W. L. RICHARDSON, M. R. BARUSCH, G. J. KAUTSKY, and R. E. STEINKE California Research Corp., Richmond, Calif.
An Improved Gasoline Antidetonant A study of the mechanism that makes TML a more effective antiknock agent than TEL in high octane g asol ines
T E T R A M E T H Y L L E A D (TML) has been found to possess antiknock properties markedly superior to tetraethyllead (TEL) in a variety of high octane gasolines. The principal benefit on replacing T E L with T M L is a gain of from one to two Road octane numbers in modern American-production automobiles. AS seen in the table this octane improvement is mirrored by a similar but somewhat smaller increase in the Motor method octane number. The substitution of T M L for TEL has very little effect on the Research octane number. The relative effectiveness of T M L improves with increasing octane number and aromatic hydrocarbon content of the base gasoline. The concentration of T M L has a pronounced influence on its relative effectiveness. Averages of the Research, Motor, and Road octane values for five west coast refineries are compared at five lead concentrations. The gasolines all have a nominal Research octane number of 100 with 3 ml. of TEL per gallon. T M L becomes superior to TEL above a molar concentration equivalent of 1 ml. of TEL per gallon by both the Road and Motor methods. There also appears to be an optimum effectiveness by the Road and Motor methods at about the 3-ml. level. A detailed discussion of the performance of tetramethyllead in a variety of gasolines and pure hydrocarbons is presented elsewhere ( I ) . At present, it will cost the refiner a premium to replace T E L with T M L because of initial low volume production
and development costs. Upon establishment of a large volume market, the cost of lead as T M L will approach that of TEL. Most refiners assign a value of between 0.2 and 0.5 cent per gallon for an octane number. The value of the Road octane improvement due to substitution of T M L for TEL makes T M L attractive even at its current price. Studies of the mechanism that makes T M L more effective than TEL in high octane gasolines have demonstrated that the greater stability of T M L is the most important property influencing its activity as an antiknock agent. Rifkin (2) has shown that T M L is more resistant to decomposition in a motored engine than is TEL. This same work confirms earlier findings that T E L must be decomposed (probably to an oxide) in order to exhibit antiknock activity. O n the surface, this appears to be a contradiction. How can T M L be more resistant to decomposition and show superior antiknock effectiveness when it must be decomposed in order to exhibit these antiknock properties? An attractive hypothesis is that, following the decomposition of TEL, there is a rapid agglomeration of the lead compound, which drastically curtails the antiknock reaction. With increasing engine severity-Le., increasing compression pressures and higher temperatures-TEL decomposes earlier and earlier in the engine cycle. With increasing aromatic hydrocarbon content of the gasoline, the preflame reactions which precede knock occur later in the engine cycle. Above a certain combination of octane level and aromatic hydrocarbon content, T E L decomposes prematurely and agglomeration of the resulting lead oxide reduces its surface and degree of dispersion before the critical time for the antiknock reaction to occur. The greater stability of T M L permits it to survive until later in the engine cycle, suffer less loss in effectiveness due to agglomeration, and
Effectiveness of Tetramethyllead in High Octane Gasolines Aromatic Octane Improvement (TML Minus TEL) Octane Rating with TELn Content, yo Research Motor Road Research Motor 104.7 100.5 100.3 99.4 99.3 98.8 99.2 a
96.3 88.2 90.5 88.1 87.8 86.6 87.4
3 nil. TEL per gallon.
48.1 43.0 39.0 33.0 32.0 28.0 16.0
0.3 0.3 0.5 0.1 -0.1 0.5 -1.6
1.4 0.8 0.6 0.7 0.7 0.5 0.1
2.1 1.0 1.6 0.9 0.8 0.5 C.6
thus possess greater antiknock effectiveness at the critical time. As demonstrated b y careful study through years of successful commercial use, the vapors of TEL over motor gasoline present no public health problem as long as the gasoline is used for its intended purpose. Because of the difference in the vapor pressures of TEL and TML, it was necessary to determine if any health problem would be associated with the use of T M L in motor gasolines. A program was initiated in conjunction with the TEL industry which has demonstrated that there is no health hazard associated with the replacement of T E L with TML. R. A. Kehoe, of the University of Cincinnati Medical School, served as consultant in the study and evaluated the test results. Studies over the years since the introduction of TEL have demonstrated that persons in contact with leaded gasolines more than the general urban population exhibit only a barely significant increase of absorbed lead associated with the use of TEL. Our studies have shown that the amount of lead that garage workers absorb does not increase when TEL is replaced with TML. The test data were presented to the Surgeon General of the United States to demonstrate that no public health problem is to be anticipated from the marketing of gasolines containing T M L . Because of its superior antiknock qualities, T M L may be expected to replace TEL in a large fraction of the world's premium motor gasolines. Two features brighten the prospects for TML. First, it functions to suppress knock with improving effectiveness as the aromatic content of gasoline increases. This effect will assume even greater importance as the trend toward highly aromatic gasolines continues. Secondly, since the relative effectiveness of T M L increases with gasoline octane level, the extension of the historical upward trend in octane quality will make T M L the additive of choice in increasing numbers of gasolines.
Editor's Note
i 1 Detailed data for this article including tables, graphs, and discussion appear in the Journal of Chemical and Engineering Data, Vol. 6 (April
I
lg61).
literature Cited (1) Richardson, W. L., Barusch, M. R., Kautsky, G. J., Steinke, R. E., J. Chem. Eng. Data 6, No. 2 (1961). (2) Rifiin, E. B., Proc. Am. Petrol. Znst. 38, 111, 60 (1958). RECEIVED for review October 10, 1960 ACCEPTED October 10, 1960
Division of Petroleum, 138th Meeting, ACS, New York, September 1960. VOL. 53, NO. 4
APRIL 1961
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