I
JAMES
B.
HINKAMP and JOEL A. WARREN
Ethyl Corp., Detroit, Mich.
Surface Ignition Control Fuel Additives b Are phosphorus additives really effective? These experiments prove their value in reducing the number and severity of surface ignitions
PRIOR
to 1953, the commercial use of phosphorus additives by the petroleum industry was limited to lubricants. The ability of certain phosphorus compounds to provide lubricity, antiwelding characteristics, antioxidant properties, and bearing corrosion protection is the subject of hundreds of patents. Many different phosphorus additives are sold in substantial quantities for these purposes. The first reference to phosphorus compounds as fuel additives for altering the effects of engine deposits appeared in 1946, when use of tributyl phosphite was described as a means of controlling preignition (7). The next year, a second patent described trimethyl phosphite and trimethyl phosphate as an improvement over tributyl phosphite { 72). Several years passed, however, before the trends in automotive-engine design toward higher compression ratio and greater power output raised the automotive preignition problem to the level of commercial significance. In 1952, compounds of phosphorus were found effective in aviation (73) and automotive (4) fuels in reducing sparkplug fouling, and in 1953 two phosphorus compounds were introduced as commercial additives for automotive fuels to control spark-plug fouling and preignition (9, 70). Surface Ignition
Normal combustion in the Otto-cycle engine is spark ignited and proceeds smoothly throughout the fuel-air mixture without auxiliary ignition from deposits or other hot surfaces within the combustion chamber. Abnormal combustion may result from a number of conditions. The combustion chamber often contains deposits of such composition that part of them may become sufficiently heated to initiate oxidation, which causes them to glow and remain glowing for one or more engine cycles. If sufficient heat is thus made available, the deposit may act as a source of ignition, and start an abnormal flame front in the combustion chamber. This phenomenon is called surface ignition,
by Phosphorus
defined by the Coordinating Research Council (3) as the “initiation of a flame front by any hot surface other than the spark discharge prior to the arrival of the normal flame front.” Such abnormal flame fronts may be started prior to the normal spark discharge (as in the diagram) or after the normal flame front is already in motion. The presence of the abnormal flame front, or fronts, from surface ignition can subject the unburned fuel-air mixture to conditions of temperature and pressure more severe than those of normal combustion. If the antiknock quality of the fuel just satisfies the engine under normal conditions, the added severity caused by the presence of the abnormal flame front, or fronts, may induce autoignition of the unburned fuel-air mixture or end gas. This results in a sharp metallic sound similar to that associated with spark knock. Because of its erratic nature, knock due to surface ignition is often called “wild ping.” If the fuel is of sufficiently high. antiknock quality, knock will not occur and the only manifestation of the surface ignition that is observable without special instrumentation is a momentary roughness of the engine. This is caused by the abnormal rate of pressure rise that results from the more rapid consumption of the change by the additional flame front, or fronts, in the combustion chamber.
Equipment and Instrumentation Engines. The engine-deposit effects of various phosphorus compounds were determined in single-cylinder, Cooperative Fuel Research (CFR) L-head and overhead-valve (OHV) engines. The O H V engines employed were designed to duplicate the geometry of two automotive V-8 O H V engines currently in wide use. The compression ratio of one of these could be varied from 7.5 to 1 to 11.5 to 1 by interchanging heads. The compression ratio of the other: O H V engine was 7.0 to 1. The L-head engines normally had a compression ratio of 7.0 to 1, but it could be raised to 8.5 to 1 by interchanging heads. The special cycling schedule (Table I) was used to accumulate deposits in all tests discussed here. In the 150- to 175hour tests, the octane requirement was measured a t 24-hour intervals, while in both the 45-hour and 150- to 175-hour tests the number of surface ignitions was
registered automatically throughout the test on an electronic total surfaceignition counter. When used with the CFR L-head engine, this procedure increases both the rate of deposit accumulation and the tendency of the deposits to produce knock, increasing the sensitivity of the test method and magnifying the effects of test variables. With this procedure the effects of fuels, lubricants, and additives agree relatively with results obtained in modern engines operated in passenger-car road service (6, 7). Although no O H V engine data are presented, it was found that the effects of phosphorus in the two types of engines are analogous. Pertinent information on the fuels and lubricant used in these studies is given in Table 11. Both fuels were commercialtype premium grade automotive gasolines. The lubricant was a nonadditive, high VI, straight mineral oil. Total Surface-Ignition Counter. The incidence of surface ignition was measured by a modification of the procedure described by Hirschler, McCullough, and Hall (8). The counter automatically detects and counts uncontrolled combustion arising from ignition by the deposits. This equipment consists of an electronic circuit, an electromechanical recording counter, a camshaft-driven timing device, and an ionization gap in the combustion chamber. The ionization gap has a voltage impressed across it which is only a small fraction of that necessary to produce a spark. When a flame front passes the gap, ionization of the gas reduces the gap resistance and permits a current pulse to flow. This pulse triggers an electronic circuit and actuates a digital counter. When surface ignition is being studied, a camshaft-
Table 1.
