat release in the oxidation of each alkane (and n-C4Hl0 or n-CsH12 in particular) is small compared with the rate of heat transfer from the compressed gas to the chamber wall. The departure of the reactive pressure profiles from that of the inert gas signifies that the overall heat release rate begins to exceed the heat loss rate to the chamber walls. For n-CsH12 and
3.2~
301
the higher alkanes, this departure is associated with the rapid transition into the first stage of two-stage ignition. The acceleration of the heat release rate during n-Call]0 oxidation is much less marked. However, a similar response can be achieved to that of the higher alkanes when n-f4H10 is oxidized at higher reactant densities in the RCM [23]. The developments of the pressure profiles, and thus the average temperatures (T) attained on the "plateau" between the first and second stage of two-stage (marked as A in Fig. 5) and in the initial stage of the ignition delay in single stage ignition for n-CsH12 and n-C6H14 are given in Table 3. These data are less readily accessible from the pressure profiles for n-f7H]6. However, from the results obtained at 102.5 mol m 3 we estimate the upper limit for the average temperature in the transition from the first to the second stage to be 840 + 10 K. In support, Ciezki and Adomeit [20]
ICsHI=,950 K
3.2 ICIHll, 920 K
2.4-
ICeH18, 920 K ~H3
m a.
2.4
ICeH18, 950 K .,.,,
CH 3
1.6-
I1.
1.6
0.8
0.8 -] 0 0
20
I
l
40
60
Ums
0 0
i
i
i
i
1
2
3
4
~ms
Fig. 13. P r e s s u r e - t i m e profiles during i-CsH18 c o m b u s t i o n at Tda = 920 K and 950 K, and C ~ H s C H 3 c o m b u s t i o n at Tad = 900 K. T h e onset of each of the transient data records is m a r k e d by an arrow. T h e c o m p r e s s e d gas density was 131.0 + 1.5 mol m 3.
5
302
J. F. GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE
"l 140
W
E
700
800
900
1000
TdK Fig. 14. The dependence of overall ignition delay on the compressed gas temperature (Tc) for the n-alkanes. The compressed gas densities throughout the temperature range are 131.0 + 1.5 mol m -3. The broken lines represent the ranges of Tc which are not accessible experimentally because heat release accompanying the first stage of two-stage ignition raises the gas temperature during the final stage of compression.
observed only single stage ignition of n-C7H]6 (~b = 1) behind a reflected shock at 845 K, at 1.4 MPa (200 mol m-3). The average temperature during the first to second stage transition does not appear to exceed 870 K during n-C6H14 combustion and remains approximately constant throughout the ntc region (Table 3). During n-CsH12 combustion the average temperature during the first to second stage transition appears to decreases throughout the ntc region from its upper limit
of approximately 850 K (Table 3). The upper branch exhibiting positive temperature coefficients of the ignition delays for n-CsH12 and n - C 6 H I 4 combustion become apparent only when Tc (that is, Tad augmented by any exothermic oxidation that takes place in the final stages of compression) exceeds the upper limiting temperature on the "plateau," as may be expected. These results suggest that more quantitative attention should be paid to the structure of the temporal evolution that is predicted in numerical modeling of autoignition than appears to have been the case hitherto. The prediction of ignition delay alone is not a sufficiently rigorous criterion for the validity of a model. The failure to observe a significant ntc of ignition delay during the combustion of nCTH16 in the present study may be attributed to the occurrence of the first stage of two-stage ignition during the final stages of compression and the extent to which Tc is raised in consequence, even at low values of Tad. TO illustrate this point, the data given in Fig. 1 for the n-alkanes are presented alternatively in Fig. 14 as tign versus Tc. For n-C5H12, n - C 6 H I 4 , and n-C7H16 , there are discontinuities at the compression temperature where the first stage of reaction becomes associated with the compression stroke. At low values of Tc the data do not differ significantly from those given in Fig. 1. The data for n-C4H10 are virtually the same in Figs. 1 and 14 throughout the entire temperature range. Spatial temperature variations certainly exist
TABLE 3 Average Gas Temperatures (T) Reached at the End of the First Stage During the Ignition Delay
n-CsHz2
n-C6nl4
Tad (K) ( + 4 K) 712 746 776 810 828 864 887 908 934
Tad (K) T (K) 852 852 852 842 834 822 840 840 871
+ + + + + + + + +
10 10 10 6 10 5 10 5 4
Features
( + 4 K)
Two stage Two stage, ntc Two stage, ntc Two stage, ntc Two stage, ntc Single stage, ntc Single stage Single stage Single stage
711 757 789 805 819 859 889 910
T (K) 850 850 871 871 869 869 876 882
+ + + + + _ + +
Features 5 5 5 5 7 5 5 5
Two stage Two stage, ntc Single stage, ntc Single stage, ntc Single stage, ntc Single stage Single stage Single stage
HYDROCARBON AUTOIGNITION IN A RCM during the postcompression interval. In the negative temperature dependent region we may expect there to be an "inversion" in the spatial distribution in temperature throughout the chamber as the two-stage ignition develops [29, 33, 34]. That is, the hot stage of ignition has been predicted to develop first from the outer regions and from pockets trapped in the corner between the piston crown and cylinder wall, rather than from the core gas, as had generally been believed to be the case hitherto [35]. Effect of the Variation of Compression Rate and Relation to Shock Tube Studies
Results from studies of the variation of the rate of compression are not reported here because large changes cannot readily be made in the Leeds RCM at compressed gas pressures above 0.7 MPa. However, comments on the effects can be made which are based on supplementary studies of n-fTH16 combustion at lower compressed gas pressures. An understanding of the consequences of a change of rates of compression is important [36] because it has a bearing on the onset of autoignition in reciprocating engines and it is relevant to comparisons between RCM experiments and studies of autoignition in shock tubes [20]. The dependence of ignition delays of hydrocarbons on the rate of compression in rapid compression machines has been addressed previously, but it seems that the limited understanding of the detailed kinetics of two-stage ignition of alkanes at the time of this early work precluded clear insights into the events taking place [37]. The effects are complicated in alkane combustion (and in that of other organic compounds) because of the intervention of the negative temperature dependence of reaction rate. The origins of this kinetic feature are well established [38]. In qualitative terms the formation and subsequent chain branching of organic hydroperoxides and dihydroperoxides dominate at low reaction temperatures, but a shift of the R + O 2 / R O 2 or the R'OzH + O2/O2R'OzH equilibria towards dissociation, displaces reaction to a nonbranching mode as the reactant temperature rises. For a fuel of relatively low reactivity (e.g.,
303
n-C4H10 at the present composition and reactant density) it is unlikely that variations in compression rate to a fixed value of Tc will affect the behavior in the postcompression period very much. There is support for this from the reasonable agreement between ignition delays of stoichiometric mixtures of n-C4H10 in air when compressed to the same reactant densities and gas temperatures in the machines at Lille [28] and at Leeds. When autoignition of a very reactive fuel occurs, such as n-C7H16 under the conditions of the present study, reaction may begin during the final stages of compression, and the behavior is likely to be modified if the rate of compression is changed. At a piston speed of 12 ms-1 [39] and CR ~ 10:1, which are fairly typical for a rapid compression experiment, at 1 ms before the end of compression the gas temperature is approximately 80% of its final value. This could represent a change from 600-650 K or 800-900 K, according to the value of Tad set by the heat capacity of the reactants. Clearly the interval during which significant hydrocarbon oxidation is possible may often exceed 1 ms and, as seen in the present results, a considerable development may take place in that time. When knock occurs in a spark ignition engine (at < 2000 revs min -1, say, at an average piston speed of 4-6 ms -1 with a stroke of 6-10 cm) there is a similar time interval during which the charge temperature may be high enough for spontaneous oxidation of the fuel + air mixture to develop appreciably, especially at high gas densities. When the final compressed gas temperature exceeds that at which that the degenerate branching oxidation predominates, the extent of reaction and the rate at which the chemistry develops is governed by the time available as the reactant temperature range during the final stage of compression. That reaction takes place is clear from the augmentation of the pressure at the end of compression during n-C7H16 ignition (Fig. 5) and from the light emission from CHeO* that occurs in the final stage of compression (Fig. 12b). The lower reactivity of n-CsH12 leads to far less reaction, in identical circumstances. If the rate of compression is reduced greater extents of reaction to form peroxy species are
