Fundamentals of Antiknock
I
ALEXANDER ROSS and ELLIS B. RlFKlN Research Laboratories, Ethyl Corp., Detroit, Mich.
Theory of Tetraethyllead Action
Although tetraethyllead has been in use as an antiknock for over 30 years and much is now known about its effectiveness in various fuels and engines under a variety of operating conditions, the chemical behavior of tetraethyllead in the engine and the mechanism b y which it suppresses knock are still far from clear. Two articles introduce the reports of a comprehensive investigation of the fundamentals of antiknock action now being carried out in the research laboratories of the Ethyl Corporation.
The first contains an experimental re-examinatian and
discussion of some of the more basic work in the field to date and the other gives the kinetic equation for the decomposition of TEL.
KNOCK
is a sound caused by pressure oscillations in the combustion chamber. These oscillations are thought to result from the rapid release of chemical energy which may occur following the spontaneous ignition of that portion of the charge as yet unconsumed by the advancing flame front (2). Ignition of this portion of the charge, called the end gas, is preceded by chemical reactions
1 528
(precombustion) which are accelerated by the increasing temperatures and pressures of the end gas as the piston rises and the flame front advances. In suppressing knock tetraethyllead (TEL) must interfere with this sequence of events. Many workers have proposed specific mechanisms by ichich this interference occurs. These theories may conveniently be divided into two classes
INDUSTRIAL AND ENGINEERING CHEMISTRY
---those ichich characterize TEL as a promoter of reactions and those which consider T E L an inhibitor. Promoter Mechanisms
Sokolik and Yantovskii (79) have suggested that the ethyl radicals released during the decomposition of the T E L induce an oxidation of the de-
generate explosion type which competes Lvith the c o d flame reaction. This results in a decrease of cool flame intensity and a consequent increase in the time lag to spontaneous ignition. King (75) theorized that lead which is continuously deposited from T E L on the surface of the combustion chamber catalyzes the “flameless oxidation” of the the1 to steam and carbon dioxide. The sream then causes a reduction in the inflammability of the fuel. Badin (7) placed the seat of promoter action in a primary peroxidation product of the T E L which he pictured as similar to the known product of zinc dimethyl oxidation, (CHa)2ZnOy. This peroxidation product serves as a feeder of peroxides into the reacting charge. giving a gradual, controlled oxidation and eliminating the relatively long induction period and the rapidly accelerating reaction observed in knocking fuels. Inhibitor Mechanisms
Callendar (.5) was first to suggest that T E L had an inhibiting effect by depositing lead in “nuclear drops.” This lead then consumes the peroxides which would otherwise give rise to detonation. Egerton and Gates (70) extended this theory by proposing that the active ingredient is lead peroxide. This undergoes “mutual reduction“ \vith the fuel peroxides which would otherwise lead to knock. The resulting lower lead oxide is then reperoxidized by the available oxygen, and the catalyst is thus regenerated. Some evidence for this mechanism has been provided recently ( 7 7 ) in Ivork Lvhich shows that tertbutyl hydroperoxide is readily decomposed by lead dioxide but not by lead or lead monoxide. M’ithrow and Rassveiler (22) have suggested that the active substance is lead oxide acting as an oxidizing agent and being reduced to elemental lead in the process. This theory is based on specrroscopic xvork in a n engine where only elemental lead was derected in the end gas during antiknock action. Berl and LVinnacker ( 3 ) found lead oxide smoke to be without effect in the vapor-phase oxidation of hexane. Lead smoke. however. definitely retarded the oxidation. They, therefore, suggested that the lead actively combines with alkyl radicals or with peroxides. Norrish (76) also attributed antiknock action to atomic lead, which functions? according to his theory. by removi r g active oxygen atoms. ,411 the foregoing inhibitor theories view the active ingredient as molecular in nature. I n contrast, LValsh and corvorkers (7: 8) maintain that it is lead oxide in the form of solid particles which is the effective antiknock. These particles provide a surface upon which