Engine Cycling Schedule Idle
Full Throttle
50 150 Cycle duration, sec. Speed, r.p.m. 600 900 Fuel-air ratio 0.087 0.077 Ignition timing TDC“ Coolant temp., O F. 148 Oil temp., F. 160 Intake air temp., O F. 110 Compression ratio 7.0-1 Test duration, hours 456 a Top dead center. Tests extended to 150 to 175 hours when measurements were made of octane number-requirement increase.
VOL. 50, NO. 2
FEBRUARY 1958
251
I
1
TOTAL SURFACE-IGNITION COUNTER
I I
i-------------J
Surface ignition and mechanism with which it is detected and counted
driven switch or timing device is used to ground the electronic surface-ignition counter, making it inoperative just prior to the time of arrival of the normal flame front. The earliest normal flame fronts, in the CFR L-head engine, usually arrive at the ionization gap at about 15' after top center, if the ignition timing is at TDC and the fuel-air ratio is 0.077. For this condition, the timing device is set to close automatically at 10' after top center, and hence none of the normal flame fronts will trigger the counter. The timing device automatically opens at 70' before top center, thus
Table II. Fuel and Lubricant Inspection Data Fuel
2-4
Gravity, OAPI Vapor pressure, lb./sq. in. Distillation, F. I.B.P." 10% evaporated 50% evaporated 90% evaporated F.B.P.b Dissolved gum, mg./100 ml. Oxidation stability, min. Sulfur, wt. yo Tetraethyllead, ml./gal. Ethyl Motor Mix Octane number Research Motor Hydrocarbon type, vol. % Olefins Aromatics Saturates
60.5
Lubricating Oil Type
8.4
2D 59.5 7.2
94
100
133
154 229 311 388
226 331 390 1.6
3.9 395 0.079 0.074
780
3.0
3.0
93.1 86.0
100 88.7
9.7 18.5 71.8
20.4 32.8 46.8
1A
Mixed base, solvent re5ned SAE No. 20 30.5 Gravity, "API Viscosity Saybolt, Sec. 373.8 At 100' F. At 210' F. 58.4 Viscosity index 107.4 0.25 Sulfur, wt. % Ash, wt. % 0.012 None Metals present a Initial boiling point. 6 Final boiling point.