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J. F. GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE
able to occur in the low-temperature regime. The propensity for autoignition may be enhanced and shorter ignition delays (measured from the end of compression) could be observed in consequence. However, the response to changes of compression rate is complicated by the effect on gas motion and the consequent heat loss rates from the reactants in the post-compression period. It has been shown elsewhere that enhancement of the gas motion has a profound effect on the ignition delay [23, 27]. These aspects must also be taken into account in the interpretation of data, or when the results from a particular apparatus are being simulated numerically. Fundamental aspects of the gas motion generated by a moving piston, and its effect on heat loss to the combustion chamber walls, are discussed by Park and Keck [19]. The fastest heating rate and pressure increase occurs at the discontinuity associated with a shock wave passing through the reactant composition. Therefore this represents one definitive measurement for autoignition phenomena, insofar that reaction can only begin after the passage of the shock, regardless of the temperature that is attained in it. Heat losses may have to be taken into account if the ignition delay is of the order of milliseconds. There are supplementary contributions to the ignition delay measured either in shock tubes or rapid compression machines when liquid fuels are injected into the combustion chamber. Rates of Pressure Rise and Pressure Waves in the Combustion Chamber
The origins of the pressure instabilities reported in this paper and elsewhere [26, 28] are not yet fully understood. Similar features of autoignition occur in shock tubes [20, 21] and in the "end gas" in closed vessels following spark ignition [40], and there may be associations with the distinction between weak and strong ignitions that have also been investigated in shock tubes [41]. Several observations may be made in the context of the experimental results reported here. 1. The oscillatory pressure fluctuations in au-
toignition originate from thermokinetic interactions during the ignition delay, and are fuel dependent. 2. There appears to be a relationship to the rate of pressure rise in the early stage of ignition. However, this global measurement may be governed by the manner in which reaction centers grow at different locations in the chamber [26]. 3. Thermokinetic modeling of single or twostage autoignition, whether from full or reduced schemes, seems not to give satisfactory representation of the growth of the ignition from gas temperatures of ca. 850 K [30-32]. 4. This "failure" of solely kinetic-based descriptions suggests that the rates of development of ignition and the evolution of pressure waves in the chamber may originate in thermokinetic and fluid dynamic interactions, perhaps of the kind reported by K6nig et al. [42]. Implications of the Present Experimental Study for Kinetic Modeling
There is a qualitative similarity in the reactivity of n-hexane and n-heptane, and there is also common ground in some features of the combustion of n-pentane. The similarities between these alkanes may be attributed to the relative ease with which diperoxy species can be formed during low temperature oxidation [43]. There is direct experimental evidence [44] and considerable support from numerical analysis [2] for their part in the oxidation of n-heptane. Normal butane behaves differently from its higher molecular mass counterparts. Its much lower reactivity may be associated with the failure to form diperoxy species readily, the low-temperature chemistry being driven by the alkylperoxy radical isomerization and decomposition to oxygenated products and OH, as discussed in detail elsewhere [45]. It seems likely that the oxidation chemistry of some of the more highly branched, isomeric structures of the higher alkanes may be constrained in a similar way to that of n-butane [43]. However, there is no evidence in the light output or pressure records for vigorous development of two-stage ignition during the oxidation of
HYDROCARBON AUTOIGNITION IN A RCM isobutane or iso-octane. The much lower reactivity of isobutane compared with that of nbutane probably stems from its limited ability even to undergo alkylperoxy radical isomerization and generate OH radicals from the decomposition of the hydroperoxy product. This difference in behavior probably lies in the strain energies involved in the isomerization transition state as a result of the tighter carbon atom structure, and the opportunity only for a primary H atom, internal transfer when the initial radical is formed at the tertiary site. The very low reactivity of iso-octane (2.2.4 trimethyl pentane) at the reactant densities reported here is less clear. The predominance of methyl groups in this structure is probably of major importance [46]. The mechanism of toluene oxidation, and that of other aromatics, is quite different from that of the alkanes. There is no low-temperature activity of the kind associated with longchain aliphatic structures. Toluene oxidation has been the subject of recent experimental and numerical modelling studies, at temperatures above 1000 K [7]. These are significant contributions to the understanding of the underlying chemistry of the autoignition, although more information is required to gain further insight into the behavior in the temperature range 850-1000 K.
The authors thank the SERC and the EEC for financial support. The authors also wish to thank the Leeds University Audio-Visual Service for its considerable help in preparing the diagrams for this and associated work. The guidance of a referee is also gratefully acknowledged. REFERENCES 1. Heywood, J. B., Internal Combustion Engine Fundamentals', McGraw Hill, New York, 1988, ch 10.6. 2. Westbrook, C. K., Warnatz, J., and Pitz, W. J., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p. 893. 3. Pollard, R. T., Comprehensive Chemical Kinetics, (C. H. Bamford and C. F. H. Tipper, Eds.), Elsevier, 1977, Vol. 17, p. 249. 4. Baldwin, R. R., Lodhi, Z. H., Stothard, N., and Walker, R. W., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, p. 123.
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5. Wilk, R. D., Pitz, W. J., Westbrook, C. K., Cernansky, N. P., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, p. 203. 6. Dryer, F. L, and Brezinsky, K., Combust. Sci. Technol., 88:111 (1985). 7. Emdee, J. L., Brezinsky, K., and Glassman, I., J. Phys. Chem., 96:2151 (1992). 8. Lignola, P. G., and Reverchon, E., Twenty-First Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1984, p. 123. 9. Baulch, D. L., Griffiths, J. F., Pappin, A. J., and Sykes, A. F., Combust. Flame 73:163 (1988). 10. Westbrook, C. K., Thornton, M. M., Pitz, W. J., and Malte, P. C., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p. 863. 11. Gueret, C., Cathonnet, M., Boettner, J-C., and Gafflard, F., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p. 873. 12. Griffiths, J. F., and Phillips, C. H., Combust. Flame 82:362 (1990). 13. Morley, C., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p. 911. 14. Agnew, W. G., and Agnew, L T., Tenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1965, p. 123. 15. Griffiths, J. F., Skirrow, G., and Tipper, C. F. H., Combust. Flame 13:195 (1969). 16. Halstead, M. P., Kirsch, L. J., and Quinn, C. P., Combust. Flame 30:45 (1977). 17. Beeley, P., Gray, P., and Griffiths, J. F., Combust. Flame, 39:269 (1980). 18. Carlier, M., Corre, C., Minetti, R., Pauwels, J.-F., Ribacour, M., and Sochet, L.-R., Twenty-Third Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1990, p. 1753. 19. Park, P. and Keck, J. C., SAIE, paper 900027, 1990. 20. Ciezki, H. K., and Adomeit, G., Combust. Flame, 93:421-433 (1993). 21. Cadman, P., to be published. 22. Griffiths, J. F., and Hasko, S. M., Proc. R. Soc. Lond. A393:371 (1984). 23. Franck, J., Griffiths, J. F., and Nimmo, W., TwentyFirst International Symposium on Combustion, The Combustion Institute, Pittsburgh, 1987, p. 447. 24. Griffiths, J. F., Jiao, Q., Kordylewski, W., Schreiber, M., Meyer, J., and Knoche, K. F., Combust. Flame, 93:303-315 (1993). 25. Griffiths, J. F., Pappin, A. J., and Reid, A. N., Fuels for Automotive and Industrial Diesel Engines, I. Mech, E Publications (MEP), 1990, p. 101. 26. Griffiths, J. F. and Nimmo, W., Combust. Flame 60:215 (1985). 27. Kojima, S., and Sujuoki, T., Combust. Flame, 92:254 (1993). 28. Ribacour, M., Minetti, R., Carlier, M., and Sochet, L-R., J. Chim. Phys., 89:2127 (1992).