active radicals such as OH, H. and HOs. can be destroyed. Although there is much work in the literature bearing on this problem, there is no conclusive proof for or against any of these theories. Some of the reasons for this state of confusion are described below. It must be granted, however. that most of the evidence supports the inhibitor theories. Examples of this are the definite increase in self-ignition temperatures of fuels (70. 73: 20) and the decrease in over-all reaction rates of oxidation systems (7. 79) in the presence of T E L . Possibly the most convincing evidence, however. is supplied by Rifkin, LValcutt, and Betker (77). Their work in a n engine operated with a delayed spark shows that there is a small but definite decrease in the preflame heat release of a fuel on addition of TEL, concurrent with a postponement of the autoignition. These data cannot easily be reconciled with a promoter mechanism. It is also fairly well established that the ethyl radicals in themselves are not the active antiknock. Many organic compounds which are known to decompose into free radicals have been tested in engines (8), and all are shown to be proknocks rather than antiknocks. It can. of course. be argued that these compounds do not decompose a t the proper time and hence produce the radicals too early or too late; hoivever: the wellkno\vn antiknock effectiveness of iron carbonyl which does not yield radicals on decomposition tends to dispel that argument. Obstacles to Greater Understanding
The problem of the mechanism of T E L action has been approached in two basic \vays. O n e group of experimenters has chosen to study the phenomenon in conventional laboratory apparatus. Such studies have been impeded by inconsistent and irreproducible surface effects, the lo\\ volatility of TEL. and limitations imposed by the flammabilitv of the mixture and by the necessity of working a t low pressures Interpretation of the results becomes difficult because the pressures. temperatures. and duration of the reaction which these limitations impose on the experiment fall far outside the range of engine conditions where T E L exerts its antiknock effect I n recognition of these difficulties. another group of experimenters has chosen to make its studies in experimental engines. This approach likewise involves difficulties related to the problem of maintaining constant conditions from cycle to clcle and measuring the rapidly fluctuating pressures and temperatures Interpretation of engine data is often difficult because it is not known a t what point in the oxidation of the fuel the antiknock reactions occur.
In addition experimenters have not always recognized the need for measuring the basic variables of temperature and pressure, resorting rather to engine parameters whose effect on the basic variables is often obscure. Both techniques are confronted by problems of analysis of the many products. I n addition, in the course of the decomposition of T E L vapor, a new solid phase is formed causing great difficulties in the kinetic interpretation of the results. For these reasons the work done by the many workers in the field has not been successful in elucidating the mechanism by which T E L prevents knock. Lead Suspension Experiments
I n the light of this understanding of the complexities of antiknock research, it was decided to repeat and extend some of the earlier basic work xvhich has been the foundation of one of the most widely accepted theories ( 6 ) . This work. done by Sims and Mardles (78). reportedly shows that “colloidal sols of iron, lead? and nickel [suspended in the fuel] are just as effective as the organometallic compounds“ of these metals in reducing knock. These metallic colloids were prepared by the thermal decomposition of the corresponding organometallics in high boiling solvents, bromonaphthalene, and petroleum jelly. After dilution with fuel, the resulting suspensions were knock rated. Preparation of Colloidal Suspensions
I n our repetition of these experiments. T E L (either 3.0 or 0.75 ml.) was added to 50 ml. of liquid paraffin (Stanolind. heavy grade), I n most runs 0.5 ml. of oleic acid was added as a colloid stabilizer. These solutions were then heated under a nitrogen atmosphere at temperatures of 250’ to 300” C. for intervals of ‘ 2 to 3 hours. During this time very finely divided black particles formed and remained suspended in the medium. .4fter cooling, these SUSpensions were diluted with 900 ml. of a fuel consisting of 52.5%, iso-octane and 47.57c n-heptane by volume. With the runs in which 3.0 ml. of T E L were added, the resultant total lead level was equivalent to approximately 1 2 ml. of T E L ;gallon (about the same as used by Sims and Mardles): when 0.75 ml. of T E L was added. the final lead level was approximately 3 ml. of TEL:’gallon. T h e resulting suspensions were fairly stable with no evidence of settling for 30 minutes or longer. Analysis of Colloidal Suspensions
The solid black material was shoum to be metallic lead by x-ray diffraction VOL. 48, NO. 9
SEPTEMBER 1956
1529
Table I.
h'iin .Yo. 1 2 3 4 5
Mineral Oil None Mineral Oil
7 8 9
10 11
12 13
None
Unheated Unheated Unheated
0.75
I " 1
(L
uo Figure 1 . Electron microphotograph of particles in lead suspensions
30
Lead A U U ~ Y S Z lfiS , Gal C'ndecomposed Decomposed,
250-270
Unheated 30 60 120
300 300 300
TEL
TEL
*..
...
11.82 11.92 8.56 2.98 1.39 0.95 0.18
Knock
51.0
... ...
89.5
3.24
85.1
86.8
...
77.0 70.6
1.52 1.84
...
...
...
2.76
120
300
0.34
Centrifuged off
...
0.04
2.40
...
... Unheated Unheated
e . .
...
...
INDUSTRIAL AND ENGINEERING CHEMISTRY
52.4 56.5
51.9 53.4 62.4
...