252
making the counter operate over an 80' period of crankshaft rotation. The surface ignition illustrated resulted in the initiation of an abnormal flame front in the combustion chamber prior to ignition by the timed spark. I n this instance, the abnormal flame front has moved across the ionization gap prior to 10' after top center and hence will automatically be registered on the counter. If the antiknock quality of the fuel is too low, or if the hot deposit ignites the charge very early in the cycle, an end-gas knock may occur. This would be a case of audible or knocking surface ignition. The total surfaceignition counter does not discriminate between knocking and nonknocking surface ignitions, but it records all abnormal flame fronts, if they arrive a t the ionization gap within the specified period. Surface ignitions determined by this electronic equipment are referred to as total surface ignitions. The technique employed provided a convenient means of measuring the surface-ignition effects of fuels, lubricants, and additive materials. Additives were screened in 45-hour tests, which were usually adequate to permit the surface-ignition control effect of the phosphorus fuel additive to build up to equilibrium value. Such tests were extended to 150 to 175 hours when it was desirable to determine the effect of additives on both surface ignition and octane-requirement increase. The reproducibility of the data was subject to the normal variability observed in engine testing procedures. Hence, the data reported in this paper were in no case obtained from a single experiment but are average values obtained from a number of engine tests which have a precision well within i5%. Audible Surface-Ignition Counter. The incidence of "wild ping" or knocking surface ignition was measured by means of an audible surface-ignition counter (6). This device consists of a microphone. suspended above the head of the engine, the necessary electrical components, and an electromechanical recording counter. The microphone is isolated from external noises by an acoustical baffle. An electrical filter is incorporated. which results in the counter's being actuated by only the high-frequency disturbances resulting from audible surface ignitions or from knock. An acoustical means permits the sensitivity of the audible counter to be adjusted periodically to a standard. -4s fuels of sufficient octane number to prevent knock are used in all tests, the counter supplies an accurate count of audible surface ignitions only. Crank-Angle Time Recorder. The usefulness of the surface-ignition counter has been extended considerably by the development of the crank-angle time recorder ( I I ) , which makes it con-
INDUSTRIAL AND ENGINEERING CHEMISTRY
,
0!
00
I 01
I
02
I
03
I 04
05
WOSPHORUS CONTENTRATION, TtIEORIES
Figure 1. Surface-ignition control by phosphorus compounds Fuel, 2A
4- 3.0 ml. of tetraethyllead per gallon
veniently possible to obtain a permanent record of the time in crank-angle degrees at which a number of engine phenomena take place, including the arrival of surface-ignited flame fronts a t the ionization gap. In addition, the number of such flame fronts is recorded. This instrument makes possible the study of the effect of surface-ignition control additives on the timing or severity o f individual surface ignitions as well as on their total number. The signal received by the total surface-ignition counter, when the gap is ionized, triggers a spark generator which jumps a spark between an electrode and a crankshaft-driven cylinder covered with sensitized paper. The electrode is moved along the longitudinal axis of the cylinder and shows each ionization as a dot on the sensitized paper. This record makes it possible to tell at what crank-angle degree in the cycle the gap was ionized, and it also shows the time interval between successive surface ignitions. Variations in Surface-Ignition Control by Phosphorus Compounds
The phosphorus concentration of interest as a gasoline additive is in the range of 0.2 to 0.5 of the quantity theoretically necessary to convert the tetraethyllead present to lead orthophosphate. This may be conveniently described as 0.2 to 0.5 theory ( T ) of phosphorus. I n fuel containing 3.0 ml. of tetraethyllead per gallon, this is equivalent to 0.06 to 0.15 gram of phosphorus per gallon. Unless otherwise stated, the work reported here was done with 0.2T of phosphorus in fuel containing 3.0 ml. of tetraethyllead per gallon as Ethyl antiknock compound (Motor Mix). The ability of different phosphorus compounds to control surface ignition varies widely-one material tested a t a concentration of 0.2T in a premium
SURFACE I G N I T I O N CONTROL
'PHOSPHATE NONE
TRIOCTYL
TRIAMYL
TRIBUTYL
TRIMETHYL
Figure 2. Surface-ignition control by trialkyl phosphates
+
Base fuel, 2A 3.0 ml. of tetraethyllead per gallon; oil, 1 A; phosphorus concentration, 0.2T
base stock gasoline reduced surface ignition to of the base line or phosphorus-free fuel value: another phosphorus compound of no commercial potential reduced surface ignition to 6% of base line value. The extension of this relationship to other phosphorus concentrations is shown in Figure 1. Attempts to correlate the surfaceignition control of different phosphorus compounds with volatility, structure, stability, or other physical or chemical properties have been a t best only partially successful. For example, a given class of compounds will show good correlation between volatility and effectiveness. I n another class, sych correlation may break down completely or even be reversed. Trialkyl Phosphates. The surfaceignition control of trialkyl phosphates appears to be a simple function of molecular weight or one of its attendant properties, s ch as volatility. Trimethyl phosphate, t e most volatile member of the series, also proved to be the most effective by reducing surface ignition to 21% of base line value. Tributyl, triamyl, and trioctyl phosphate followed in that order by reducing surface ignition to 23, 27, and 39%, respectively. These data (Figure 2) show that increasing the size of the alkyl group progressively decreases the effectiveness of the molecule. Alkyl Aryl Phosphates. An interesting relationship between structure and surface-ignition control was found in the methyl-tolyl phosphate series (Figure 3). Either trimethyl phosphate or dimethyl tolyl phosphate reduced surface ignition t o 21% of base line value. Under identical conditions of test, methyl ditoIyI phosphate reduced surface ignition to 28y0 and tritolyl phosphate to 32y0 of base line. Effectiveness would appear to be related to paraffinicity or perhaps to volatility.