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29. Griffiths, J. F., Jiao, Q., Schreiber, M., Meyer, J., and Knoche, K. F., Twenty-Fourth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1992, p. 1809. 30. Halstead, M. P., Prothero, A., and Quinn, C. P., Proc. R. Soc. Lond. A322:377 (1971). 31. Cox, R. A., and Cole, J. A., Combust. Flame 60:109 (1985). 32. Cowart, J. S., Keck, J. C., Heywood, J. B., Westbrook, C. K., and Pitz, W. J., Twenty-Third International Symposium on Combustion, The Combustion Institute, Pittsburgh, 1990, p. 1055. 33. Griffiths, J. F., Rose, D., Schreiber, M., Meyer, J., and Knoche, K. F., Combust. Flame, 91:209-212 (1992). 34. Griffiths, J. F., Rose, D., Schreiber, M., Meyer, J., and Knoche, K. F., I. Mech. E, C448/30:29 (1992). 35. Hu, H., and Keck, J. C., SAE paper 872110, 1987. 36. Martinengo, A., and Homann, K. H., Oxidation and Combustion Reviews (C. F. H. Tipper, Ed.), Elsevier, Amsterdam, 1967, Vol. 2, p. 207. 37. Sokolik, A. S. Self-Ignition, Flame and Detonation in Gases, Israel Program for Scientific Translations, 1963, p. 125.
38. Griffiths, J. F., and Scott, S. K., Prog. Energ. Combust. Sci. 13:173 (1987). 39. Griffiths, J. F., and Perche, A., Eighteenth International Symposium on Combustion, The Combustion Institute, Pittsburgh, 1981, p. 893. 40. Gabano, J., Kayegama, T., Fisson, F., and Leyer, J. C., Twenty-Second Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1988, p. 447. 41. Vermeer, D. J., Meyer, J. W., and Oppenheim, A. K., Combust. Flame 18:327 (1972). 42. K6nig, G., Maly, R. R., Bradley, D., Lau, A. K. C., and Sheppard, C. G. W., SAE paper 902136, 1990. 43. Morley, C., Combust. Sci. TechnoL 55:115 (1987). 44. Cartlidge, J., and Tipper, C. F. H., Combust. Flame 5:87 (1961). 45. Griffiths, J. F., Advances in Chemical Physics Vol L X I V (I Prigogine and S. A. Rice, Eds.), Wiley, New York, 1986, p. 203. 46. Westbrook, C. K., Pitz, W. J., and Leppard, W. R., SAE paper 912314, 1991. Received 26 November 1992; revised 27 May 1993
291
Fundamental Features of Hydrocarbon Autoignition in a Rapid Compression Machine J. F. GRIFFITHS*, P. A. HALFORD-MAW, AND D. J. ROSE School of Chemistry, The University, Leeds LS2 9JT, U.K. Results are ?eported for the autoignition characteristics of n-butane, n-pentane, n-hexane, and n-heptane and also of /-butane, /-octane, and toluene in stoichiometric mixtures with air following mechanical compression to gas temperatures in the range 600-950 K and pressures up to 0.9 MPa. Emphasis is placed on the dependence of ignition delay on compressed gas temperature, on the evolution of reaction as portrayed in the pressure-time records and on features of light output associated with single and two-stage ignition. Two-stage ignition is a clear feature of the n-alkane combustion at low compressed gas temperatures. Single-stage ignition is apparent at somewhat higher compressed gas temperatures, but there is evidence of the first stage reaction having occurred during the final stages of compression in some circumstances. Engine-knock related pressure waves are associated with the autoignition of the n-alkanes, to a lesser extent with the branched chain structures, but not at all with toluene under the present experimental conditions. These general features and also the relationship of the measured pressures to average gas temperatures attained during the ignition delay period are discussed. These data are relevant to the validation of numerical models for the autoignition of hydrocarbons.
INTRODUCTION The spontaneous ignition (or autoignition) characteristics of hydrocarbons have been of interest to chemists and engineers for many decades. The current interest relates predominantly to combustion in reciprocating engines, and the occurrence of knock in spark ignition engines in particular, although the potential for autoignition hazards in industrial environments is also of importance. In experimental studies, commonly it is the duration of ignition delay and its dependence on the reaction conditions of temperature, pressure and composition that is studied, and a given experimental system may be used to obtain comparative behavior of different classes of hydrocarbons or of different members within a class. Sometimes this information is then collated to give empirical relationships for the prediction of behavior in other circumstances, or derived in the form of kinetic expressions based on overall Arrhenius parameters and reaction orders with respect to components [1]. There is little evidence that such relationships have ever been
* Corresponding author. Copyright © 1993 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc.
used successfully (or at least in full confidence of the outcome) to predict behavior in circumstances or applications differing appreciably from those of the original investigations. Over the last decade the development of computers and computational methods has fostered the alternative, more fundamental approach to prediction of behavior through the simulation of events from thermokinetic models representing the detailed chemistry and its associated heat release. This is not without its limitations however, since the coupling of detailed kinetic models to complex fluid mechanical models that represent the gas dynamics associated with every type of experimental system in general use has not yet been fully mastered. In fact, a spatial uniformity of temperature and concentration of species is normally assumed in the kinetic analysis in order to restrict the numerical integration to that of ordinary differential equations. Nevertheless, numerical analysists have helped to enhance considerably the understanding of the intricate mechanisms associated with hydrocarbon combustion over wide ranges of temperature and pressure [e.g., 2]. Perhaps the most useful experimental data of all for the purpose of testing kinetic models 0010-2180/93/$6.00
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J . F . GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE
are detailed chemical histories, though often concentration profiles and elementary rate data are obtained under isothermal conditions, such as from experiments in closed vessels [3-5] or turbulent flow systems [6, 7], and are usually restricted to certain temperature ranges. Kinetic and mechanistic data from different isothermal studies then have to be combined if they are to be used for the construction of models for the quantitative prediction of behavior under the markedly nonisothermal conditions that exist during oscillatory cool flames or spontaneous ignitions. Thus the onus remains on the experimentalist to provide more comprehensive data, but it is so much more difficult to win detailed quantitative chemical information under conditions of fast changing temperatures, and rarely (if ever) are these data obtained under spatially uniform conditions. The stirred flow reactor [8-11], or wellstirred closed vessel [12], are the closest approaches to the model idealization. Flat flame burners [13, 14] or flow tubes [15] offer viable alternatives as sources of chemical information in nonisothermal reactions, but often the move into other types of combustion systems, such as rapid compression machines [16-19] or shock tubes [20, 21], makes direct access to detailed chemistry much more difficult and there is an inclination to settle for less than the full chemical story from the measurements made. Nevertheless there is scope to validate the overall response of kinetic models from ignition delays and other global measurements in these more "applied" circumstances. In this paper we present data on ignition delays, pressure-time records and light output associated with the spontaneous ignition of the lower molecular weight n-alkanes (n-butane, n-pentane, n-hexane, and n-heptane) and also of/-butane,/-octane, and toluene, over a range of compressed gas temperature in a rapid compression machine (RCM). Stoichiometric fuel + air mixtures were studied. The experimental temperatures covered the range in which an overall negative temperature coefficient of rage was observed, at least for the substances that are capable of exhibiting this phenomenon. The pressure-time records are an extremely important addendum because they reveal strik-
ing differences in the evolution of single and two-stage ignition over the range of compounds selected for study. Our purpose was to obtain data that we could use to test "reduced" kinetic models of alkane combustion designed for application to the computational fluid dynamic interpretation of autoignition following rapid compression. However, features of general interest with regard to alkane combustion were observed, comparisons may be made with results obtained from other experimental systems, and the data may serve also as a basis for validation of modeling studies other than our own.