11.60
e . .
......
Effectiveness of Suspensions
Decomposition of T E L in mineral oil under the specified conditions \vas sloxv and incomplete (Table I ) . Even after treatment at 300' C. for 2 hours about 6% of the T E L remained undecomposed. LVith 30-minute heating a t 250" to 270' C. (conditions used by Siins and Mardles), almost 75% of the T E L remained unchanged. In every case decomposition of the T E L resulted in a reduction of the octane number of that particular mixture; the greater the extent of decomposition. the lower the resulting octane number. Because it can be inferred from these data that the undecomposed TEL is the effective ingredient of the mixture, a comparison was made betiveen the antiknock value of the suspensions and the antiknock value of a series of unheated solutions containing various amounts of T E L in the mineral oil-iso-octaneheptane solution used. T h e results are shoivn graphically in Figure 2. From the close agreement between the suspension values and the calibrating curve. it is quite clear that all the antiknock effectiveness is accounted for by the undecomposed T E L ; conversely. the decomposed, suspended lead is completely ineffective as a n antiknock. In order to check this conclusion. the solid material in t\vo of the runs was separated by centrifuging and redispersed in fresh fuel. I n each case the resulting suspension appeared to be as stable as the original. These next suspensions showed no antiknock activity whatsoever. M'hen T E L and a-bromonaphthalene (the decomposition medium used by Sims and Mardles) were added to the heptane-iso-octane mixture without prior decomposition, knock ratings of the resultant solution shoived that most
64.6 54.0
Centrifuged off
...
None
-
I .c,'
-
r
1
f?atwljb
(Octane S o )
0.75
3.0
measurements. By extraction of the suspensions tvith hydrochloric acid all the lead present was converted to lead chloride which was then determined by the standard colorimetric molybdate method (4). T o distinguish betn.een T E L and its decomposition products, another sample of each suspension was treated with a mixture of ammonium hydroxide and ammonium acetate. This reagent extracts all forms of lead present except TEL. (\$'here solution of the lead metal was slow, 5% nitric acid was carefully added.) The lead thus extracted \vas precipitated as lead chromate and deter(72). The mined gravimetrically amount of T E L present was then found by difference. The size of the lead particles was found by electron microscopy to be between 0.05 and 0.3 p in diameter with a median estimated a t 0.2 p (Figure 1). The knocking tendencies of the suspensions were determined by the standard ASTM D-357 method (Motor Method). T h e same determinations were run on a number of appropriate blanks which \yere not heated.
1 530
Tzme, mw
0.75
Solid material of run 8 redispersed in 950 ml. of fuel (52.5 O.N.) Mineral Oil Solid material of run 10 redispersed in 950 ml. of fuel (52.5 O.N.) a-Bromonaphthalene
DKompOS&tlO,i__ TPmp OP
T E L Added, -1f1. 3.0 3.0 3.0 0.75 0.75
6
Effectiveness of Lead Suspensions
4
5
-
...
~7
__ '
9
0
'
I% T E_ P E P r - - L ~
Figure 2. pensions
Knock ratings-lead
sus-
of the antiknock effectiveness of T E L
was destroyed by the high bromine concentration. For this reason, no extensive studies of T E L decomposition in e-bromonaphthalene were made.