1
PHOSPHATE NONE
DITOLYL METHYL
TOLYL DIMETHYL
TRIMETHYL
Figure 3. Surface-ignition control by alkyl aryl phosphates
+
Base fuel, 2 A 3.0 ml. of tetraethyllead per gallon; oil, 1 A; phosphorus concentration, 0.2T
Alkyl aryl phosphates may be readily prepared by the reaction of phosphorus oxychloride with phenol, cresol, or xylenol to form the mono or diary1 intermediate. ArOH Poc13 + ArOPOClz HC1 2ArOH
TRITOLYL
+ + + POCli +(Ar0)zPOCI +
2HC1
This is followed by reaction with the appropriate alcohol to yield the desired alkyl aryl product. Triaryl Phosphates. No comparison was made of the relative surface-ignition control of various triaryl phosphates. However,. an interesting effect resulted when sulfur atoms were introduced into the tritolyl phosphate molecule (Figure 4). Tritolyl phosphate reduced surface ignition to 320/, of base line, while either tritolyl thionophosphate or tritory1 tetrathiophosphate reduced surface ignition to 23y0/,. Replacement of the oxide oxygen atom with sulfur increased the effectiveness of the molecule. Replacement of the three remaining oxygen atoms produced no measurable change. The thionophosphates may be prepared from phosphorus thiochloride and the appropriate phenol, while the thiolo derivatives are made from phosphorus oxychloride and thiophenols. 2-Chloroalkyl Phosphates. Differences in surface-ignition control brought about by the interchange of oxygen and sulfur atoms were also demonstrated by a group of chloroalkyl phosphates (Figure 5 ) . Replacement of oxide oxygen by sulfur in both the 2-chloroethyl and 2chloroisopropyl phosphates markedly increased the effectiveness of the compound. Tris(2-chloroethyl) phosphate reduced surface ignition to 42% of base line, tris(2-chloroethyl) thionophosphate to 17%, and tris(2-chloroisopropyl) phosphate and tris(2-chloroisopropyl)
thionophosphate to 27 and 17%) respectively. The thiono derivatives were more effective as in the case of the triaryl phosphates. However, the relative effectiveness of the sulfur-free materials was contrary to that expected from the previous examples cited. The ethyl derivative of lower-molecular weight was less effective than the propyl analog. The chloroalkyl phosphates are conveniently prepared by the reaction of phosphorusoxychloride with olefin oxides. For example, POCIS CHz-CHz + \O/ P ( 0 ) (O-CHz-CH2Cl)a
+
The analogous thiono derivative requires a two-step procedure. The first step involves the reaction of phosphorus trichloride and an olefin oxide to form a chloroalkyl phosphite, PCla
+ CHz-CHz "/
+ P(O-CH2-CH&I)a
followed by reaction with elemental sulfur which converts the phosphite to the thionophosphate. P(O-CHz-CHzCl)s S+ P(S) (0-CHz-CHzCI)~
+
The analogous chloroisopropyl thionophosphate has been successfully developed as a commercial additive, Phosphonates. The rather limited data in Figure G on phosphonates show that completely aromatic phosphonates may be more effective than the mixed dialkyl arylphosphonates. Ditolyl phenylphosphonate reduced surface ignition to 3170 of base line, while dimethyl phenylphosphonate reduced surface ignition to 49%. Hence, both phosphonates have a rather low level of surface-ignition control. Although phosphonates may be prepared in a number of ways, probably VOL. 50, NO. 2
FEBRUARY 1958
253
ADDITIVE NONE SULFUR
ADDITIVE' SULFUR:
NONE
Figure 4. phates
-TRITOLYL PHOSPHATE' NONE THIONO TETRATHIO
Surface-ignition control by triaryl phos-
+
Base fuel, 2 A 3.0 ml. of tetraethyllead per gallon; oil, 1 A; phosphorus concentration, 0.2T
the most versatile method involves the Arbuzov reaction. (R0)aP
+R'X+
(RO)zP(O)R' 4-RX