EXPERIMENTAL METHODS, APPARATUS AND DETERMINATION OF Tc Experimental Aspects Full details of the apparatus and experimental procedures have been described previously [17, 22, 23]. The salient points are that the hydrocarbon gases or vapors were mixed with "air" consisting of 21% oxygen and a 79% composite of nonreactive components comprising argon, nitrogen, or CO 2. The proportions of these three were varied to alter the overall heat capacity of each mixture, and hence the ratio cv/c~ (= T). Thus the range of compressed gas temperatures 600-950 K could be covered at a fixed compression ratio of the machine (CR = 11.00 + 0.15"1). This variation in CR results from very small variations in the final position of the piston as the operating conditions are changed. The hydrocarbons used were of analytical or research grade. Each fuel + "air" mixture was premixed and then transferred, at an initial pressure of either 26.7 or 33.0 kPa, from a storage reservoir to the combustion chamber prior to compression. The gaseous reactants were then compressed by a piston driven by compressed air, the stroke taking 22 ms. The cylinder and combustion chamber temperature were maintained at 345-355 K, with a precision of + 1 K. Thus the compressed gas densities from the respective initial pressures were 102.5 + 1.5 and 131.0 + 1.5 mol m -3. Compressed gas pressures in the respective ranges 0.6-0.75 MPa or 0.75-0.9 MPa over the compressed gas temperature
HYDROCARBON AUTOIGNITION IN A RCM range 600-950 K were obtained as 3' was successively increased in each fuel composition at a constant initial pressure. The respective compositions for each fuel are given in Table 1. Pressure-time data during the compression and throughout the post compression period, 200 ms in total, were measured by Kistler transducer and recorded digitally on a pc at a sampling rate of 200/zs per point, via an A / D converter. The ignition delay (t~), measured from each record, was defined as the time from the end of compression to the maximum rate of pressure rise in ignition. This information could be calculated automatically from the digital data, but, as discussed below, there are circumstances in which some subjective judgment was required. On this account we normally made measurements of t~ from the analogue display on the computer screen of each experimental result via a "zoom" facility in the software, which enabled any part of the picture to be expanded and the time or pressure at any point then to be obtained as a digital output from the cursor location. The accuracy of this measurement for a single experiment was limited to the interval of the digital data collection (0.2 ms). In general the ignition delays were reproducible, over 3-6 experiments at each reactant composition and compressed gas temperature, to +_2.0 ms at long durations and + 1.0 ms at short durations. Pressure records were also obtained via a transient data recorder so that the time resolution could be magnified × 103. This was important in the identification of the conditions at which extremely high rates of pressure rise accompanied ignition and gave rise to an "engine knock" related phenomenon. Light output was also detected in some experiments by use of a photomultiplier (1P28)
293
through a polymethyl methacrylate window that comprised the cylinder head. These signals were also recorded digitally on a second channel of the pc. The purpose of this part of the study was to observe the chemiluminescence (CH20*) associated with the first stage of two-stage ignition and to study how the time of its occurrence was effected by the experimental conditions. Results are reported briefly here.
Determination of the Compressed Gas Temperature The ideal, adiabatic compression temperature (Tad) is given by Tad = T/(CR)V-
1,
where CR is the mechanical compression ratio and y is the ratio of the heat capacities of the reactant mixture at Tad. The heat capacities of each component are expressed as temperature dependent polynomials. Thus Tad for a given mixture is derived by an iterative calculation. The initial temperature (T/) is the measured chamber and cylinder wall temperature. Since departures from ideality may occur during compression as a consequence of heat losses and boundary layer effects, a more natural reference for the adiabatic temperature and nonreactive gases, TEd, may be regarded to be that associated with the core gas within the combustion chamber. T~d is derived from the measured pressures at the start and end of compression, Pi and Pc, such that T~ d = T ~ ( p c / p i ) ~ - 1 ~ / ~ .
When the value of 7 for the reactants is temperature dependent, the response to an ideal compression is encapsulated in the foregoing equations in the form
TABLE 1 Reactant Compositions for Each Fuel at ~ = 1 1.0 n C 4 H l 0 + 6,5 0 2 + 24.5(Ar + N 2 + CO 2) 1.0 iC4H10 + 6.5 0 2 + 24.5(Ar + N 2 + C O 2) 1.0 nCsH12 + 8 , 0 0 2 + 30.1(Ar + N 2 + C O 2) 1.0 nC6H14 + 9.5 0 2 + 35.7(Ar + N 2 + C O 2) 1.0 nC7H16 + 11.0 0 2 + 41.4(Ar + N 2 + C O 2) 1.0 iC8H18 + 1 2 . 5 0 2 + 47.0(At + N 2 + C O 2) 1.0 C 6 H s C H 3 + 9 . 0 0 2 + 33.9(Ar + N 2 + C O 2)
ln(CR) = f7~"~
1
din T
and
ln( pc/Pi) = fTr~a- - d"Yl n T . 7--1 In practice, T~d and T."d may be derived by an iterative procedure in which the compres-
294
J . F . GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE
sion is considered to occur incrementally (> 100 steps) with the value for 3, taken to be that at the average temperature in each step. In the present application this method yielded compressed gas temperatures that were within 0.5 K of the correct value. A comparison of the values for Tad and Tad, related to a particular experiment, gives an indication of the closeness of the approach to an ideal, adiabatic compression. However, the evaluation of T"d is valid only if no reaction occurs during the final stages of compression. As we shall see, this condition is not satisfied in many experiments reported here. Thus in order to maintain consistency throughout this paper the value of Tad is quoted as the reference temperature for the end of compression, based on CR = 11.0. There is an error of + 4 K, which arises from a combination of the uncertainties in the combustion chamber temperature and in the compression ratio. Another parameter that also represents the temperature at the end of compression is the average compression temperature, Tc, which is derived from the ideal ga s law applied to initial and final compression conditions: T~ T/
small. The experiments are performed under relatively dilute conditions and, in general, the moles of product approximately equal the moles of reactant during the "slow oxidation" of hydrocarbons. With the precision of the experimental measurement the compressed gas pressure (s.d. + 1%, in general), Tc never fell much below Tad (Table 2). However, in many circumstances the value of Tc exceeded Tad quite considerably, which was attributed to exothermic reaction occurring in the final stages of compression (see Table 2 and later discussion). A study of the series of pressure-time records for each fuel over a range of compressed gas temperature also revealed the conditions at which there were detectable extents of reaction during compression. An illustration of this feature can be seen in Fig. 5 in connection with the combustion of n-heptane in relation to the other fuels at a similar prescribed value of Tad. RESULTS
Ignition Delay as a Function of Compressed Gas Temperature The primary experimental data comprise the pressure-time records. However, it is easier to assimilate these results when they are put in perspective in terms of the overall dependence of tg on Tad for each of the fuels investigated. These are represented in Figs. 1 and 2 for each fuel within the two compressed gas pressure ranges, which correspond to the reactant densities 102.5 and 131.0 mol m -3, respectively. Common to the n-alkanes, at both pressure (or
Pc pi(CR)
"
This is the only relationship that is applicable to reactive as well as nonreactive conditions during the compression stroke. We assume that not only departures from ideal behavior but also the change in the number of moles of reactants as a consequence of reaction are
TABLE 2 Comparisons of Tad and T c for the Results Shown in Figs. 3 - 9 (T,"d is the predicted core temperature for nonreactive gases) Nonreactive ~ d (K) (±4K)
( ± 2 K)
678 708 758 785 813 856 895
681 712 761 787 812 847 881
nC4H m
T~a (K)
~ (K) 676 706 754 782 807 846 886
± ± ± ± ± ± ±
3 3 5 3 3 3 4
Too (K) ( ± 4 K) -717 755 785 815 858 906
nCsH12
T~ (K) -721 ± 751 ± 771 ± 808 ± 850 ± 885 ±
5 4 5 6 7 7
Tad (K) ( ± 4 K) 673 704 746 776 810 864 908
nC6HI4
Tc (K) 707 711 758 801 847 857 895
± ± ± ± ± ± ±
4 6 4 7 13 3 5
Tad (K) ( ± 4 K) 680 711 757 789 819 859 911
nC7H16
T¢ (K) 682 705 763 829 861 891 929
± ± ± ± ± ± ±
4 4 2 3 7 6 5
Tad (K) ( ± 4 K) 672 709 753 802 816 864 910
Tc (K) 714 ± 748 ± 892 ± 878 ± 911 ± 960 ± tign =
4 14 7 15 7 6 0
HYDROCARBON AUTOIGNITION IN A RCM
160
C4H1° i ]C4H10
1
I
60 (/)
~nC4H1o
m~'ll' nCsH,2\
/~ilC~H~° /
,\ \ \\
• •
i 2oJ
o
10C~
r
6O0
7OO
8CO
9O0
T,w'K
Fig. 1. The dependence of overall ignition delay on the ideal adiabatic compression temperature (Tad) for reactant mixtures as given in Table 1. The compressed gas densities throughout the temperature range are 131.0 + 1.5 tool m-3.
