Critical compression ratio

Gaseous paraffins and olefins with low molecular weight and short chain length have relatively high critical compression ratios. Their octane numbers must all be well above 100. Normal paraffins have the lowest octane numbers of any of the members of their... 

The engine data for the normal olefins, pentenes to octenes inclusive, are recorded in Table IX. The data are stated as octane numbers or critical compression ratios. The table is fragmentary, the work being still incomplete, but it is sufficiently complete to show the trends. 

The boiling point, refractive index, and density of the olefin derivative of any paraffin were shown, by use of Table III, to stand in the onier of their olefin type. Table X contains the engine data of the olefin derivatives of 2-methylpentane and 3-methylpentane, recorded in the order of their olefin type. No consistent relations between octane numbers or critical compression ratios are obvious—but the blending octane numbers of these branched olefins, as measured by both the research and Motor methods, do generally stand in the order of their type. Two olefins of type III form exceptions, the exceptions being in one case too high and in the other case too low. 

Aromatic hydrocarbons have exceptionally high engine characteristics. The Research octane numbers of all aromatic hydrocarbons thus far measured are above 100. Those measured by the Motor method are a little lower, but in all cases are above 95. The critical compression ratios at 600 revolutions per minute and 212° F., jacket temper-... 

This datum constitutes, or is partly derived from, an extrapolated, octane number based on critical compression ratios, measurement of which is not limited to the octane range as defihed (0-100). 

The blending octane members of the first five members of the monoalkylbenzenes as measured by the Motor method show alternation. The same is true for the four mono-butylbenzenes, as the butyl group is telescoped on the aromatic nucleus. This is also true of their critical compression ratios (Table XI). 

One of the most fundamental elements of molecular structure is chain length. It serves to fix the hydrocarbon s position on its own subseries curve and thus becomes a factor in determining its physical constants. It also is a major factor in determining the hydrocarbon s rate of combustion and hence its octane number and critical compression ratio. 

The double bond functions in a very analogous manner. It too interrupts effective chain length and determines the principal point of oxidative attack. In a manner quite analogous to the methyl group, through changes in the combustion velocity, the double bond also alters the octane number and critical compression ratio. 

Hydrocarbon Structure Olefin Type Research Octane No. Motor Octane No. Blending Octane No. (Research) Blending Octane No. (Motor) Critical Compression Ratio (600/212° F.) Critical Compression Ratio (600/350° F.)... 

The aromatic hydrocarbons form a unique case of cyclic structure. The benzene ring, like the methyl group and double bond, exerts a powerful influence upon the properties of any hydrocarbon of which it is a part. All aromatic hydrocarbons have high boiling points, high densities, and high refractive indices. They also have high octane numbers and critical compression ratios. 

Fig. 6.20. Critical compression ratios (CCR) for selected alkane isomers, measured in a CFR engine as discussed in Chapters 1 and 7. (After Lovell [140].)...

To determine the octane number, a standard CFR engine is run with the test fuel under specified conditions [9] and the compression ratio is increased until the engine knocks, as indicated by a meter based on an accelerometer attached to the cylinder block. The air/fuel ratio is adjusted for the conditions of maximum knock, which occurs slightly rich of stoichiometric. The test is repeated with a series of reference fuels to find a pair whose critical compression ratios are just above and below that of the test fuel, and the composition of the reference fuel which would exactly match is found by interpolation. 

Since the early work of Ricardo [7], it has been recognized that some fuels, including many aromatics and ethers, knock less easily than isooctane. This presents no difficulty for an assessment based on the HUCR, or the critical compression ratio (CCR), widely featured in Lovell s comprehensive review of fuel properties in 1948 [10]. However, it does present difficulty for one based on the octane number, because some fuels have octane numbers larger than 100. The octane number scale has been extrapolated in a way which is necessarily arbitrary, and there are secondary standards based on iso-octane containing the anti-knock, lead tetra-ethyl, discussed in Section 7.2.6 [9]. 

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