Re-evaluation of Previous Work on Suspensions
By using 3.0 ml. of T E L per liter of fuel (equal to about 12 ml./gal.), Sims and Mardles \cere operating at a level where T E L increments show quite small incrrases of antiknock effect (Figure 2 ) . Thus their runs approximate our run 4 (Table I), which 27.57, of the T E L was decomposed. If all the lead \cere effective, the octane number of this suspension Ivould be 87.5. If only rhe undecomposed T E L ivere
efective and the suspended lead completely inert, the octane number \\zould be 85.8,a difference of only 1.7 octane numbers. At this level then, it is difficult to evaluate the effectiveness of decomposed versus undecomposed T E L . It is for this reason that most of the Tvork reported here was done at a concentration of 3 ml. of TEL ‘gallon. Furtherinore the assumption that the T E L was completely decomposed with the time and temperatures of decomposition used by Sims and Mardles is definitely erroneous, as shown by our results. A11 the above factors show that the antiknock effect of the “colloidal sols“ of Sims and Mardles \vas due only to the undecomposed TEL. It must no\v be concluded that suspensions of finely divided lead particles (0.05 to 0 . 3 ~ ) are not effective antiknocks. T h e implications of this conclusion are discussed below l e a d Smoke Experiments
.%nother piece of evidence often quoted in support of the surface theory of inhibition (27) \vas supplied by Egerton and Gates in 1927 (9). They state that “lead vapor distilled from an arc in a stream of nitrogen and passed into the engine cylinder produced a rise in H.V.C. (highest useful compression ratio) Mhereas a carbon arc gave no such effect.” Since no experimental details or quantitative data were presented, this work has been repeated and extended. Lead smoke was generated in the arc \vhich resulted when a 48-volt d.c. potential was applied between a pointed carbon electrode of 1;’4-inch diameter and a lead electrode made by- filling a crater in the head of a 1-inch diameter carbon rod with lead (Figure 3). The smoke was carried by a stream of metered prepurified nitrogen (0.5 cu. ft.,’min.) into the intake manifold of a modified CFR single-cylinder, variable compression ratio engine. Approximately 10% of the smoke was removed by isokinetic ;ampling through a probe in the center i f the stream. T h e sampled smoke was collected on a “millipore” filter (manufactured by Love11 Chemical Co., IVatertown, Mass.) which is extremely effective in removing particles as small as 0.2 micron in diameter. T h e filter was iveighed before and after each run. T h e engine was run under the folloiving conditions: Engine speed, r.p.m. 900 F/A ratio 0.081 3 Ignition timing, degrees before top center 20 Inlet air temp., O F. 150 Jacket temp., O F. 205 Oil temp., O F. 150 Fuel: 60y0 iso-octane in n-heptane (0.43 gal./hour)
NITROGEN INLET
INLET AIR AND FUEL
I
VALVE
Figure 3. Apparatus for evaluating antiknock effectiveness of lead smoke
The knock-limited compression ratio \vas determined by visual reference to the pressure-time trace on an oscilloscope This procedure \vas carried out with and Lvithout the arc. For reference purposes a series of leaded fuels !vas knock rated under the same conditions but iLith no arc. This gave a T E L response curve which alloived knock ratings of the smoke. made in terms of compression ratio. to be expressed in terms of equivalenr T E L concentration. Effectiveness of l e a d Smokes
T h e results of four runs (Table 11) sho\v that lead smoke generated in a stream of nitrogen is bet\veen 10 and 19T6as effective as an equivalent amount of lead in the form of ?’EL. Electron microscope examination showed the smoke particles to be 0.02 to 0.4,~ in diameter \vith a median of about 0 . 1 ~ . X-ray diffraction patterns showed the deposit on the filters to be primarily metallic lead ivith some yellow lead oxide present. T o ascertain that the antiknock effect was due to the lead and not to the electrical properties of the arc, the runs were repeated using only a carbon arc. This required a higher voltage for op-
Table II.
eration. S o antiknock effect was observed, In fact there was a slight increase in the knocking tendency attributable to the larger amount of heat released by the carbon arc and a consequent rise in the inlet air temperature. \Vhen the lead arc was struck in air rather than nitrogen: the smoke formed \vas yello\v lead oxide. This smoke also exhibited definite antiknock properties; however it \vas impossible to get a quantitative estimate of its effect becauze of difficulty in maintaining the arc in air. Once extinguished, the arc could not be restarted. This behavior was due to a layer of lead oxide which formed on the surface of the molten lead and cut off‘ the flow of current to the lead electrodc. Quantitative results from these experiments are only approximate because of difficulties in sampling and collecting the smokes, but the conclusion that the output from a lead arc can have a n antiknock effect, as reported by Egerton and Gates. is definite]!. confirmed. Conclusions
