Experimental simulation of engine knock by means of a preheated static combustion chamber

Twenty-Second Symposium (International) on Combustion/The Combustion Institute, 1988/pp. 447-454

EXPERIMENTAL SIMULATION OF ENGINE KNOCK BY MEANS PREHEATED STATIC COMBUSTION CHAMBER

OF

A

J. D. GABANO, T. KAGEYAMA, F. FISSON AND J. C. LEYER Laboratoire d'Energ~tique et de D~tonique Ecole Nationale Sup~rieure de M~canique et d'A~rotechnique U.R.A. 193 au C.N.R.S. 20 Rue Guillaume VII, 86034 POITIERS (France)

Vibratory combustions showing the same features as those of reciprocating engine knock are observed in an electrically preheated, two dimensional stainless steel spark ignited combustion chamber, using a mixture of n-butane and oxidizer (21% of oxygen and 79% of argon in volume) with equivalence ratio of 0.9. Combustions are performed for different initial temperatures, varying from 303 K up to 513 K, for a given initial density, so as to keep a constant energy content. The results of the experiments, undertaken for several initial densities between 6.2 kg/m 3 and 12.3 kg/m ~ show that for an initial temperature lower than 423 K, only smooth pressure oscillations, whose amplitude is gradually amplified but never exceeds 1 bar, and due to acoustic-flame interaction, are observed. In contrast, with initial temperature over 423 K, pressure vibrations, initiated by strong pressure pulses which break into pronounced damped pressure waves, characteristic of knocking combustion, are detected. The two types of vibratory phenomena are clearly distinguished by pressure-timehistory signal analyses, and autoignition of the end-gas, which is the last part of the fresh mixture to be consumed by the normal flame propagation. The autoignition is evidenced by high speed schlieren cinematography during knocking combustions. Finally, means to characterize and quantify knock intensity related to the mixture used are studied.

Introduction A good understanding of reciprocating engine knock inducing mechanisms is a very important problem when lead-free fuel is used with high compression ratio engines. Detected by a sharp metallic noise, knock is correlated with the presence of pressure oscillations which are known to cause damage to pistons and other parts of engine structure. The appearance of pressure instabilities has been first assigned to detonation waves propagating within the end-gas. 1-3 Curry, 4 by using multiple ionization probes and a pressure transducer in a CFR engine, suggested that the knock might be the result of sudden flame acceleration. It is now widely accepted that knock is due to the abnormally high rate of heat release resulting from autoignition of the end-gas before the flame passes through it. 5'6 The pressure oscillations are at frequencies characteristic of acoustic reasonance modes 7 and are thought to be caused by relaxation of a pressure pulse starting in the end-gas when autoignition occurs, as shown by Haskell and Bame. 8 In low speed engine conditions, knock always seems to occur for end-gas temperature below 1000 K as indicated by Gluekstein and Waleutt 9 who 447

measured the speed of sound in end-gas to obtain its temperature and confirmed by Marie and Cottereau, 1° who used the recent single-shot temperature measurements by Coherent Anti-Stokes Raman Scattering technique in an engine. Ohta et al. 11 have shown that the importance of the low-temperature oxidations of the end gas might grow when the compression is slow. So it appears that the temperature-pressure-time history is a fundamental parameter in the sense that long compression duration might provide ample development of slow reactions and low-temperature flames (cool and blue), which results in the conversion of mixture constituents and alteration of the ignition delay time. Cool flame occurrence has been well established on laboratory time scales by using a rapid compression machine nAz or motored engines which have same time scale as fired engines. 13-15 Smith et al.16-1r have concluded, from an engine study with n-butane fuel, that low-temperature reactions can also have influence on the onset of knock in relatively high speed running conditions. In their experiments, in effect, the end-gas was heated up very rapidly (2.6 ms) from 725 K to 1125 K through the compression by the burned-gas expansion generated by four converging flame fronts in a cylindeF.

ENGINE COMBUSTION

448

As recently proposed by Ohta et al.,18 in high speed engine conditions, autoignition can also be triggered thermochemically, in assistance with active radicals, by turbulent mixing of preflame mixture and the partially reacting gases in a highly distorted flame. The purpose of the present work is to study the onset and the characteristics of autoignition of end gas under conditions where initial turbulence and swirl are excluded. So, a preheated closed vessel with optical access has been designed to observe knocking-like combustion and observations are realized by schlieren cinematography and pressure analyses. Pressure vibrations due to knock are clearly distinguished from those due to acoustic-flame interation. Finally, knock intensity variation is studied versus initial temperature and pressure.

