Preflame oxidation

These studies provide comprehensive information on the effects of many halogen compounds on the combustion behavior of various hydrocarbons. The combustion behavior included preflame oxidation, ignition over a wide range of temperature, and the flame process. The effect of iodine compounds on preflame combustion, which had not been previously investigated, is now reported. 

Preflame Oxidation. The principal reactions responsible for promoting preflame oxidation are considered to be the following, where X represents a halogen atom ... 

Of Reactions 1 to 5, Reactions 2, 3, and 4 probably play the more important role because they favor the formation of the kinetically important peroxides and enhance their contribution to chain branching. Their ease of reaction is expected to decrease in the order HI > HBr > HC1, and this accounts for the fact that the order of effectiveness of the halogen compounds in promoting preflame oxidation is iodine > bromine > chlorine. 

Sampling Studies. To obtain a better understanding of the chemistry of preknock reactions, a number of investigators have sampled and analyzed hydrocarbon-air mixtures undergoing preflame oxidation in engines. The types and concentrations of some of the more stable intermediates taking part in these reactions have been determined. While... 

Combustion chemistry in diffusion flames is not as simple as is assumed in most theoretical models. Evidence obtained by adsorption and emission spectroscopy (37) and by sampling (38) shows that hydrocarbon fuels undergo appreciable pyrolysis in the fuel jet before oxidation occurs. Eurther evidence for the existence of pyrolysis is provided by sampling of diffusion flames (39). In general, the preflame pyrolysis reactions may not be very important in terms of the gross features of the flame, particularly flame height, but they may account for the formation of carbon while the presence of OH radicals may provide a path for NO formation, particularly on the oxidant side of the flame (39). 

Seakins (16) has reported that the low temperature oxidation of propane is promoted by chloroform but not by carbon tetrachloride. Our studies, however, show that chloroform and carbon tetrachloride have generally similar effects on all preflame stages (Figure 3) and that their patterns of oxidative degradation are also similar (Figure 8). Under the conditions of Seakins experiments the following reaction, which he suggested, probably initiates the sequence of reactions responsible for promotion. 

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). 

Several hydrocarbons, including benzene, diisobutylene, and methane, do not form cool flames in engines (26, 37, 105). The absence of cool flame radiation does not indicate the absence of preflame reaction, as oxidation products have been isolated from a diisobutylene-air mixture in a motored engine (105). At lean air-fuel ratios, benzene, diisobutylene, and methane have been observed to form blue flames (36). 

Tetraethyllead is believed to act as an antiknock by changing the course of the complex hydrocarbon oxidations which precede knock (69). It also may improve fuel-ignition resistance by its effects on these preflame reactions it is most effective in fuels that undergo considerable preflame reaction and has little effect on fuels that do not. Some of the products formed in preflame reactions—aldehydes, for example—sensitize fuels to ignition (47, 69, 86). 

There is no direct experimental evidence for this complex decomposition and it may well occur by several steps [107]. However, substantial yields of unsaturated carbonyl compounds are formed particularly at high pressures [78] under initial reaction conditions where cool flames propagate. For example, the cool-flame oxidation of 2-methylpentane at 525 °C and 19.7 atm in a rapid compression machine [78] yields no less than 14 unsaturated carbonyl compounds viz acrolein, methacrolein, but-l-en-3-one, pent-2-enal, pent-l-en-3-one, pent-l-en-4-one, trans-pent-2-en-4r one, 2-methylbut-l-en-3-one, 2-methylpent-l-en-3-one, 4-methylpent-l-en-3-one, 2-methylpent-l-en-4-one, 2-methylpent-2-en-4-one, 2-methyl-pent-2-enal and 4-methylpent-2-enal. Spectroscopic studies of the preflame reactions [78] have shown that the unsaturated ketones account for ca. 90 % of the absorption which, occurs at 2600 A. At lower initial temperatures and pressures acrolein and crotonaldehyde are formed from n-pentane [69, 70] and n-heptane [82], and acrolein is also formed from isobutane [68]. 

---