density) ranges, are three distinct regions of the ignition delay as Tad is raised. That is, the low- and high-temperature regions exhibiting a positive temperature dependence of t~ as a function of Tad, separated by the intermediate range in which there is a negative temperature coefficient (ntc) of t i. The considerably greater reactivity as the carbon number within the n-alkane series is increased is evident both
~//4
6O
nCsH14
/ .e,.,, E
.IT
°i
nC4H10
i
CH3i ~" i
'~"
i
2O
Fig. 2. The dependence of overall ignition delay on Tad as in Fig. 1, but at the lower compressed gas densities of 102.5 + 1.5 mol m -3.
295
from the minimum adiabatic compressed gas temperature at which spontaneous ignition first occurs and from the minimum and maximum duration of the ignition delay within the ntc range. The negative temperature dependence of t i is especially prominent for n-C4H10. There appears to be a trend towards a lower compressed gas temperature at which the minimum point of the ntc exists, but the overall range of Tad within which the ntc region of t i exists does not differ greatly for n-C4H10 to n-C6H12. The behavior of n-heptane is qt~ite different and the negative temperature dependence of t i is hardly perceptible at the higher reactant pressure range (Fig. 1). The ignition delays of i-C4H lO mixtures were considerably greater than those of n-C4Hlo at compressed gas temperatures below about 850 K. An inflexion was observed in the dependence of t i on Tad for i-C4H10 at 800 < Tad < 850 K and t i = 120 ms, which is outside the range displayed in Fig. 1. There is a striking similarity in behavior between the C4H10 isomers at Tad > 870 K, as also reported previously [23]. At Pc = 0.9 MPa (Fig. 1), the measured ignition delays for i-C8HI8 were also rather similar to those of n-C4H10 and i-C4Ht0 at T.dd> 900 K, but this may be fortuitous since there was less close accord between iCsHI8 and n-C4Hi0 at the lower compressed gas pressure of 0.75 MPa (Fig. 2). At the relatively low compressed gas pressures reported here, autoignition of stoichiometric mixtures of i-C8Hls occurred only at compressed gas temperatures greater than 930 K. Autoignition occurred at Tad > 750 K and a negative temperature dependence of t i was observed in the intermediate temperature range when the compressed gas pressure was raised to 1.3-1.5 MPa (molar density > 200 mol m-3). The shape of the curve representing the dependence of the ignition delay of toluene on the adiabatic compressed gas temperature in our rapid compression machine differs appreciably from that of the alkanes (Figs. 1 and 2). The shortest delays were less than 2 ms at the highest accessible compressed gas temperatures. There was a very high sensitivity of ti to compressed gas temperature at the threshold of ignition of stoichiometric mixtures of
296
J . F . GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE
toluene, Tad > 850 K. The longest measured duration did not exceed 40 ms, by contrast to the ability of alkanes to sustain ignition delays of up to 150 ms in the RCM at their minimum compressed gas temperatures for autoignition. The response of toluene is reminiscent of autoignition which is strongly dominated by thermal feedback [24], chain branching playing only a limited part in the range 850 K < Tad < 900 K. The response is similar to that of methanol [25]. The reproducibility in the behavior of toluene was not as good as that of the alkanes.
32]
"!
2, ~. ~ '~
=rage I
mR 2
A5
o
;0 t/ms
Two-Stage Ignition as a Feature of the Pressure Records
Pressure-time records for each of the n-alkanes are shown in Fig. 3-9 at a given values of Tad over the range 670-910 K. Because the compositions of the reactant mixtures varied considerably for each fuel, it was not possible to match Tad exactly in each of the figures. The composition and initial temperature for compression of the equivalent nonreactive mixture is given in each figure legend. The proportion of these diluting gases in each of the reactive compositions was varied to accommodate the effect on the overall heat capacity of the hydrocarbon present and of the oxygen which displaced part of the nitrogen. The correspond-
Fig. 4. Pressure-time profiles showing the compression stroke and the subsequent two-stage ignition of (a) nC7HI6 at Tad = 709 K, (b) n-C6H14 at 7~d = 711 K, (c) n-CsHt2 at Too = 704 K, and (d) n-C4Hj0 at Tad = 717 K. The nonreactive mixture 0.47 CO 2 + 0.53 N 2 at Tad = 708 K (Ti = 351 K) is represented by curve e. Stages 1 and 2 of ignition are identified on the pressure record for n-CsHlz.
ing values of Tad and Tc are given in Table 2. The diagrammatic and tabulated results are a limited representation of a considerable number of experiments for each of the alkanes at many more compression temperatures. A pressure record associated with toluene ignition is also shown in Fig. 9.
32[
\
2'1
2.4!
\
b
,b\
1.6
¢
~"
el
16
d
•
",
K
0
°I
10
J
20
30
40
50
t/mS
o
40
20
60
t/ms
Fig. 3. Pressure-time profiles showing the compression stroke and the subsequent two-stage ignition of a) n-C 7H16 at Tad = 672 K, (b) n-C6H14 at 7~a = 680 K, and (c) n-CsHlz at Z~d = 673 K. The nonreactive mixture 0.62 CO 2 + 0.38 N~ at Tad = 678 K ( T i = 351 K)is represented by curve e.
Fig. 5. Pressure-time profiles showing the compression stroke and the subsequent single ignition of (a) n - C 7 n l 6 at Tad = 753 K, and the two-stage ignition of (b) n-C6Hl4 at Tad = 7 5 7 K, (c) n-CsH12 at Tad = 746 K, and (d) n-Call10 at T~a = 755 K. The nonreactive mixture 0.27 C O : + 0 . 7 3 N: at Tad=758 K (T~=348.5 K) is represented by curve e. A typical condition at which the average gas temperatures given in Table 3 were calculated is identified by point A.
H Y D R O C A R B O N A U T O I G N I T I O N IN A RCM
32]
3.2
\
2,: I1.