Experimental A rectangular stainless steel combustion chamber of 169.6 ml capacity (cross section:40 mm × 53 mm~ length:80 mm, weight:9 kg) is maintained at a constant wall temperature, by means of electrical heating wires (Fig. 1). Wall temperature Ti, measured about 1 mm from the wall surface, is regulated between 303 K and 523 K around a chosen value. Simultaneous measurements of the temper-

atures at four different positions in the chamber wall and in the gas, performed during preliminary tests, have shown that the temperature of introduced mixtures becomes uniform and reaches Ti in less than 1 minute with an accuracy of +5 K. As was mentioned by Girard et a1.,19 cavities in a constant volume chamber can cause the onset of pressure vibrations during combustion. For this reason, a water cooled poppet type valve and a water cooled pressure transducer adapter were developed specially to avoid any cavity in the chamber except that of the spark plug which is installed at the center of an end wall. This cavity, located in the burned gas from the beginning of the flame propagation, has no noticeable effects on the vibratory phenomena. 19 The ignition system was a conventional automotive transistorized inductance type igniter associated with a standard spark plug. The mixture used for this study was n-C4Hlo + 34.39 (0.21 02 + 0.79 Ar) which corresponds to an equivalence ratio of 0.9. Argon has been used to replace nitrogen of air in order to increase the compressed end-gas temperature. The mixture was prepared manometrically in a tank under 15 bar with laboratory grade gases. Pressure measurements were performed by means of a thermally protected piezo-electric transducer Kistler 7055 B, flush mounted at the center of the end wall opposite to the spark plug. Signals were recorded by a Nicolet 3091 digital 12 bits oscilloscope, with a sampling speed of 5 Ixs/points. Then, the data were transferred to a computer which allowed different numerical treatments. Observations of flame motion and of autoignition onset were made by means of a schlieren cinamatography system including an Argon CW laser, fitted with an acousticoptic deviator and a 60 mm wide rotating drum camera. Framing rates from 7500 to 10000 fr/sec were used to observe the autoignition phenomena. The field of view was a 5 mm height zone along the chamber axis (see Fig. 1).

Sp~

Results and Discussion Pressure Analysis:

lregulatloll Schlieren

field

FIG. 1. Sketeh of the wall heated combustion chamber.

Two kinds of vibratory combustion have been observed, whose origin and aspect are completely different: one is due to autoignition of end-gas, the other due to hydrodynamic instabilities of the flame and acoustic-flame front interaction. A typical example of the pressure record corresponding to the latter case is shown in Fig. 2, where the initial pressure, temperature and density are respectively Pi = 7 bars, Ti = 303 K, Pi = 10.8 kg/m 3. On Fig. 2.a the raw signal is displayed. By processing the data through the numerical band pass filter (221 kHz), a smooth gradual increase of the vibration

EXPERIMENTAL SIMULATION OF ENGINE KNOCK

75 "-c 6O izl

45 u~

P 30 121..

151 0 '

4' '

20

2

Time [ ms ] (a)

500 "-C 300 1:3 E 100 _100 EL

_300 _

5 3

0 10

0

12

,

~ 14

16

18

Time [ ms] FIG. 2. Acoustic-flame interaction regime; Pressure record (a) and pressure vibration (b). The latter was obtained by processing the former through a band pass filter (2-21 kHz). Pi = 7 bar, T~ = 303 K, Pi = 10.8 kg/m ~.

449

other acoustic modes of the chamber are scarcely detected. This last phenomenon has been observed previously in closed vessels and explained, on the basis of coupling between acoustic and flame motion, (see i.e. Ref. 20). Given the same initial density Pi = 10.8 kg/m 3 as above, so as to keep a constant mixture energy content, the other kind of vibratory phenomenon can be observed as the initial temperature T~ is increased over 453 K. This phenomenon is characterized by abrupt pressure variations whose amplitudes increase with the initial temperature. Figure 4.a shows an example of such a pressure record where Ti = 513 K. The main differences from the vibratory combustion due to hydrodynamic-acoustic instabilities are: 1) the onset of pressure vibrations is abrupt and the pressure change can reach a rate higher than 8 bars/Ixs and then breaks into pronounced damped pressure waves, whose amplitude can be of the order of the mean pressure (Fig. 4.b); 2) the pressure variations are so abrupt that the recorded signal contains the transducer natural frequency (47 kHz); 3) the vibration energy of the damping pressure waves is distributed over a number of acoustic modes of the chamber, not restricted to the fundamental longitudinal mode; 4) finally, as shown by FFT analysis, the amplitude of the different modes is time evoluting. For example, Fig. 5 shows two spectra of the same signal (Fig. 4.b), taking 512 samples into account and corresponding to two different time intervals (9.725 - 12.280 ms), (12.280 - 14.835 ms) respectively. In the first period (Fig. 5.a), the fundamental longitudinal mode remains prominent, whereas in the second (Fig. 5.b), when the pressure waves damp, the fundamental longitudinal mode is not the most