This work shows that finely divided lead suspended in a fuel has no antiknock properties. O n the other hand
Effectiveness of Lead Smokes dfktikriock
Duratioir of Run, R u n -1-0. .\fin. 1 2 3 4
5.37 6.55 5.34 4.95
Lead Sampled, -7lg. 13.1 7.9 19.7 8.0
Lead Input, Mg. 134 81 194 79
-4rltikllocii Equiia-
Lead EffecticeInput, n PSS , lence, G./Gal. Fuel A C. R. J i l . TELjGal. 3.52 1.74 5.05 2.21
0.16 0.14 0.32 0.22
~~~~
0.34 0.30 0.57 0.43
Relatire EffertizPiicss,
% of TEL 10 17 11 19
~
VOL. 48, NO. 9
SEPTEMBER 1956
153 1
the products from a lead arc d o have
a measurable antiknock effect! even though the particles in the two experiments are apparently in the same size range. There are some plausible explanations for this apparent contradiction. T h e first explanation is based on the possibility that some of the lead particles entering the engine from the lead arc were actually very much smaller than is indicated from the analysis of the material collected on the filter. O n e cannot rule out the possibility that some lead may have been atomically dispersed. T h e existence of lead vapor or of agglomerates u p to 500 -4.> in concentrations of about lOy0 of the total lead, would then easily explain the difference betlveen the two sets of experiments. .4lthough this is very easy to conceive. it is very difficult to verify experimentally. O n the other hand. it may be assumed that the smoke particles on the filter are of a size truly representative of the material entering the engine. Since these particles are of the same size as those obtained in the suspension experiments, the difference in behavior must be attributed to the nature of the particles. I t is possible that the greater age of the suspended particles or their oleic acid coating in some way inactivated the lead surfaces. Another possibility is that the lead smoke particles \yere quickly oxidized to lead oxide in the hot inlet air stream, while the suspended particles were protected from oxidation a t this stage by the oleate coating. If lead oxide is indeed the effective antiknock and lead is not! as previously pro-
Fundamentals
posed (7). this difference in oxidation state would explain the apparent contradiction. T h e major conclusion to be draivn from this !vork is that inorganic lead in some form has antiknock properties. T h e evidence available to date. however. does not allow one to distinguish betneen a mechanism based on a chemical reaction as suggested by Egerton ( 9 ) or a surface inhibition as postulated b) Chamberlain and LValsh (7) T h e observed effectiveness of inorganic lead suggests that the decomposition products of T E L are important in the anriknock mechanism. For this reason a study of the factors Lrhich affect TEL decomposition is being made.
Acknowledgment T h e authors express their appreciation to R. C. Getoor for his help \vith some of the experimental Ivork. They also wish to acknowledge \rith gratitude the electron microscope work. done by C . Lavintal of the Physics Department, Cniversity of Michigan. literature Cited ( 1 ) Badin? E. J.. "'Third Symposium on Combustion, Flames, and Explosion Phenomena." p. 386. Williams 8r Wilkins. Baltimore, 1949. (2) Ball, G. A,. SAE National Fuels and Lubricants Meeting, Tulsa, Okla., S o v . 5 %1954. (3) Berl. H. E.. Winnacker. %. physik. Chem. A145, 161 (1929). (4) Calingaert, G.. Gambrill. C. M.. IND. ENG. CHEM.. ANAL. ED.. 11,324(1939).
( 5 j Callendar. H . L.. King, R. O., Sims. C. .J., ;\eronautical Research Committee. Reuorts and Memoranda. 1013, 1062: 1925 [Engineering 121, 475 (1926)l. (6) Chamberlain. G. H . K.,Hoare. D. E.. Walsh. A . D.. Discussions Faradav SOC.S o . 14. 89. 1953. ( 7 ) Chamberlain. G. H. Walsh. .A. D., Proc. Roy. Soc. (London) A215, 175 (1952). ( 8 ) Downs, D.: Walsh. A . D.: IYheeler. R . W., Trans. Roy. SOC.(London) 243,463(1951). (9) Egerton, A , . Gates. S. F., .J. Inst. Petrolfum Technoi. 13. 250 (1927 I. f l 0 i Z b i d . . ~ .281 ( 1 1 ) Egerton, A . C.. Rudrakanchana, S., Fuel 33,274 ( 19 54 1. (12) Ethyl Corp.. Detroit. kfich.. Analvtical Method 10-53. (13) Frank, C. E.. Blackham. A . L'.,I s u . EKG.CHEM. 4 4 , 862 (1952). (14) Hoare, D. E.. Walsh. A . D., "Fifth Symposium on Combustion,'' Abstracts. p. 77. Pittsburgh, Pa.. September 1954. (15) King. R . 0.. Can. J . Research F26, 125 (1948). (16) Norrish. R . G. W.. Ptoc. Roy. Snc.. ( L o n d o n ) A-150, 26 (1935). (17) Rifkin, E. B.. Walcutt. C., Betker, G. W..Jr.. SAE Quart. Trons. 6 , 472 (1952). (18) Sims. C. J.. Mardles. E. W..I.. T r a n j . Faraday Soc.. 22, 363 (1926). (19j Sokolik. A . %Yantovskii. S. Acta Physicochim. (C',R,S,S.) 19, 329 (1944). (20) Sortman. C. W.. Beatty. H. A , , Heron, S. D.. I N D . ENG. CHEM.33, 357 (1941 ), (21) Walsh. A. D., "Third Symposium on Combustion. Flames; and Explosion Phenomena," p. 389, Williams and Wilkins. Baltimore. 1949. (22 j Withrow. L.. Rassweiler. G. M., IYD. ENG.CHEM.27, 872 (1935).
