Antiknock performance

Although being regulated out of use due to its toxicity and exhaust emission contribution, alkyl leads and other organometallics are still used in some parts of the world to improve automotive gasoline antiknock performance. Alkyl lead is also used in aviation gasoline. 

No single measurement can be used to characterize antiknock performance. It is a physical and chemical phenomena interrelated with engine design and operating conditions. 

Road Octane—A numerical value based upon the relative antiknock performance of an automobile with a test gasoline as compared with specified reference fuels. Road octanes are determined by operating a car over a stretch of level road or on a chassis dynamometer under conditions simulating those encountered on the highway. 

H.E. Hesselberg and W.G. Lovell, What Fuel Antiknock Performance Means in Engine Performance, SAE Journal 59[IV] (1951) 32. 

Reforming and Isomerization Feedstocks. Some saturated fractions contain substantial amounts of low octane-number normal pentane or normal hexane. These chemical species can be isomerized for antiknock performance enhancement. It is, however, useful to remove branched compounds from the feedstock by sharp distillation. A de-isohexanizer, for example, may need 60 or more actual valve plates and a high reflux ratio. The modeling of equipment for this purpose follows standard distillation design procedures, but because of the narrow relative volatilities, special care is needed with the basic data. 

The various types of hydrocarbons in gasoline behave differently in their preflame reactions and thus, their tendency to knock. It is difficult to find any precise relationship between chemical structure and antiknock performance in an engine. Members of the same hydrocarbon series may show very different antiknock effects. For example, normal heptane and normal pentane, both paraffins, have antiknock ratings (octane numbers) of 0 and 61.9, respectively (Table 5.5). Very generally, aromatic hydrocarbons (e.g., benzene and toluene), highly branched iso-paraffins (e.g., iso-octane), and... 

Abbreviations Btu British thermal units MJ megajoules RON research octane number, the number used in predicting antiknock performance during low-speed acceleration and MON motor octane number, the number used in predicting antiknock performance at high constant speed... 

This value is primarily used to rate the antiknock values of aviation gasolines with octane numbers over 100. It is defined as the maximum knock-free power output obtained from fuel expressed as a percentage of the power obtainable from isooctane. The relationship between octane number and performance number is listed as follows ... 

Wiese Formula An empirical formula developed by General Motors Research for expressing motor fuel antiknock values above 100. This value is obtained by measuring the number of milliliters of tetraethyl lead added per gallon to achieve the same knocking tendency as the fuel being tested. The determined value is then expressed as a performance number. 

W.L. Richardson et al, Ibid, 1023-33 (Organolead antiknock agents - their performance and mode of action) 5) A.D. Walsh, Ibid, 1046-55 (The knock rating of fuels) 6) S. Curry,... 

Standard Oil of California added the petrochemicals of Gulf Oil, purchased in 1984, to its subsidiary Chevron Chemical. Other United States petrochemical producers took advantage of special circumstances. Amoco was served by a strong terephthalic (TPA) base and its good performance in polypropylene Arco, by its Lyondell subsidiary in Channelview, Texas, and by its development of the Oxirane process through which propylene oxide could be produced by direct oxidation with styrene as a coproduct. The process also led to MTBE (methyl tertiary-butyl ether), the antiknock agent used as a substitute for tetraethyl lead. 

Both types of anti-knock are more effective in paraffinic fuels then in olefinic or aromatic fuels, and can even promote knock when added to some alcohols. In Fig. 7.9 the response of some pure hydrocarbons to the addition of 3ml/US gal of tetra-ethyl lead is shown, in terms of Performance Number. Almost all the alkanes lie on a steeper line than the alkenes. The exceptions are low octane number alkenes, which are largely straight alkane chains, and a few highly-branched alkanes (which also have high sensitivity, see Section 7.2.3). Notwithstanding the subtleties of lead additives, a broad explanation in chemical kinetic terms is that the antiknock acts to increase radical termination rates and, consequently, has proportionately less effect in those fuels where the termination rates are already high. 

The higher grades of fuel are thus classified by their specified antiknock ratings under both sets of test conditions. For example, 100/130 grade fuel has an antiknock quality of 100 minimum by the weak mixture test procedure and 130 minimum by the rich mixture procedure. Octane numbers are used to specify ratings of 100 and below, whereas performance numbers are used above 100. 

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