Knock resistance

In order to characterize the behavior of motor fuels or their components with regard to knocking resistance but without involving chemical composition criteria which are complex and not easy to quantify, the traditional method that has been universally employed for more than 50 years consists of introducing the concept of octane number. 

Eor a considerable period, >90% of the new cars in Brazil operated on E96 fuel, or a mixture of 96% ethanol and 4% water (82). The engines have high compression ratios (ca 12 1) to utilize the high knock resistance of ethanol and deUver optimum fuel economy. In 1989 more than one-third of Brazil s 10 million automobiles operated on 96% ethanol/4% water fuel. The remainder ran on gasoline blends containing up to 20% ethanol (5). 

Thomas Midgley, working with a group of researchers under Charles F. Kettering, discovered a fuel-additive, tetraethyl lead that enhanced the knock resistance of existing fuels. By the time this additive entered commercial use in 1923, the average compression ratio of new U.S. cars had advanced to 4.3. 

An octane rating scale was devised for fuels to quantify their knock resistance. Further research led to cataloguing the antiknock qualities of the myriad individual hydrocarbon species found in gasoline. 

Considerably knock resistant, (4) As a consequence of its burning cleanly, residue and oil contamination is small,... 

Both motor octane number (MON) and research octane number (RON) values of finished fuel blends are measured using the whole fuel, not individual fuel fractions. However, the octane number contribution of the various fuel fractions can influence the overall knock resistance. Finished fuels with identical RON values may contain fractions with either very similar or widely different octane number values. 

For example, the T-10 through T-40 fractions of available gasolines may possess significantly different octane numbers. This difference may be manifested in various levels of knock resistance under low-temperature engine starting and warmup operation. 

The engine characteristics of aromatic hydrocarbons seem strongly dependent upon the nature of the side chain. The alignment of the knock resistance of the polymethyl-benzenes with structure corresponds quite closely with that found to be characteristic of the physical constants. The vicinal derivatives o-xylene, hemimellitine, and prehnitene... 

One cannot help being impressed by the dominant character of the methyl group. It would seem that when the electron release of the methyl groups is balanced across the benzene nucleus the knock resistance is increased this indicates that the velocity of combustion is slowed down. On the other hand, when the electron releases of the methyl groups supplement each other, as in the case of the vicinal derivatives, knock resistance is decreased this indicates that the combustion velocity is increased. An accumulation of methyl groups either upon the side chain, as in ferf-butylbenzene, or upon the nucleus, as in isodurene, seems to increase the knock resistance. 

Ignition delays, investigated in adiabatic compression machines (89, 91, 92, 106, 182, 209, 212, 213), have been correlated with knock (90, 93-5, 111, 158, 160, 194, 203). Ignition occurs in two stages. Levedahl (106) examined the effects of temperature, density, and fuel type on the induction periods, ti and r2, corresponding to each ignition stage. A close correlation existed between total delay, r, and knock resistance. Sensitivity was explained in terms of the relative partial contributions of n and r2 to r. 

Knock resistance has also been correlated with other preflame reaction properties such as the rate of pressure development during adiabatic compression (17), the temperature coefficient of preflame reactions (202), and the pressure developed prior to firing (34). Estrad re (59) made a correlation between the temperature of initial exothermic oxidation in a tube and knock. No quantitative connection exists between apparent activation energy (160) or the total heat (179) of the precombustion reactions and knock. 

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). 

Other Properties. THERMAL STABILITY. Several attempts have been made to correlate knock resistance with thermal stability. Petrov (162) attempted to account for the knock characteristics of various gasoline fractions in terms of their cracking products. Rice (177) showed that a parallelism existed between yields of cracking products and knock tendency. Estradgre (60) did not find a direct relation between temperature for initial cracking and detonation characteristics. 

The decomposition of the alkoxy radical by Reaction 8 occurs by a-scission at the C—C bond attached to the largest hydrocarbon group (185, 229). Straight-chain paraffins produce aldehydes, while highly branched paraffins yield ketones (Reaction 11). The knock resistance of naphthenes may be caused by the stability of the naphthene ring to C—C scission (25). 

The importance of hydrogen-containing species in knock reactions is strikingly shown in knocking combustion studies of carbon monoxide by Anzilotti and Tomsic (6). The presence of 1.4 mole % of water markedly lowered the knock resistance of nearly anhydrous carbon monoxide. Other studies show that water enters into the combustion reactions of carbon monoxide and contributes hydrogen to the intermediate combustion products 125). 

Antiknocks. Antiknock agents are used to improve the natural knock resistance of gasolines used in Otto-cycle gasoline engines, knock being the power-robbing, po-... 

Reforming is a process which, while not greatly altering the size of the molecules, increases their knock resistance. Among these processes are isomerization of straight chain alkanes to branched hydrocarbons, cycliz-ation and dehydrogenation to aromatics and removal of the side chains of aromatics. The process produces much of the H2 used elsewhere in the refinery. 

Alkylation is a synthetic process in which lower alkenes (from catalytic cracking) are reacted in an acid medium (sulphuric or hydrofluoric acid) with small branched alkanes to produce Cg to Cg branched alkanes. These are probably the most desirable constituents of a gasoline, with good knock resistance and with fewer undesirable properties, such as tendencies to... 

In Germany, however, bioethanol will mainly be used as low blend of bioethanol and petrol (E5, ElO) and for the production of ETBE (ethyl-tertiary-butyl-ether). Similar to methyl-tertiary-butyl-ether (MTBE), ETBE is used to enhance the octane index and improve knock-resistance and combustion properties of gasoline." It is less challenging to handle, does not induce evaporation of gasoline and does not absorb moisture like ethanol does. ETBE is produced by reacting ethanol and isobutylene via acid catalyst ... 

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