Phosphites. The relative instability of phosphites limits their value as fuel additives, Consequently, only a few engine tests of this class of materials were made. The data available showed that tri-n-butyl phosphite and triphenyl phosphite reduced surface ignition to approximately 17 and 25YC, respectively, of base line value. Inorganic Compounds. Only a few inorganic gasoline-soluble phosphorus compounds are suitable for consideration as fuel additives. The materials examined displayed the same wide range of effectiveness which was shown by organic phosphorus compounds. For example, phosphorus sesquisulfide, P&, is quite effective, while phosphonitrile dichloride trimer, (P;\C12)8, does not possess unusual activity. These materials reduced the incidence of surface ignition to 18 and 3870, respectively (Figure 7). Phosphonitrile dichloride trimer is the volatile product of the reaction of phosphorus pentachloride and ammonium chloride. 3PClj
+ 3NH4Cl+
(PNC1z)a
4-12HC1
A mixture of polymers is formed, of which approximately one third is the trimer. Influence of Phosphorus on Incidence of Surface Ignition
The foregoing examples demonstrate the ability of certain phosphorus compounds to reduce markedly the incidence of surface-ignited flame fronts. T o elucidate the manner in which this was accomplished, it was reasoned that any factor which reduced the frequency of these early flames would do so by inhibiting the ease by which they are
254
ALKYLGiiOdF
T R I S (2-CHLOROALKYLI PHOSFHATETHIONO NONE 'H13NO
C E-HYL
-
-1SCPROPYL-
Figure 5. Surface-ignition control chloroalkyl phosphates
by
2-
Base fuel, 2A f 3.0 ml. of tetraethyllead per gallon; oil, 1 A; phosphorus concentration, 0.2T
ignited, or by delaying abnormal ignition. Thus, if the timing of each abnormal flame front was retarded somewhat, the later surface ignitions would not occur at all because the fuelair charge would already be consumed by the normal flame. Consequently, both the total number of surfaceignited flame fronts would be reduced, and the average timing of those remaining would be retarded. T o test this theory, an engine was equipped with a crank-angle time recorder to measure the time in the engine cycle a t which the surfaceignited flame fronts reached the ionization gap in the combustion chamber. The base line fuel normally used was replaced by a similar fuel of higher octane quality to ensure knock-free engine operation at the 8.5 to 1 compression ratio employed for these tests. The engine was operated both with and without tris(chloroisopropy1) thionophosphate in the fuel. The data obtained (Figure 8) show that the earliest surface ignitions occur in the 23' to 30" before top center (BTC) period when no phosphorus is used and that a large and fairly constant number occur in each 5' increment from 10' BTC to 10" after top center (ATC). \Yhen 0.2T of phosphorus was added to the fuel, the earliest surface ignitions at the equilibrium conditions occurred in the 15" to 20" BTC period. about 10' later in the cycle than for the test with phosphorus-free fuel. The number of surface ignitions in each of the other periods was reduced at least 507, by the action of the phosphorus. However, the greatest percentage reductions occurred in the early periods of the cycle. Tris(chloroisopropyl) thionophosphate eliminated the earliest surface ignitions that otherwise take place and greatly reduced the frequency of those that occurred later in the cycle. I t has been established
INDUSTRIAL A N D ENGINEERING CHEMISTRY
-
NONE
(77) that the earliest surface ignitions are those most prone to result in knock or wild ping. Consequently, surface ignitions which remain in the presence of phosphorus are of relatively minor importance.