297
2.4q
1.6 ! o.8~ e
J
0.8
e 0
0
17.5
7o
s2.s
t/ms 0
10
20
30
40
50
t/ms
Fig. 6. P r e s s u r e - t i m e profiles showing the compression stroke and the subsequent single ignition of (a) n-C7H16 at Tad = 802 K and (b) n-C6H14 at Zdd = 789 K, and the two-stage ignition of (c) n-CsH]2 at Z~d = 776 K and (d) n-Call10 at Tad = 785 K. The nonreactive mixture 0.19 CO 2 + 0 . 8 1 N 2 at Zdd = 7 8 5 K ( ~ = 3 4 8 . 5 K) is represented by curve e.
The distinction between a two-stage and a single stage ignition of n-CsH12 is exemplified in Figs. 7 and 8. The transition occurs at Tad ~ 850 K. From the more extensive set of results available two-stage ignition of n - C 6 H 1 4 r e -
3.2!
\
\
Fig. 8. P r e s s u r e - t i m e profiles showing the compression stroke and the subsequent single ignition of (a) n - C 7 n l 6 at Tad = 864 K, (b) n-CaHI4 at Tad = 859 K and (c) n - C s H j 2 at Tad = 864 K, and the two-stage ignition of (d) n-CaHlo at Ta0 = 858 K. The nonreactive mixture 0.95 N 2 + 0.05 Ar at Tad = 856 K ( ~ = 336 K) is represented by curve e.
mains distinguishable in the postcompression period only to Tao = 775 K (i.e. at conditions between Figs. 5 and 6). The "limiting" Tad for two-stage ignition is less than the adiabatic compressed gas temperature at the upper end
32~
1
2.4~ 1
?
24,
I a
a.
b
c c
O8~
~ e
0
10
2O
30
40
5O
60
t/ms
Fig. 7. P r e s s u r e - t i m e profiles showing the compression stroke and the subsequent single ignition of (a) n - f 7 H 1 6 at Tad = 816 K and (b) n-C6HI4 at Z~d = 819 K, and the two-stage ignition of (c) n-CsH12 at T~d = 810 K and (d) n-C4H10 at Tad = 815 K. The nonreactive mixture 0.125 C O 2 + 0 . 8 7 5 N 2 at Zad = 8 1 3 K (T, = 3 5 0 K) is represented by curve e.
0
• 0
5
lO
15
~
25
~ms
Fig. 9. P r e s s u r e - t i m e profiles showing the compression stroke and the subsequent single ignition of (a) n-C7H16 at Tad = 910 K, (b) n-C6HI4 at Tad = 911 K, and (c) n-CsH12 at Tad = 908 K, (d) n-CnHx0 at Tad = 906 K, and (f) C 6 H s C H 3 at Ta0 = 900 K. The nonreactive mixture 0.95 N 2 = 0.05 Ar at Tdd = 895 K (T/ = 354 K) is represented by curve e.
298
J. F. GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE
of the ntc region of the ignition delay in nC5H12 and n-C6H14 combustion (Fig. 1). The ignition of n-CTH16 is clearly resolved in two stages at Tad in the range 610-670 K, similar to that for n-f6Hl4 in Fig. 3. However, the exceptional reactivity of n-C7H16 at higher compressed gas temperatures tends to create distinctions of its behavior from that of the other alkanes, as shown in Figs. 3-9. The two stage ignition is not apparent at Tad > 750 K. In common with previous work the first stage is interpreted as the interval from the end of compression to the point of in flexion in the pressure rise, as marked in Fig. 4 [16, 23]. From the sequence of events displayed in Figs. 3-9 it is clear that, for the n-alkanes of C > 4, "single stage ignition" is observed when the duration of the first stage becomes short compared with the duration of the final stages of compression. This does not necessarily mean that no "first stage chemistry" is able to occur. In fact, the excess pressure attained at the end of compression of the n-fTH16 mixture compared with that from the other alkanes, shown in Fig. 5, signifies considerable reactivity during the final stage of compression. Similar remarks apply to n-C6HI4 at Tad = 789 K (Fig. 6). There seems to be no significant contribution to the gas temperature at the end of compression when single stage ignition of nC5H12 occurs (Table 2), suggesting that oxidation in the first stage is not sufficiently vigorous at these reactant densities for there to be much heat release within the timescale of the final stages of the compression stroke. The behavior of n-C4H10 is in marked contrast to that of n-CsH~2, n-f6Hl4 , or n-C7H16. The development of the first stage of two-stage ignition is appreciably slower and the overall ignition delay during n-C4H10 combustion is extremely long. At the present composition and reactant density, the first stage of reaction develops only after the end of compression. Even so, two-stage ignition degenerates to a single-stage ignition at higher compressed gas temperatures: the first stage is no longer apparent when Tad exceeds 850 K (e.g., Fig. 9). In this case the adiabatic compressed gas temperature at which the maximum ignition delay in the ntc region is measured corresponds to that at the disappearance of two-stage ignition.
Light Output in Two-Stage Ignition The two-stage temperature and pressure rise during ignition are accompanied by light output during the ignition delay. This is exemplified in Fig. 10a for n-CTH16 combustion at Taj = 720 K. The first peak in the light output occurred close to the maximum rate of pressure rise in the first stage of ignition. It was very weak compared with the emission which accompanies the second stage. Undoubtedly the first stage of light emission corresponds to that of "cool flames," that is the deactivation of CH20*. At higher compressed gas temperatures, where single-stage ignition of n-C7HI6 is observed in the RCM, the initial light pulse was associated with the final stage of the compression stroke (Fig. 10b). Qualitatively similar behavior was observed in the light-output from n-C6HI4 as Tad was raised. No cool flame emission was detected in the final stage of compression of n-CsH12 when single-stage ignition was observed. There were two peaks in the emission accompanying n-Call10 combustion at Tc < 860 K (Fig. lla), although the intensity of the first peak was appreciably lower than that from n-CTH16 and its rise and fall was much slower. However, at T~ > 860 K, at which single-stage ignition was observed, although there is feeble, continuous light emission after the end of compression there was no significant peak in the light output prior to the luminescence that accompanies the hot stage of ignition (Fig. llb). The light output accompanying i-C8H18 ignition in the present study was similar to that from n-C4H10 shown in Fig. llb. Rates of Pressure Rise at Ignition and Pressure Instabilities As the compressed gas temperature was raised an instability evolved at the peak pressures in ignition of n-CsH12, n-C6H14, or r/-C7H16 (e.g. Figs. 5-9). These examples do not constitute a quantitative record of the phenomenon, but data obtained by use of a transient data recorder showed these to be oscillatory pressure fluctuations (e.g., Fig. 12). The increasing intensity of the oscillatory pressure instability accompanying n-C7H16 appeared to be associ-
HYDROCARBON AUTOIGNITION IN A RCM
299
,,
1.80-
3.00-
ii
1.35-
2.25 -
m
5
--.... eL
0.90 -
0.45 -
1.50-
0.75 -
i UGHT
INTENSITY
o
/
Ia
0
0
]
I
I
10
20
30
0
0
I
I
I
10
20
30
t/ms
(a)
(b)
Fig. 10. The development of pressure (broken lines) and light intensity (solid lines) during n-CTH]~ combustion at (a) T~d = 720 K and (b) T~ = 850 K. The compressed gas density is 102.5 + 1.5 tool m -3.
ated with an increase in the rate of pressure rise in the early stages of ignition (Fig. 12). There was no evidence of this oscillatory instability in the combustion of n-C4H~0 at the gas densities studied in the present work, but similar features were reported from n-CaHt0 corn-
bustion in the RCM at higher gas densities [26]. The effect can also be seen in the pressure records reported in more recent studies of the autoignition of n-C4Hl0 [27] and of nC5H12 [28] in rapid compression machines. Although these events are not identical to "en-
2.0
1.5 1.50.
t~ n
t~ a.
~= 1.o
=E
D.
I1. ........................
