Anti-knock rating

Many of the ashless anti-knocks are amines or phenols [26] and are related to liquid-phase oxidation inhibitors. They probably work by reacting with active radicals (particularly OH) to produce radicals which are inert. For instance, N-methyl aniline (NMA) CeHsNHCHs probably produces stabilized CeHsNCHs radicals which, because of their resonance stabilization, are unable to react to regenerate active radicals again and may undergo only radical recombination reactions. The rate of radical removal by this process is likely to be limited in the most favourable case by how fast the additive can react with OH to produce stabilized radicals. Although exact rates are not known, this is probably already a fast process for NMA, and unlikely to be very much faster for any other substance. Indeed, the most effective ashless anti-knock found by MacKinven [26] in an extensive study of 970 substances was a tetra-aryl hydrazine, with a molar effectiveness 2.9 times that of NMA. 

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 use of MTBE as a fuel additive leads to improvements in the gasoline quality (so-called oxyfuels oxygen-containing gasoUnes). First of all, burning of the fuels is optimized as the content of harmful components in exhaust gases is reduced (benzene, ozone, NOx and CO). A second positive effect is the improvement in the anti-knock property of the fuel (increase in the octane rating). 

In contrast to the development in the USA, in Germany MTBE was added with the objective of replacing the lead compoimd (lead-tetra-ethyl) in gasolines in order to guarantee the anti-knock property of the gasolines (increase in the octane rating). 

Aromatic molecules are now not so acceptable as components of ICE fuel, due to the carcinogenic behaviour of benzene, although they are still wanted by the petrochemical industry. Other additives including tetraethyl-lead and oxygenated molecules (alcohols, ethers) have been approved from time-to-time as anti-knock or octane-rating enhancers, but each has its disadvantages or environmental hazards as well, and none is totally free from objection. Palladium has sometimes been used with platinum to control carbon formation. 

Biological catalysts are known as enzymes (Chapter 23 on the accompanying website) and consist of proteins, often associated with metal ions. A substance that decreases the rate of a reaction is called an inhibitor. An example of an inhibitor is the anti-knock compound, tetraethyl lead(iv), which was used to prevent pre-ignition in leaded petrol vapour but has now been banned in practically all countries (Chapter 10). There are many specific and general inhibitors known for enzymes many nerve gases and poisons, for example cyanides, operate as enzyme inhibitors, often by interacting with the active site of the enzyme (Figure 6.13). 

The trials confirmed that it the anti-foam injection equipment failed, the rate of foam carry-over would greatly exceed the knock-out capacity of existing header slops facilities and compressor suction drums. 

The anti-rollback water spray system (USBM 1985a) is particularly suited for exhaust ventilation faces without a methane problem, with low face airflow, and where dust standards are more stringent because of sdica. With this system, a moderate spray pressure of 692 kPa (100 psi), measured at the nozzle, is a practical maximum. Although higher pressure sprays have the potential to knock down more dust, they can also increase the dust blown back to the operator. However, water-flow rates should be as high as possible, within a 1.6- to 2.2-L/s (25- to 35-gpm) range. 

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