1
2
Figure 1.
3
4
Normal combustion
E. C. HUGHES, P. S. FAY, L. S. SZABO, and R. C. TUPA Chemical and Physical Research Division, The Standard Oil Co. (Ohio), Cleveland 6, Ohio
Effect of Boron Compounds on Combustion Processes
BORON
compounds have been reported by several authors to have a variety of interesting effects on the oxidation of hydrocarbons, gasoline. and carbon (79, 20, 22). In engines a reduction in the octane requirement increase has been observed (6, 9, 73, 75). More recently it was reported that tributylborine, in two tests. did not consistently affect the equilibrium octane requirement of a single cylinder engine (24). While the increase in octane requirement is a serious result of combustion zone deposits in engines, there is considerable evidence that it is caused by several properties of the deposits (8, 25), of which only the catalytic may be affected by boron compounds. Another phenomenon of combustion in reciprocating engines is “surface ignition,” which results from ignition of the charge in the cy!inder at some
1
2
Figure 2.
1858
INDUSTRIAL AND ENGINEERING CHEMISTRY
point other than the spark plug (23). -4s indicated by the name, it is believed to initiate on the walls or deposit surface in the combustion zone. As compression ratios increase in gasoline engines, this property, rather than the effect of simple knock, tends to become limiting (27). This surface ignition phenomenon is susceptible to catalytic influences. I n particular, materials that will influence the ignition temperature of carbon are active in enhancing or hindering this phemonenon. Sabina and coworkers (27) point out also that the prevention in the preflame period of the conversion of carbon monoxide to carbon dioxide is important and that this is normally a catalytic step. We have noted that boric oxide coated on porcelain or steel would eliminate their catalytic influence on the conversion of carbon monoxide to carbon dioxide in the oxidation of heptane (72, 73).
3
Combustion with surface ignition
4
A D D I T I V E S IN FUELS
5
7
6
Figure 1.
Normal combustion
Nebel and Cramer (76) of General Motors showed that whereas their sample of carbon would glow in air at 1030” F., the common lead basic halides which occur in leaded gasoline deposits would reduce the glow point to 605’ to 650’ F. Lead phosphates had no effect and, as pointed out, might prevent the “burning-out” ( 3 ) of carbon normally desired as a result of the lead content of combustion zone deposits. However, Nebel and Cramer point out that “If the preignition problem is to be solved. . . by the use of fuel additives, compounds containing phosphorus would be most efficient for this purpose.” They also found that lead borate gave an ignition temperature in their carbonaceous pellets of 795’ F. This would place it about halfway between the most active lead compound found in deposits (oxychloride) and the lead phosphates. This suggests that it would allow both some burning-out of carbon and some repression of surface ignition, thus providing a good “engineering compromise” to the over-all problems of the reciprocating engine. In an effort to improve the understanding of the effect of boron compounds on combustion reactions we have carried on further studies, both in the laboratory and in the engine. The work in the latter employed a new gasolinesoluble, volatile boron compound. These investigations include observations on surface ignition, preflame reactions, octane requirement, scavenging, and carbon-lead glow temperatures.
5
6
Figure 2.
