Knock rating

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,... 

Among isomers the knock rating increases as the number of side chains increases. This effect, too, is very large, more than spanning the full octane number scale. Adding chains in such a way as to increase compactness or centralization raises the rating most. Neopentane is an exception. It is worse than isopentane under all test conditions. 

If a methyl group is added to a given paraffin and it introduces an added branch, it usually raises knock rating. The effect is larger if the methyl is added nearer the center of the chain. The paraffins are the hydrocarbons least sensitive to engine conditions. 

Among straight-chain olefins, knock rating increases as the double bond is moved toward the center of the molecule. 

Introduction of a double bond in a branched paraffin raises the knock rating for slightly branched paraffins but lowers the rating for highly branched hydrocarbons. 

Centralization of the double bond in branched olefins tends to raise knock ratings. 

Introduction of a double bond in a paraffin has more tendency to raise knock rating if it is added next to the methyl in a monomethyl paraffin or near the quaternary carbon in a dimethyl paraffin. 

Aliphatic olefins are much more sensitive to engine conditions than the corresponding paraffins except at low knock ratings. 

CYCLOPENTANES AND CYCLOHEXANES. These naphthenes cover gen-erally the same knock rating range as the paraffins. Naphthenes rate better than corresponding n-paraffins, although none are as good as the best branched paraffins. 

Adding a straight side chain lowers knock rating. The effect increases with chain length. 

Branching in a side chain of given length raises knock rating. 

Among two-branched cyclohexane isomers, the order of decreasing knock rating tends to be 1,1 > 1,2 > 1,3 > 1,4, and cis > trans. 

The cycloparaffins are more sensitive than the paraffins. Sensitivity is greater at high knock ratings. 

AROMATICS. The aromatics Have high knock ratings, usually above 100 octane number. 

Adding a straight side chain to benzene lowers knock rating. The effect increases with chain length. As an exception, ethylbenzene falls below n-propylbenzenc by most methods. 

Change from one straight side chain to an isomer with two chains meta or para to each other raises knock rating. 

Acetylenes vary widely in knock rating relative to the corresponding paraffins. Acetylene is very bad. Centralization of the triple bond raises the rating. 

Compactness or centralization in a molecule results in increased knock rating. Kobayashi and Mibashan have used this observation to develop empirical quantitative correlations between structure and knock. 

Kobayashi used molecular dimensions directly (101-4). For paraffins he defined an unstability factor which was a measure of approach to a sphere. A fair correlation was obtained between his factor and blending octane number. The calculated factors indicated the observed rise in knock rating with centralization of the double bond in a straight-chain olefin. Gaylor (69) applied Kobayashi s method to aromatics and compared calculations with both clear and blending octane numbers. It is difficult to select molecular dimensions of a flexible molecule representative of its configuration during reaction. 

Ignition pressures, determined at one temperature for a series of fuels, increase as the knock ratings of the fuels increase (125, 159). 

The preflame reactions include slow oxidation and cool flame reactions (110). Slow oxidation threshold temperatures and reaction rates have been considered important factors in controlling knock resistance (43, 75, 133). Knock ratings have been related to cool flame intensities and temperature limits (36, 43, 153). Recently, Barusch and Payne (9) have found striking correlations between octane number and the position of the cool flame in a tube (a parameter which should be a function of Ti). The heat evolved during cool flame reaction may also be a vital factor in determining the occurrence of knock (106,156,179). 

Free energy (85) and heat of formation (216) both correlate with knock rating but neither in itself is sufficient to account for the differences in reactivity found (216). 

In particular cases bond strengths may be controlling—for example, the extreme resistance of benzene to oxidative attack may be explained by the very high C—H bond strength (12, 208). The anomalous low knock rating of neopentane may be correlated with the low strength of the neopentyl C—H bond. C—H bond strengths for methane, ethane, and neopentane are, respectively, 102, 98, and 96 kcal. per mole (82, 204). 

It is of interest to review ideas as to the point of hydrogen abstraction. Through 1939 most investigators believed that attack of paraffins was at the primary C—H bonds at the end of a chain (75, 109, 167, 168, 217, 220, 224), Attack at the a-carbon atom of substituted benzenes (217) and at the end methyl of olefins (109) was proposed. Preferential attack at 1° C—H bonds fitted in with the comparative ease of oxidation of n-paraffins and their low knock ratings. 

Evidence exists that Reaction 3 may occur as an intramolecular reaction (8, 131). This provides a possible explanation of the extremely low knock rating of o-xylene com-... 

The following arc believed to be important in determining knock ratings Thermodynamic properties such as heat content and vibrational coupling. 

A large number of experimental engine techniques have been developed in the course of investigating the knock phenomenon. Highly instrumented, precision knock-rating engines have been used in studying the knock characteristics of various hydrocarbon... 

Micro Research octane numbers were obtained using the standard CFR (Cooperative Fuel Research) knock rating unit. 

Motor octane method a test for determining the knock rating of fuels for use in spark-ignition engines see also Research octane method. 

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