100 10-1

amplitude from 25 mbar to 600 mbar is clearly observed (Fig. 2.b) from the time t = 10 ms till the end of combustion. The corresponding spectrum of the squared pressure amplitude, whose values are normalized to the summation over all vibration modes with a frequency f comprised between 0 and 50 kHz, obtained by FFT (Fast Fourier Transform) calculation, is shown in Fig. 3. The prominent peak is observed at a frequency about 5.9 kHz, which corresponds to the frequency of the fundamental longitudinal mode of the chamber, calculated with the sound speed of gas in the burned state. The square of the amplitude of the second harmonic is only 0.2% of that of the fundamental mode and the

_

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102

~o-3

N 10_4 10-5 10-6

10

20 30 40 Frequency [ k Mz ]

50

FIG. 3. Spectrum of gas vibration caused by acoustic-flame interaction corresponding to Fig. 2.

E N G I N E COMBUSTION

450

200' 160 f,_.

tJ 120

.£3

(IJ

~- 80

cL 40

0~ - - - - - ~ 0 3

fign\,~ 12 6 9 Time [ ms ] (o)

15

msec (Ti = 303 K; Pi -- 6 bar) decreasing to 13 msec when the initial temperature is augmented up to 453 K. The ratio of final to initial chamber pressure decreases from 10 to 6.5 about, in the same conditions. The case of autoignition in the end gas is illustrated, at constant density (pi = 10.8 kg/m3), by the two typical examples of Fig. 6 and 7. The essential new feature, compared to the previous case, is the appearence on schlieren pictures (7692 frames per sec.), at time tc after ignition, of a significant refractive index change in the unburned gas volume, resulting in luminous zones clearly seen on frames 7 to 9 of Fig. 6 (Ti = 513 K), and on frames 3 to 6 of Fig. 7 (Ti = 493 K). Then, after some delayed time, a burst in pressure occurs at t = tig~ followed by the strong pressure oscillations similar to knock. At the same time, the pictures show that the end gas luminous zone and the normal flame

80

L t~ t'a

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10-5 I

io

I

11

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1'3

6

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1.

Time [ms] (b)

10- 2 10-? 10-4 IO-E 10 -(

In the case of acoustic-flame front interaction regime, the flame propagation exhibits quite classical characteristics. The total propagation time which varies with initial conditions, is of the order of 16

50

10-1

~

Photographic Observations and Correlation With Pressure Records:

L

10 o

F]c. 4. Heavy knock: Pressure record (a) and pressure vibrations (b). The latter was obtained by processing the former through a band pass filter (221 kHz). P~ = 11.85 bar, T~ = 513 K, p~ = 10.8 kg/m s.

prominent one anymore, and there exist quite a number of detectable modes of higher frequencies, which approximately agree with the calculated values based on the law of simple acoustic theory.

10 20 30 40 Frequency [ k Hz ] (o)

o

i;

2'o

3'0

so

Frequency [ kHz ]

(b) £1C. 5. Dependence upon time o£ spectrum of gas vibrations caused by heavy knock (a) t = 9.725 ms and (b) t = 12.280 ms.