Effect of Phosphorus Additives on Phosphorus-Free Deposits
All the work described u p to this point involved engine tests started with clean combustion chambers, which adequately demonstrate the relative ability of phosphorus additives to control surface ignition when all the fuel consumed during equilibrium-deposit accumulation contained a phosphorus additive. In actual practice, this situation does not always exist. Therefore, it was of interest to determine the usefulness of phosphorus fuel additives for engines which contained equilibrium deposits laid down without the benefit of phosphorus. To do this, experiments were carried out in which phosphorus was added to the test fuel only after deposit equilibrium was reached on phosphorusfree fuel. The engine was operated on fuel containing 3.0 ml. of tetraethyllead per gallon. After 75 hours, the effects of the deposits on engine operation had become stabilized (Figure 9). After 150 hours, 0.2 T of tris(chloroisopropy1) thionophosphate was added to the fuel without interrupting engine operation. Six hours later the surface-ignition rate had been reduced to 637, of the former equilibrium rate. After 30 hours of operation, a new equilibrium was established at only 28% of the former level. The instrumentation used in this experiment permitted a separate determinationof the total number of premature flame fronts, or total surface ignitions, and the end-gas detonations ("wild ping") or audible surface ignitions.
80 I-
w
2E 60 J
2
;40 I-
9 k
20
ADDITIVE
NONE
Figure 6. phonates
DIMETHYL
D ITOLYL
Surface-ignition control b y phenylphos-
+
Base fuel, 2 A 3.0 ml. of tetraethyllead per gallon; oil, 1A; phosphorus concentration, O.2T
Figure 9 shows that 57% of the total surface ignitions resulted in “wild ping” when no phosphorus was present. In the presence of phosphorus only 43% led to “wild ping.” Therefore, phosphorus not only reduced surface ignition to approximately one fourth of its former value but also reduced the proportion of audible surface ignitions in the number that remained. This latter effect is probably the result of the later timing of the surface ignitions in the presence of phosphorus. Surface ignition which occurs late in the comoression
60
0 ADDITIVE
NONE
PHOSPHO-NITRILIC DlCHLORlDE
PHOSPHORUS SESOUISULFIDE
Figure 7. Surface-ignition control b y inorganic phosphorus compounds
+
Base fuel, 2 A 3.0 mi. of tetraethyllead per gallon; oil, 1A; phosphorus concentration, 0.2 T
accompanied by an increase in the octane quality of the fuel necessary for knock-free operation. The change in octane requirement from clean-engine conditions to equilibrium-deposit conditions is termed “octane-requirement increase’) (ORI). A large number of 150- to 175-hour tests have been run in the single-cylinder CFR engines to determine the effect of various phosphorus additives on the magnitude of this phenomenon. The presence of phosphorus additives increased the octane-reauirement in-
240-
I
wide variety of phosphorus compounds tested. For example, tritolyl phosphate and tris(2-chloroisopropyl) thionophosphate have surface-ignition effects which differ widely, yet 0 . 2 T of either material increased the octane-requirement increase an equal amount, to 109% of the phosphorus-free fuel value. I t is possible that untested phosphorus structures might behave differently. Surface-Ignition Control at Different Tetraethyllead Concentrations
I
0 2 T PHOSPHORUS
I
1
I
i
i
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TOTAL
-
4
45
(r
\
‘