0.75
;'/
05
LIGHT
~s
o
/
INTENSITY
/ s"
;o
'
4'o
T
io l/ms
t/ms (a)
(b)
Fig. 11. The development of pressure (broken lines) and light intensity (solid lines) during n-C4H i(i combustion at (a) Tad = 750 K and (b) Tad = 880 K. The compressed gas density is 131.0 + 1.5 m o l m -3.
300
J . F . GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE 3.2-
3.2-
896K
2.4-
2.4-
8O5 K 1.6-
1.6
0.8-
0.8
0
20
40
60
0
I
896 K
/ f
662K
I
[
I
I
I
1
2
3
4
5
t/ms
t/ms
Fig. 12. Pressure-time profile during n-C7H16 combustion at a compressed gas density of 131.0 + 1.5 mol m -3 and Tad = 662, 805, and 896 K, respectively. The corresponding pressure records obtained by a transient data recorder over a 5-ms time interval show the increasing amplitude of the oscillatory pressure instabilities that occur as T~o is raised.
gine knock," because there is no corresponding spark ignition in the present experiments, there is a similarity insofar that autoignition from reaction centers that develop at different rates in different locations of the chamber are believed to be the cause [26]. Oscillatory pressure fluctuations were virtually absent from i-C4H10 oxidation and entirely absent from the combustion of C6HsCH 3 throughout the entire range of compressed gas temperatures and reactant densities investigated (Fig. 13). This phenomenon was apparent in the combustion of i-C8H18 only a t Tad ~" 930 K (Fig. 13).
resulting from the piston motion [29]. As has been discussed previously, the heat loss rates are highest just after compression ceases because the residual gas motion as a result of the rapid compression is greatest at this stage [23]. Since there is very little change in numbers of moles of material, especially during the ignition delay, an average gas temperature in the closed, constant volume combustion chamber can be interpreted from the measured pressure (p) during the postcompression period. Thus, for ideal gases, the average gas temperature (T) is given by the relationship T
DISCUSSION
Assessment of Gas Temperature and' Average Temperatures Reached in the First Stage of Ignition Spatial temperature variations exist within the combustion chamber during the postcompression period, despite the residual gas motion
T/
p pi(CR) '
where subscript i signifies initial conditions and CR is the compression ratio ( = 11.00 _+ 0.15). Of particular interest is the average gas temperature that is attained during the transition from the first to the second stage of twostage ignition. Since most modeling is based on
HYDROCARBON AUTOIGNITION IN A RCM an assumed uniform temperature, when either full or reduced kinetic schemes are implemented [30-32], this information is relevant to the quantitative validation of numerical modelling of the two-stage autoignition of alkanes. At conditions in which the first stage of two-stage ignition of the n-alkanes is well resolved (e.g. Figs. 3 and 4), a pressure fall occurs after the piston stops, and the initial pressure record from each of the reactive systems follows very closely the decay exhibited in the corresponding record of a nonreactive mixture. Thus in the initial stage the rate of heat release in the oxidation of each alkane (and n-C4Hl0 or n-CsH12 in particular) is small compared with the rate of heat transfer from the compressed gas to the chamber wall. The departure of the reactive pressure profiles from that of the inert gas signifies that the overall heat release rate begins to exceed the heat loss rate to the chamber walls. For n-CsH12 and
3.2~
301
the higher alkanes, this departure is associated with the rapid transition into the first stage of two-stage ignition. The acceleration of the heat release rate during n-Call]0 oxidation is much less marked. However, a similar response can be achieved to that of the higher alkanes when n-f4H10 is oxidized at higher reactant densities in the RCM [23]. The developments of the pressure profiles, and thus the average temperatures (T) attained on the "plateau" between the first and second stage of two-stage (marked as A in Fig. 5) and in the initial stage of the ignition delay in single stage ignition for n-CsH12 and n-C6H14 are given in Table 3. These data are less readily accessible from the pressure profiles for n-f7H]6. However, from the results obtained at 102.5 mol m 3 we estimate the upper limit for the average temperature in the transition from the first to the second stage to be 840 + 10 K. In support, Ciezki and Adomeit [20]
ICsHI=,950 K
3.2 ICIHll, 920 K
2.4-
ICeH18, 920 K ~H3
m a.
2.4
ICeH18, 950 K .,.,,
CH 3
1.6-
I1.
1.6
0.8
0.8 -] 0 0
20
I
l
40
60
Ums
0 0
i
i
i
i
1
2
3
4
~ms
Fig. 13. P r e s s u r e - t i m e profiles during i-CsH18 c o m b u s t i o n at Tda = 920 K and 950 K, and C ~ H s C H 3 c o m b u s t i o n at Tad = 900 K. T h e onset of each of the transient data records is m a r k e d by an arrow. T h e c o m p r e s s e d gas density was 131.0 + 1.5 mol m 3.
5
302
J. F. GRIFFITHS, P. A. HALFORD-MAW, AND D. J. ROSE
"l 140
W
E
700
800
900
1000
TdK Fig. 14. The dependence of overall ignition delay on the compressed gas temperature (Tc) for the n-alkanes. The compressed gas densities throughout the temperature range are 131.0 + 1.5 mol m -3. The broken lines represent the ranges of Tc which are not accessible experimentally because heat release accompanying the first stage of two-stage ignition raises the gas temperature during the final stage of compression.
observed only single stage ignition of n-C7H]6 (~b = 1) behind a reflected shock at 845 K, at 1.4 MPa (200 mol m-3). The average temperature during the first to second stage transition does not appear to exceed 870 K during n-C6H14 combustion and remains approximately constant throughout the ntc region (Table 3). During n-CsH12 combustion the average temperature during the first to second stage transition appears to decreases throughout the ntc region from its upper limit
of approximately 850 K (Table 3). The upper branch exhibiting positive temperature coefficients of the ignition delays for n-CsH12 and n - C 6 H I 4 combustion become apparent only when Tc (that is, Tad augmented by any exothermic oxidation that takes place in the final stages of compression) exceeds the upper limiting temperature on the "plateau," as may be expected. These results suggest that more quantitative attention should be paid to the structure of the temporal evolution that is predicted in numerical modeling of autoignition than appears to have been the case hitherto. The prediction of ignition delay alone is not a sufficiently rigorous criterion for the validity of a model. The failure to observe a significant ntc of ignition delay during the combustion of nCTH16 in the present study may be attributed to the occurrence of the first stage of two-stage ignition during the final stages of compression and the extent to which Tc is raised in consequence, even at low values of Tad. TO illustrate this point, the data given in Fig. 1 for the n-alkanes are presented alternatively in Fig. 14 as tign versus Tc. For n-C5H12, n - C 6 H I 4 , and n-C7H16 , there are discontinuities at the compression temperature where the first stage of reaction becomes associated with the compression stroke. At low values of Tc the data do not differ significantly from those given in Fig. 1. The data for n-C4H10 are virtually the same in Figs. 1 and 14 throughout the entire temperature range. Spatial temperature variations certainly exist
TABLE 3 Average Gas Temperatures (T) Reached at the End of the First Stage During the Ignition Delay
n-CsHz2
n-C6nl4
Tad (K) ( + 4 K) 712 746 776 810 828 864 887 908 934
Tad (K) T (K) 852 852 852 842 834 822 840 840 871
+ + + + + + + + +
10 10 10 6 10 5 10 5 4
Features
( + 4 K)
Two stage Two stage, ntc Two stage, ntc Two stage, ntc Two stage, ntc Single stage, ntc Single stage Single stage Single stage
711 757 789 805 819 859 889 910
T (K) 850 850 871 871 869 869 876 882
+ + + + + _ + +
Features 5 5 5 5 7 5 5 5
Two stage Two stage, ntc Single stage, ntc Single stage, ntc Single stage, ntc Single stage Single stage Single stage
HYDROCARBON AUTOIGNITION IN A RCM during the postcompression interval. In the negative temperature dependent region we may expect there to be an "inversion" in the spatial distribution in temperature throughout the chamber as the two-stage ignition develops [29, 33, 34]. That is, the hot stage of ignition has been predicted to develop first from the outer regions and from pockets trapped in the corner between the piston crown and cylinder wall, rather than from the core gas, as had generally been believed to be the case hitherto [35]. Effect of the Variation of Compression Rate and Relation to Shock Tube Studies