a
Combustion Effects in Gasoline Engines
In addition to the annoyance of “wild ping” due to surface ignition in engines, it has been recognized that serious consequences to power, engine life, and smoothness can result from silent surface ignition ( 7 7). Tongberg and coworkers (24) point out that with proper octane number wild ping can be avoided. However, it has been observed by all investigators in this field that much surface ignition can occur without pinging or detonation and is not necessarily controllable by the same means. Surface ignition becomes particularly observable in the “after-running’’ of automobile engines (running after the ignition is cut off). We have used an experimental 10 : 1 compression ratio Oldsmobile engine in which wild ping and after-running were induced by a series of full throttle accelerations under load. Primarily, however, for closer study, two single-cylinder techniques were used. In one, surface ignitions detected by an ionization gap were counted in a 12: 1 compression ratio CFR overhead valve engine, using principles described by Hirschler ( 7 7 ) . The other employed a high speed camera and quartz-head engine to record surface ignition. Both the addition of the boron compound and a change from chloridebromide to all-bromide lead mix reduced the number of wild pings and shortened the after-running time in the
7
Combustion with surface ignition
a
10: 1 Oldsmobile engine. The combination was better than either alone. A further study was made comparing this combination with the commonly used chloride-bromide mix of tetraethyllead in the quartz-head single cylinder engine. This type of observation was chosen because the quartzhead and high speed photography made it possible to count directly the number of nonspark ignition centers, whereas the ionization gap technique counts only one such incidence per cycle. Bowditch (7) emphasized that frequently several centers develop per cycle. This proved to be the case. Figure 1 is a typical explosion sequence in which no surface ignition occurred, and Figure 2 shows one in which there were six centers of surface ignition. These pictures and other similar observations were obtained on a CFR L-head engine operated under the conditions shown in Table I. An equilibrium deposit was built up in this engine using leaded fuel (chloride bromide mix) while the cast-iron head was in place, usually over a period of 40 hours. Then the quartz head was installed, and fuel measurements were made. Each fuel was run 1 to 2l/2 minutes at about 20” BTC spark advance. As soon as the jacket temperature reached 205’ F., the spark was advanced to borderline knock (29” BTC), and about thirty cycles were photographed with a Fastax camera on DuPont 931 film at 1100 frames per second during 10-20 seconds. Each frame, covering 6’ of crank angle, was then examined for number of surface ignitions and other properties. Table I summarizes the surface ignition data from these studies. Surface ignition before normal spark ignition time (29” BTC) was nearly absent from the fuel containing the boron compound, although the engine conditions Were severe enough to produce many ignition centers with the nonboron fuel. Frequency of surface ignition remained considerably lower for the boron fuel during the first two frames after the spark but progressively approached that for the fuel without boron until they were substantially equal in the fifth frame (12’ to 6 ” BTC). The higher frequency of surface ignition for the boron fuel in the last frame probably results from the greater volume of unburned gas in which new centers can arise. Combustion was complete by TDC in 4070 of the nonboron cycles but only in 20% of the boron cycles. The total number of urface ignitions up to the 12” BTC crank angle was 3570 less for the boron fuel. Moreover, it is believed that the most serious ignitions are the ones that occur early in the combustion, since they have nearly the whole VOL. 48, NO. 10
OCTOBER 1956
1859
~
~~
Table I.
Quartz Head Surface Ignition Data
Fuel: 94.5 F-1 O.N. blend of SR and cat. cracked naphtha, 3 Oil: SAE 1OW-20,solvent extracted mid-cont., S-1 level CFR: L-head engine Borderline knock conditions:
29' BTC spark advance 30 inches Hg manifold pressure 205' P. jacket temp.
a
TEL/gal.
150' F. sump temp.
1050 r.p.m. CR 51/2:1
Spark Crank Angle' BTC 36-S0$50-%$ Frame 1 2 Flame Centers Obsvd." No boron; CI-Brmix 19 12 0.004% Boron ; all-Brmix 1 7 Decrease for boron, % ' 95 42
CC.
250" F. inlet air temp.
0.07 fuel/air ratio
Cycles Showing Total Surface One or More 18-18 12-6 6-TDC Ignitions U p Surface Ignitions 4 6 6 to 12O BTC by18aBTC
24-18 S
55
72
95
41
158
50
34
61
98
58
103
30
38
15
-41
35
40
-3
Distinct flame centers observed in 294 cycles per fuel.