EXPERIMENTAL SIMULATION OF E N G I N E KNOCK

451

Sparl plug N: frame 1

2 3• /+, 5~ 0, 7,

8 9 10 11 12 /13 / %

N'-frame

1

2

3 t~ 5 6

7

8

9 10 11 12 13 %

,

86

9.2 98 Time [ ms]

1(1#

110

FIG. 6. Pressure record and schlieren photographs of heavy knocking combustion, 7692 frames per second, spark plug at lower side, end-gas at upper side. P~ = 11.85 bar, T~ = 513 K, p~ = 10.8 k g / m 3.

ikont zone fuse together as in sudden explosion. It should be noticed that the p h e n o m e n o n occurs abruptly for T i = 513 K (Fig. 6), the propagation of the normal flame being apparently not influenced by the change observed in the end gas. In contrast, for Ti = 493 K (Fig. 7), the normal flame is pushed backwards during the time interval (tign to), while p r e s s u r e oscillations d e v e l o p with moderate amplitudes before the burst. As Ti is further reduced (Ti = 473 K), the influence of the end gas change on the flame propagation is much more pronounced and the pressure burst disappears. Finally, when T~ < 450 K, the acoustic regime is recovered without any observable change in the end gas. The relevant parameters and results of the experiments are summarized in Table I. The listed -

t,0 1(15

I

11.1

11.7 12.3 Time [ ms ]

I

I

12.9

13.5

FIG. 7. Pressure record and sehlieren photographs of medium knocking combustion, 7692 frames per second, spark plug at lower side, end-gas at upper side. Pi = 11.39 bar, T~ = 493 K, p~ = 10.8 k g / m 3.

TABLE I Parameters involved in the autoignition experiments Pi = 10.8 k g / m 3 T~ (K) p~ (bar) t~ (msec) Pc (bar) T,. (K) b,c tlg, (msec) Pil~n (bar)

473 10.9 14 59.7 888 0.11 ---

Tig n (K)

--

"r = t~g, - tc (msec)

>2

493 11.4 11.32 55.4 887 0.28 12.22 67.6 954 0.90

513 11.9 9.36 49.7 873 0.38 9.72 53.9 900 0.36

452

ENGINE COMBUSTION

values are measured ones, excepted for temperatures Tc and Tign, calculated with the pressure ratio and isentropic compression law (constant ~/ - 1.59) and for b, the unburned gas mass fraction, estimated assuming a linear dependance betwen the burned fraction and the mean overpressure. The following remarks hold: i) the observed change of the end gas region occurs at time tc corresponds to a narrow interval of the temperature (T¢ -- 875 K) suggesting strongly the development of significant chemical reactions; ii) at the end of an induction-like delay x = tign to, the reactions turn to be of an explosive eharacter, and severe pressure oscillations result; iii) the delay "r is a decreasing function versus Ti, despite of the tendaney of pressure and temperature measured at time tc to decrease suggesting that the complete end gas history from Ti to Tign (above 900 K) must be taken into account for the prediction of the burst occurrence. Finally it should be noted that the chemical changes in the end gas after to, imped more and more the normal flame propagation as Ti is decreased (~ increased).

t._

t:l_ - -

00

, 3

Pi

f~ign

, , 6 9 Time [ ms ]

I 12

15

FIG. 8. Mean pressure time-history obtained by low pass filtering (0-1 kHz) process. P~ = 11.85 bar, T~ = 513 K, Pi = 10.8 kg/m3: heavy knock.

,~ =

(dP/dt)k - (dP/dt) v (dP / dt )v

(2)

Characterization of the Autoignition Development: The pressure oscillations consecutive to the burst explosion is, as shown earlier, very dependant on the value of the initial temperature, the mixture density being constant. To quantify the intensity of the vibratory motion of the gas the parameter: Pe

- - Pign Pe - Pi

KI - -

(1)

was selected. In Eq. (1), Pigs is the chamber pressure reached at t = t i g n (i.e. just before the occurrrence of the severe pressure oscillations as shown on Fig. 4.a), Pe is the final mean pressure, obtained by processing the data of the pressure signal through the numerical low pass filter (0-1 kHz) as shown in Fig. 8. So defined, KI is somewhat representative of the mixture mass fraction involved in autoignition. The calculated values of KI, plotted on Fig. 9.a, increase strongly with T~ and p~, as it would be expected from the dependance of the global autoign i t i o n delay time on local p r e s s u r e P a n d temperature T: the higher Pi and T/are, the shorter the delay becomes during compression by the flame, so as to be insignificant compared to the remaining time for normal flame propagation. To quantify the effect of heat release rate related to autoignition in the end-gas, responsible for the pressure unbalance which then equilibrates through strong vibrations, 7 another parameter ct is chosen and defined as:

In Eq. (2), P is the filtered mean value of the pressure, (dP/dt)k, the maximum pressure increase rate during the knocking period and (dP/dt)p, the pressure increase rate due to normal flame propagation immediately before knock occurrence, estimated around t = tign. Figure 8 shows how (dP/dt)k and (dP/ dt)p are estimated. The observed correlation between KI and ct is shown in Fig. 9.b. Clearly, when KI is less than about 0.1 (light knock), a is lower than 0.5. Then, increases with KI and becomes larger than 5 for heavy knock conditions (KI > 0.25). The former observation might be explained by the fact that spontaneous ignition occur in a very small fraction of the initial mass confined in a very reduced volume by the flame so that the global effect is just to shorten slightly the time of combustion but not to increase significantly the rate of mean pressure augmentation. In the latter case, the end-gas which autoignites represent a non negligible part of the initial mass so that the mass burning and the heat release rate due to autoignition can be observed not only by a shortening of combustion time, but also by a singificant increase of the mean pressure rate.

Conclusion

A simple experimental device which reproduces different types of unstable constant volume combustion regimes starting from acoustic-flame interaction and growing to knock, has been developed.

EXPERIMENTAL SIMULATION OF ENGINE KNOCK I

0.4

9 9

0.3

9

0.2

A 9

A

0

0.1

420

0

0 I

I

440

G60

Q

I

480 Ti [K] (a)

I

I

500

520

FIG. 9.a. Characterization of knock intensity in function of initial temperature T~ for different initial densities.

453

nition, estimated under adiabatic compression assumption, were always lower than 1000 K. This fact, c o n f i r m e d by s c h l i e r e n c i n e m a t o g r a p h y , has strengthened the idea that low temperature reactions play certainly a significant role on the onset of knock, but further experiments, especially chemical species analysis of end-gas during the flame propagation, are still needed to clarify the observed two-stage mechanism. In order to approach real reciprocating engine conditions, this study should be extended by adding initial turbulence so as to evaluate its influence on the onset of knock. Nevertheless, the wall-heated bomb is a useful simple mean to analyze rapidly the resistance to knock of new fuels.

Acknowledgment The authors are greatly indebted to the technical team of the laboratory for its effective collaboration and to the Agence Fran~aise pour la Maltrise de l'Energie and to P.S.A. Etudes et Recherches fbr their financial support.

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REFERENCE

6 V

[3

5

o

V 9

A

A OVA

0

01

0.'2

o13 o14 KI

(b) FIG. 9.b. Evolution of the knock intensity criteria KI versus ct for different initial densities, o p~ = 6.2 kg/m 3, A p~ = 9.2 kg/m 3, V p~= 10.8kg/m 3, [] p~= 12.3kg/m 3

With Ti < 450 K, smooth acoustic pressure waves have been clearly identified as the result of hydrodynamic instabilities of the flame front related to its contact with the chamber walls. With higher initial temperature spontaneous ignition of end-gas accompanied by abrupt pressure waves are observed. In our experiments, the temperature of the end-gas at the moment of autoig-

1. MIDGELY, T. JR, AND BOYD, T. A.: SAE Transactions 17, Part 1, p. 7 (1922). 2. SOKOLIK, A. S.: Self Ignition, Flame and Detonation in Gases, Israel Program for Scientific Translations, Jerusalem, 1963. 3. MALE, T.: Third Symposium (International) on Combustion, p. 721. The Combustion Institute, 1949. 4. CURRY, S.: Ninth Symposium (International) on Combustion, p. 1056. The Combustion Institute, 1963. 5. AFFLECK, W. S., AND FISH, A.: Comb. Flame 12, 243, (1968). 6. GRIFF1TrfS,J. F., AND NIMMO, W.: Comb. Flame 60, 215 (1985). 7. KONO, M., SHIGA, S,, KUMAGAI,S., AND IINUMA, K.: Comb. Flame 54, 33, (1983). 8. HASKELL, W., AND BAME, J. L.: S.A.E. Transactions 74, 772, (1966). 9. GLUCKSTEIN, M. E., AND WALCUTV, C.: SAE

Transactions 69, 529, (1961). 10. MARIE, J. j., AND COTrEREAU, n . J.: SAE Technical paper n°870458, (1987). 11. OHTA, Y., HAYASHI, A. K., TAKAHASHI, H., FUJlWARA, T.: Consequence of Temperature-Pressure-Time History for Autoignition. Dynamics of Reactive Systems Progress in Aeronautics & Astronautics (J. R. Bowen, J. C. Leyer, and R. I. Soloukhin, Ed.), Part. 1, AIAA, 1986. 12. SRIGA, S., KONO, n . , I1NUMA, K., KARASAWA, T., KURABAYASHI, T.: Further Investigation of

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13.

