Octane number sensitivity

Because of the existence of numerous isomers, hydrocarbon mixtures having a large number of carbon atoms can not be easily analyzed in detail. It is common practice either to group the constituents around key components that have large concentrations and whose properties are representative, or to use the concept of petroleum fractions. It is obvious that the grouping around a component or in a fraction can only be done if their chemical natures are similar. It should be kept in mind that the accuracy will be diminished when estimating certain properties particularly sensitive to molecular structure such as octane number or crystallization point. 

Adding lead to a fuel increases octane numbers by several points. From an RON of around 92, the increase is on the order of 2 to 3 points for 0.15 g Pb/1 and of 5 to 6 points for 0.4 g Pb/1. For higher concentrations the effect of saturation appears and additional improvement in the octane number becomes more modest. The preceding values concern the RON as well as the MON. Nevertheless, one more often observes slightly larger increases for the RON. In other words, lead addition tends to increase the sensitivity slightly (on an order of one point for 0.4 g Pb/1). 

The octane number requirement (ONR) of a car is the octane number which causes barely audible, ie, trace knock when driven by a trained rater. The Coordinating Research Council (CRC), a research organi2ation funded joindy by the American Petroleum Institute (API) and the American Automobile Manufacturers Association (AAMA), has defined test procedures for measuring ONR. Each car is driven under a set of light and heavy accelerations until the most sensitive driving mode is determined. Then a series of fuels is mn in the car until trace knock is determined. Each year, CRC members measure ONR of more than 100 cars and pubHsh the results. 

On account of its faint odour, leakage is not easily identified, (2) Handling must be under pressurized conditions, (3) Suitability under high compression ratios, (4) Exhibits a low octane number and a high load sensitivity, (5) Shows very poor response to blending. Overall LPG seems suitable as a fuel for trucks and tractors. 

Sensitivity The difference between the research octane number (RON) and the motor octane number (MON) of a gasoline. Fuels blended with alkylate material have low sensitivity values. 

The significance of the equilibrium relationships become more apparent to the refiner when the unleaded research and motor octane values for each carbon group are volumetrically blended and plotted vs. temperature. Such a curve is shown in Figure 4. The sensitivity for the C5 and C6 paraffins is about 1-2 numbers on a clear basis vs. 10-13 for the C6 naphthenes. All of the octane numbers for these components are shown in Table IV. 

Octane Number—The octane rating of a motor fuel is defined in terms of its knocking characteristics relative to those of blends of isooctane (2.3,4-trimethylpentane) and n-heptane. and a rating of 100 to isooctane, The octane number of an unknown fuel is numerically equal to the volume percent ot isooctane in a blend with -heptane which has the same knocking tendency as the unknown fuel when both the unknown and the reference blend are run in a standard single-cylinder engine operated at specified conditions. Motor Method octane numbe-is are measured at moie- severe engine conditions and are numerically lower than those determined by the milder Research Method. The difference between the two numbers is termed sensitivity. 

Fig. 7.2. Motor octane number (MON) and research octane number (RON) for alkanes and alkenes. Alkenes and especially dialkenes have greater sensitivity = RON-MON (data...

The difference between the RON and MON values for a fuel is called the sensitivity. The sensitivity for most practical paraffinic (e.g., alkylate) fuels is close to zero (and exactly so, by definition, for n-heptane and isooctane), but Fig. 7.2 shows that some low octane number alkanes have values of MON > RON. However, in the practical range of ON (>70) for aromatics and especially alkenes, the MON is usually less than the RON. For commercial gasolines the sensitivity is about 10 octane numbers, depending on the aromatic and alkene content. Although the RON of fuels is more usually quoted and is determined in the CFR test more... 

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. 

Sensitivity The difference between the research and the motor octane numbers. 

Gasoline obtained by alkylation of isobutane with C3-C5 olefins is an ideal blending component for reformulated gasoline, since alkylate has a high octane number with a low octane sensitivity (difference between RON and MON), and is mainly formed by multibranched paraffins. If replacement of the environmentally hazardous sulfuric and hydrofluoric acids used as commercial alkylation catalysts by more friendly solid-acid catalysts becomes technically and economically feasible, this would greatly enhance alkylation capacity (Corma and Martinez 1993). 

Due to the Clean Air Act, increasing attention is paid to the production of alkylates, which is a very clean burning fuel and has a high MON (motor octane number) with a low octane sensitivity and moderate vapor pressure. Commercially operated alkylate production uses a liquid acid catalyst such as H2SO4 or HE, resulting in problems associated with cost, apparatus and the environment [47]. New synthetic methods utilizing solid acid catalysts have been developed but no commercial process has emerged due to fast catalyst deactivation [48]. 

The lead susceptibility of reformates is shown in Table 3-12, the relationship (spread or sensitivity) between Research and Motor method octane numbers in Table 3-3, and the blending octane number in Fig. 4-46. 

The amounts of produced hydrocarbons widi a branched cliain do not overwhelm diose with a normal chain. Assuming that sensitivities in analyses for hydrocarbons of the same catbon number (n) remain identical, the molar ratio of branched Cn/nonnal C can be calculated from respective peak areas on the gas chromatogram. The estimated values were less than 1.5. In contrast, the ratio was more than 5 (4-metiiyl octane/n-nonane) in the pyrolysis experiment by Domine [160]. The pyrolysis tliat proceeds sluggishly under a high-pressure condition prefers the formation of branched hydrocarbons to tiiat of normal ones, probably because of the small molar volume of tiie former. Thus, tiiis experiment demonstrates that some mechanism different from that in pyrolysis is involved in the shock process for an instant. 

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