charge to affect (26). This was shown by planimeter measurements of the total flame area in successive frames. O n the average, about 40ycof the charge was burned by 18" BTC (end of the third frame) when no surface ignition occurred. Total surface ignitions for the boron fuel were less than half those for the fuel without boron a t this point. Surface ignition usually occurred more than once in any cycle where it occurred a t all. The ionization gap technique cannot tell the difference between one and several surface ignitions. As a matter of interest, the last column of Table I shows the number of cycles in
Table II. Surface Ignition Frequency in 12:l Compression Ratio CFR Overhead Valve Engine 900 r.p.m. timing 10' ATCa Sump temp., 185' F. Jacket temp. 1 12' F." 'Test period 4 hr. per fuel 95 octane fuels, 3 cc. TEL
No. Surface Ignitions i n 4 Hours N o Boron 0.004% Boron All-Br Lead m i x Cl-Br All-BT Cl-Br 1578 1237 1416 1919 1983 2039 1860 2777 1753 2636 2024 2191 1874
1870 2361 2388 1879 1819 3124 2157 1987 1854 2636 1948 1760 1558
Fuel average 1945 2103 Grand average 2024
1012 1322 1106 I485 1124 1225 1461 1880 2063 2059 1710 1228 1338 1455
820 1165 1718 1322 1624 1560 1746 2543 1658 1484 1903 2001 1255 1600 1527
a Chosen to avoid detonation with 95 0.N. fuels.
1 860
which one or more surface ignitions occurred in the first three framesi.e., by 18' B T C s u c h as might have been observed by an ionization gap setup. The total for the boron fuel is about 40% less than for the nonboron fuel, in good agreement with results from a count of total ignition centers observed. An ionization gap operating up to 18' BTC (1 1 O after spark ignition) would therefore have given roughly the same result. This experiment compared the combination of the boron compound and allbromide lead mix with the usual chloridebromide lead mix. We have since determined the effect of adding the boron compound to the chloride-bromide lead mix in the same engine. Total surface ignitions counted were reduced by 50yO. In another study the 12 : 1 compression ratio CFR engine was operated for a test period of 4 hours per fuel starting with an equilibrium deposit developed from gasoline containing the usual chloridebromide lead mix. The experiment covered 13 runs with the boron compound present and 13 without in each of two fuels, one containing chloridebromide, the other all-bromide lead mix, arranged in a randomized plan. Table I1 shows the results. The fuels containing the boron compound averaged 1527 counts per 4 hours as compared to 2024 counts for the fuels containing no boron or a reduction of 2570. This result was significant a t well over the 20: 1 level. Therefore, 0.00470 of boron in gasoline effectively reduced surface ignition under these severe conditions. The two lead mixes were not significantly different in their effect on surface ignition. Thus, both ionization gap measurements at 12 : 1 compression ratio and surface ignition counts in the quartzhead engine a t lower compression ratio
INDUSTRIAL A N D ENGINEERING CHEMISTRY
show that the boron compound will significantly reduce the important types of surface ignition. Hence, a boron fuel should be effective in both current and future engines. One of the surprising observations from all the surface ignition studies was the rapidity with which the ignitions were reduced when the boron gasoline was introduced. I t would be expected that this process would be slow, requiring considerable modification in the deposit (70). Actually, a plot of surface ignition counts for successive 15-minute intervals of the 4-hour running period in our 12: 1 engine showed both an immediate reduction in surface ignition frequency on changing to boron fuel and a further progressive decrease from deposit modification This rapidity suggested that in addition to deposit change other combustion phenomena were being affected by the boron compound Reduction of preflame reactions has been shown to modify the combustion behavior of fuel (4, 5, 7, 77, 78, 27). Engine and chemical work a t Ohio State University has indicated that carbonyl content might be a useful clue to the extent of preflame reaction (7). A sampling valve, therefore, was installed in a CFR overhead valve engine and the gas sampled immediately ahead of the flame front. During a 30-minute run, 2 cubic feet of gas was withdrawn and passed through a 2,4-dinitrophenylhydrazine scrubber. The data in Table I11 show that the boron compound reduced the average carbonyl content by 14.2%, while the combination of the boron compound and the all-bromide lead mix extended the reduction to 21.3%. This average difference was based on five determinations for each of four fuels and is significant at the 20: 1 level. The results may relate closely to the observed valuable effects on surface ignition, after-running, and wild ping. A reduction in preflame reactions should reduce not only the tendency of the fuel
Table 111. Analysis of Preflame Gas Samples for Carbonyl CFR overhead valve engine, 7.5:l CR 900 r.p.m. Timing 10' BTC A/F 11:i Sump, 160-1 90' F. Test period: 3 0 min. with sampler valve open between 15' and 20° ATC Fuel: 95-octane commercial blend 3 cc. TEL
Curbonyl, M g . Avo Boron
0.004% Boron
Cl-Br
All-Br
Cl-Br
All-Br
mix
mix
mix
mix
316 375 516 520 526
303 363 527 424 581
294 353 492 395 510
242 375 422 361 373
Av. 451
440
409
355
A D D I T I V E S IN FUELS to surface ignite but also its tendency to knock. Figure 3 shows that the boron compound does, in fact, increase the F-1 octane number of a commercial leaded fuel.