14.

15.

16.

E N G I N E COMBUSTION Knock Intensity by a Thermodynamic Model and Experiments Using a Rapid Compression Machine. Paper presented at COMODIA, Tokyo, Sept. 1985. BALL, G. A.: Fifth Symposium (International) on Combustion, p. 366, The Combustion Institute, 1955. LEVEDAI~L, W. J.: Fifth Symposium (International) on Combustion, p. 372. The Combustion Institute, 1955. OHTA, Y., AND TAKAHASHI, H.: Homogeneity and Propagation of A u t o i g n i t e d Cool a n d Blue Flames. Dynamics of Flames and Reactive Systems. Progress in Aeronautics and Astronautics (J. R. BoweR, N. Manson, A. K. Oppenheim, and R. I. Soloukhin; Ed.) 95, p. 236, AIAA, 1984. SMITH, J. R., GREEN, R. M., WESTBROOK, C. K., AND PITZ, W. J.: Twentieth Symposium (International) on Combustion, p. 91. The Combustion Institute, 1985.

17. GREEN, R. M., CERNaNSKY, N. P., PITZ, W. J., WESTBnOOK, C. K.: S.A.E. Technical p a p e r n°872108 (1987). 18. OHTA,Y., AND TAKAHASHI, H.: Mixing of Flame Zone Mixture with U n b u r n e d Mixture. Another Initiation of Engine Knock than Autoignition of End Gas. Paper presented at the l l t h International Colloquium on Dynamics of Explosions and Reactive Systems. August, 1987. 19. GIRARD, A., FISSON, F., AND LEYER, J. C.: Vibratory Combustion Triggered by a Small Cavity in the Wall of a Constant Volume Combustion C h a m b e r . Dynamics of Flames and Reactive Systems. Progress in Aeronautics and Astronautics (J. R. Bowen, N. Manson, A. K. Oppenheim, and R. I. Soloukhin, Ed.) 95, p. 433, AIAA, 1984. 20. LEYER, J. C.: Contribution ~ l'6tude de la naissance et du d~veloppement des r6gimes vibratoires de combustion, Th~se de Doctorat ~s Sciences Physiques, Universit6 de Poitiers, 1970.

COMMENTS C. K. Wu, Chinese Academy of Science, China. Since the pressure and temperature of the e n d gas vary during the ignition delay period, and the rate of compression varies with speed of flame travel, how can the knocking or non-knocking behavior of the fuel-air mixture be quantitatively correlated with some characteristic pressure and temperature of the end gas?

Author's Reply. It is obvious that we did not intend to determine the autoignition delay of the endgas from the static combustion c h a m b e r experiments. Indeed, it would be then necessary to take the complete pressure and temperature history into account. Nevertheless, our results showed that autoignition of end-gas occurred at temperatures below 1000 K, assuring an isentropic compression law, and for pressures ranging from 40 up to 70 bars. These considerations, in addition to the results of Schlieren photographs showing preignition reactions in end gas at temperatures below 900 K, give indications on the necessity to take low temperature into account in order to describe properly the autoignition development, particularly when the endgas is brought to temperature and pressure domains close to those of e n g i n e s r u n n i n g u n d e r knocking conditions.

A. K. Oppenheim, Univ. of California, USA. The

concept of using a constant volume vessel for the study of knock is most commendable, and the authors should be congratulated for the impressive manner in which they have demonstrated its virtues. So far, however, they employed only gaseous fuels. It would be of great practical interest to apply their technique to liquid fuels, especially those in commercial use. At sufficiently high initial temperatures one should have no problem in maintaining their mixtures with air at a homogeneous state. Do you intend to extend your studies in this direction, and, if so, what are your plans for the future?

Author's Reply. The use of gaseous fuels at roomtemperature allowed us to perform experiments with mixtures of fuel-oxygen-argon, whose equivalence ratio is perfectly and easily controlled by preparing them manometrically in a tank u n d e r 15 bars, without any special vaporizing device. The use of n-butane, chosen because it has the lowest octane numbers among gaseous hydrocarbons, allowed us to demonstrate the aptitude of the preheated static combustion chamber for simulating engine knock and determining the conditions of its onset. So, we now plan to study and to compare the resistance to knock of commercial liquid fuels, by designing a preheated mixing tank system which allow to prepare properly homogeneous vaporized mixtures under pressure up to 15 bars.