Results from studies of the variation of the rate of compression are not reported here because large changes cannot readily be made in the Leeds RCM at compressed gas pressures above 0.7 MPa. However, comments on the effects can be made which are based on supplementary studies of n-fTH16 combustion at lower compressed gas pressures. An understanding of the consequences of a change of rates of compression is important [36] because it has a bearing on the onset of autoignition in reciprocating engines and it is relevant to comparisons between RCM experiments and studies of autoignition in shock tubes [20]. The dependence of ignition delays of hydrocarbons on the rate of compression in rapid compression machines has been addressed previously, but it seems that the limited understanding of the detailed kinetics of two-stage ignition of alkanes at the time of this early work precluded clear insights into the events taking place [37]. The effects are complicated in alkane combustion (and in that of other organic compounds) because of the intervention of the negative temperature dependence of reaction rate. The origins of this kinetic feature are well established [38]. In qualitative terms the formation and subsequent chain branching of organic hydroperoxides and dihydroperoxides dominate at low reaction temperatures, but a shift of the R + O 2 / R O 2 or the R'OzH + O2/O2R'OzH equilibria towards dissociation, displaces reaction to a nonbranching mode as the reactant temperature rises. For a fuel of relatively low reactivity (e.g.,
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n-C4H10 at the present composition and reactant density) it is unlikely that variations in compression rate to a fixed value of Tc will affect the behavior in the postcompression period very much. There is support for this from the reasonable agreement between ignition delays of stoichiometric mixtures of n-C4H10 in air when compressed to the same reactant densities and gas temperatures in the machines at Lille [28] and at Leeds. When autoignition of a very reactive fuel occurs, such as n-C7H16 under the conditions of the present study, reaction may begin during the final stages of compression, and the behavior is likely to be modified if the rate of compression is changed. At a piston speed of 12 ms-1 [39] and CR ~ 10:1, which are fairly typical for a rapid compression experiment, at 1 ms before the end of compression the gas temperature is approximately 80% of its final value. This could represent a change from 600-650 K or 800-900 K, according to the value of Tad set by the heat capacity of the reactants. Clearly the interval during which significant hydrocarbon oxidation is possible may often exceed 1 ms and, as seen in the present results, a considerable development may take place in that time. When knock occurs in a spark ignition engine (at < 2000 revs min -1, say, at an average piston speed of 4-6 ms -1 with a stroke of 6-10 cm) there is a similar time interval during which the charge temperature may be high enough for spontaneous oxidation of the fuel + air mixture to develop appreciably, especially at high gas densities. When the final compressed gas temperature exceeds that at which that the degenerate branching oxidation predominates, the extent of reaction and the rate at which the chemistry develops is governed by the time available as the reactant temperature range during the final stage of compression. That reaction takes place is clear from the augmentation of the pressure at the end of compression during n-C7H16 ignition (Fig. 5) and from the light emission from CHeO* that occurs in the final stage of compression (Fig. 12b). The lower reactivity of n-CsH12 leads to far less reaction, in identical circumstances. If the rate of compression is reduced greater extents of reaction to form peroxy species are
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able to occur in the low-temperature regime. The propensity for autoignition may be enhanced and shorter ignition delays (measured from the end of compression) could be observed in consequence. However, the response to changes of compression rate is complicated by the effect on gas motion and the consequent heat loss rates from the reactants in the post-compression period. It has been shown elsewhere that enhancement of the gas motion has a profound effect on the ignition delay [23, 27]. These aspects must also be taken into account in the interpretation of data, or when the results from a particular apparatus are being simulated numerically. Fundamental aspects of the gas motion generated by a moving piston, and its effect on heat loss to the combustion chamber walls, are discussed by Park and Keck [19]. The fastest heating rate and pressure increase occurs at the discontinuity associated with a shock wave passing through the reactant composition. Therefore this represents one definitive measurement for autoignition phenomena, insofar that reaction can only begin after the passage of the shock, regardless of the temperature that is attained in it. Heat losses may have to be taken into account if the ignition delay is of the order of milliseconds. There are supplementary contributions to the ignition delay measured either in shock tubes or rapid compression machines when liquid fuels are injected into the combustion chamber. Rates of Pressure Rise and Pressure Waves in the Combustion Chamber
The origins of the pressure instabilities reported in this paper and elsewhere [26, 28] are not yet fully understood. Similar features of autoignition occur in shock tubes [20, 21] and in the "end gas" in closed vessels following spark ignition [40], and there may be associations with the distinction between weak and strong ignitions that have also been investigated in shock tubes [41]. Several observations may be made in the context of the experimental results reported here. 1. The oscillatory pressure fluctuations in au-
toignition originate from thermokinetic interactions during the ignition delay, and are fuel dependent. 2. There appears to be a relationship to the rate of pressure rise in the early stage of ignition. However, this global measurement may be governed by the manner in which reaction centers grow at different locations in the chamber [26]. 3. Thermokinetic modeling of single or twostage autoignition, whether from full or reduced schemes, seems not to give satisfactory representation of the growth of the ignition from gas temperatures of ca. 850 K [30-32]. 4. This "failure" of solely kinetic-based descriptions suggests that the rates of development of ignition and the evolution of pressure waves in the chamber may originate in thermokinetic and fluid dynamic interactions, perhaps of the kind reported by K6nig et al. [42]. Implications of the Present Experimental Study for Kinetic Modeling
There is a qualitative similarity in the reactivity of n-hexane and n-heptane, and there is also common ground in some features of the combustion of n-pentane. The similarities between these alkanes may be attributed to the relative ease with which diperoxy species can be formed during low temperature oxidation [43]. There is direct experimental evidence [44] and considerable support from numerical analysis [2] for their part in the oxidation of n-heptane. Normal butane behaves differently from its higher molecular mass counterparts. Its much lower reactivity may be associated with the failure to form diperoxy species readily, the low-temperature chemistry being driven by the alkylperoxy radical isomerization and decomposition to oxygenated products and OH, as discussed in detail elsewhere [45]. It seems likely that the oxidation chemistry of some of the more highly branched, isomeric structures of the higher alkanes may be constrained in a similar way to that of n-butane [43]. However, there is no evidence in the light output or pressure records for vigorous development of two-stage ignition during the oxidation of
HYDROCARBON AUTOIGNITION IN A RCM isobutane or iso-octane. The much lower reactivity of isobutane compared with that of nbutane probably stems from its limited ability even to undergo alkylperoxy radical isomerization and generate OH radicals from the decomposition of the hydroperoxy product. This difference in behavior probably lies in the strain energies involved in the isomerization transition state as a result of the tighter carbon atom structure, and the opportunity only for a primary H atom, internal transfer when the initial radical is formed at the tertiary site. The very low reactivity of iso-octane (2.2.4 trimethyl pentane) at the reactant densities reported here is less clear. The predominance of methyl groups in this structure is probably of major importance [46]. The mechanism of toluene oxidation, and that of other aromatics, is quite different from that of the alkanes. There is no low-temperature activity of the kind associated with longchain aliphatic structures. Toluene oxidation has been the subject of recent experimental and numerical modelling studies, at temperatures above 1000 K [7]. These are significant contributions to the understanding of the underlying chemistry of the autoignition, although more information is required to gain further insight into the behavior in the temperature range 850-1000 K.
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