Table IV.
Carbon in Cylinder Head Deposits
(Medium duty; equilibrium deposits after 140 hr.; 94-octane commercial fuel containing 3 cc. TELjgal., SAE 1 O W - 2 0 solvent extracted mid-continent S-1 oil)
Carbon.. %
I "
Deposits in Combustion Zones
Full scale engine tests were carried out in the laboratory and on the highway to determine the effect of the boron compound on engine performance. These consisted of a laboratory medium-duty cycle test under the conditions shown in Table VI, a high speed turnpike test in passenger cars (Table VII), and a valve-life test in trucks overloaded and run at high speed. Earlier results (73) had shown much less octane requirement increase with a fuel containing a boron compound. However, these tests did not 'show any important reduction in equilibrium octane requirement. The boron compound affected the composition and the ignition temperature of the deposit, the amount of deposit in hotter areas, and the life of the exhaust valves in severe service. Lamb (74) has made an x-ray diffraction study of the complex compounds of boron oxide with lead oxide, lead halides,
No Boron Cl-Br mix All-Br mix Engine Type Oldsmobile Cadillac Chevrolet Lincoln Buick Pontiac
14.0 16.0
0.004% Boron CI-Br mix ALl-Br mix
10.4 12.9 14.9 16.2 17.8 11.6
.. 18.4
18.1 14.1
Average
13.8 17.2 14.1 16.3 20.7 14.2
14.9
and lead sulfate which might form in engine deposits. One of these compounds was unusually friable and might be thought of as facilitating mechanical scavenging. Lamb (74) has supplied analyses of some of the deposits accumulated in the medium duty tests. These analyses, based on comparison of x-ray diffraction patterns of known compounds synthetically prepared, showed that boron appeared in the deposits in a number of complex crystalline compounds,
Ianition
Linco1n
T E L Mix Cl-Br All-Br
Pontiac
0
.. ..
..
(Engine deposits from leaded fuel in medium duty service)
Cl-Br All-Br
96.2
..
15.9
Table V. Effect of Boron on Ignition Temperature in Oxygen
0
0
..
10.6 13.5 18.8 14.2 18.6 9.8 15.2
Buick 96.4
0.020 boron, all-Br mix
Cl-Br All-Br
Temp., a F . No 0.00L% boron boron 463 467 466 471 463 466 465 471 488 502.5 497 517
472 480 46 1 478 470 486 471 473 490 525 500 540
Average 478.0 487.2 Average Difference 9.2O F.
96.0
&
n
95.8
B
Z
S B
95.6 c
h
95.4
95.2
95.0
0
.004
Figure 3.
-01 2
,008 Weight
.016
.020
7 0B
Effect of boron on octane number of leaded fuel Commercial blends plus 3 cc. of TEL as ol!-Br mix
containing the elements lead, halogen, oxygen (and occasionally sulfur) as well as boron. These crystalline compounds in some cases constituted as much as 30y0 of the deposit. The xray diffraction patterns obtained for the deposits were more diffuse than those for synthetic preparations, suggesting to Lamb the presence of amorphous compositions of boron as well. Lead salts normally present in combustion chamber deposits promote the burning out of carbonaceous material. This is desirable on the one hand but promotes surface ignition by the glowing deposit on the other. As mentioned earlier, Nebel and Cramer (76) show that lead borate in carbon gave ignition temperatures 80" to 190' F. higher than the conventional lead compounds found in engines. We have found similar results and have also noted that boric oxide would raise the ignition temperature of (unleaded) carbon. The work of Burk and coworkers ( 2 ) shows similar results on a carbon which is evidently VOL. 48, NO. 10
OCTOBER 1956
1861
roble Vi. Exhaust Volvc Deposits fmm Medium Duty Cycle 2.N. sMmercId godine with 0.004% boron, 3 CL EL! durolim. 140 how" R.P.M. am Load, Lb. Tint o Ai 7 % h w r i
able VII.
Piston Top Deposits from Turnpike Tests
tributed to the preparation of thii paper -in particular, R. I. Potter and E. H. Scott of the Automotive Engineering Division for the turnpike data; H. F. Hostetler of the same division for afterrunning and wild ping studies; W. Wotring of the Pmceap and Pmduct Development Laboratory and G. Scott of the Cleveland Refinery Control Laboratory foroctane number data; R. R. Decker, J. E. Robinson, and V. Frances Gaylor of the Chemical and Physical Research Division.
leas).
This result is significant at the Similarly, Table VI1 shows that in the turnpike test a d u c t i o n in piston top deposit fmm an average 5.62 grams per piston to 4.42 grams (21% less) occurred. At the same time less exhaust valve channeling was noted, possibly as a result of the deposit reduction. A valve-life test under exceptionally severe duty in a truck fleet reached the end point of two valve failures per truck in an average of 28,000 miles on nonboron fuel containing chloride-bromide lead mix. It reached 55,000 miles with all-bromide lead mix but was stopped at 60,000 miles before reaching any end point in the trucks using the boron compound as well as the allbromide l a d mix. The boron compound used in these tests significantly reduced the h q u e n c y of surface ignition over deposits built up with ordinary fuels. It affected the preflame reactions occumng in an engine and increased the F-1 Octane number of commercial leaded fud. When used in a clean engine it resulted in leas deposit on exhaust valves and longer valve life under severe operating conditions. 20: 1 confidence level.
mix
Oik SAE IOW-20 d m n t exkomd midmntln.nt, s-1 IWd
Am. Dcpo&/Pi.lon Top.
amtM
No &on Eord Chsnolet
OldamobUe C@8C
Average
0.004% h
5.97 5.65 4.53 6.33
4.05 4.57 4.10 4.96
5.62
4.42
n
more active. The ignition temperature% of these lead wmponnd-carbon mixes are in the range 510' to 700' F. in air at atmosphuie pnaaure and are probably well below this range a t the higher oxygen concentration of presaurized air. Table I V shows that the average carbon content of engine deposits fmm the d i u m duty cyde test was 15%. Those made when the boron compound was used in the gasoline contained the same amount as those without it. Thus, the boron content of the depoait did not prevent normal carbon burnout. On the other hand, Table V shows that the "glow" temperature of deposits from the same engines w a s higher whun the boron wnipnmd was used. The dif. fmnce, though small, was statistidly significant. Even a small difference could be decisive in reducing the frequency of m&ce ignition or delaying it until later in the cyde. Table V I shows that in the medium duty engine study, the boron compound reduced exhaust-valve deposit weight from an average of 0.49 gram per valve (16 " ,I 4 engines) to 0.29 gram (41%
1862
Acknowledgment The authors acknowledge the help of C. E. Ebord, E. W. Malmberg, and M. L. Smith of Ohio State University in setting up p r e h m e gas sampling and analysis and C. D. M i k r and C. A. Anderson of Battelle Memorial Institute for high speed photography of surface ignition phenomena in a project sponsored by The Standard Oil Co. (Ohio). In addition we would like to acknowledge the effort of those employees of The Standard Oil Co. (Ohio) who col:
INDUSlNAL AND E W N E E R I W OlEMlSTRY ~
Q-r-*.mn
far -nv November 2, 1955 A m m o April 27,1956
