Preflame reactions

NFPA now normally refers to autoignition as the Hot Flame Ignition Temperature, as a more precise definition. Subsequently the following two additional terms are being adopted by NFPA to further refine the ignition properties of materials. The lowest temperatures at which cool flame ignitions are observed are named the Cool Flame Reaction Threshold (CFT). The lowest flame temperatures at which an exothermic gas phase reaction is noticed are named the Preflame Reaction Threshold (RTT). 

PREFLAME REACTIONS. During the past 30 years, many workers have associated knock with the preflame reactions occurring prior to rapid combustion. Peroxides and aldehydes are important preflame products. It has become customary to consider these compounds, particularly the former, as important in the knock process (28, 43, 142, 143, 170, 181, 235). Motored engine experiments have tended to confirm this view (42, 46, 154, 136, 157, 174, 103, 225), although Ross (183) has obtained severe knock with n-heptane with no evidence of formation of prereaction products. 

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). 

Conclusion. Knock appears to be determined by spontaneous ignition, which in turn depends on the extent of preflame reaction. In general, both cool flame and precool flame reactions will be important. 

Emission of cool flame radiation is associated with an early stage in the preflame reactions of most hydrocarbons, as evidenced by the fact that the appearance of cool flames occurs at the same time as the initial pressure development. At a later time in the cycle a second radiation phenomena, described as a blue flame, has been observed under certain conditions (36, 124). A second cool flame also occurring late in the cycle may be the same phenomenon (81, 103, 131). While the importance of cool flame and blue flame phenomena in the over-all reaction mechanism is not fully understood, their occurrences can be used to mark certain stages in the course of the preflame reactions. This principle has been used extensively in studying the effect of hydrocarbon structure, physical variables, and additives on engine preknock reactions. 

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). 

Formaldehyde and acetaldehyde have been identified by Davis, Smith, Malmberg, and Bobbitt (33) as the major carbonyl compounds formed in the preflame reactions of... 

Pressure development due to preflame reactions of paraffin hydrocarbons is affected little by the presence of tetraethyllead even though autoignition is suppressed 25, 115). In one series of experiments 25), increasing fuel quality 10 octane numbers reduced the pressure development 10 pounds per square inch, while addition of tetraethyllead to achieve the same 10 octane number increase resulted in a decrease of only 2 pounds per square inch. These findings indicate that tetraethyllead is specific in its activity, inhibiting only certain types of reactions leading to the final autoignition step. 

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). 

Chemical delay occurs as a result of the time required for preflame reactions to accelerate and lead to ignition of a combustible mixture of fuel vapor and air. 

TEMPERATURE. The ignition delay of most Diesel fuels decreases as the temperature increases (50, 67, 94, 139). In certain temperature ranges highly-branched and cyclic compounds are exceptions (50). If it is assumed that the chemical delay is long in relation to the physical delay at low temperatures, then it is possible to estimate an apparent over-all energy of activation of the preflame reactions, as indicated in Figure 3 (71). Results similar to those shown in Figure 3 can be represented by an empirical relationship (43, 44) of the form ... 

The mechanism of ignition acceleration by small additions of additives has.been explained on the basis of chain reaction theory 21). It has been suggested 56) that vapor-phase additions are more effective than liquid-phase additions because of the initiation of preflame reactions during the compression stroke. 

Figure 1. The tendency of a fuel to engage in preflame reactions depends on the structure of its component molecules.

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]. 

The slow combustion [93] is measurable at 380 °C, but there is no low temperature mechanism, nor have cool flames been observed [45]. At 560 °C, in a flow system, mixtures of air and methyl formate ignite with explosive violence [47(a)]. The preflame reaction produces methane and methanol. 

The various types of hydrocarbons in gasoline behave differently in their preflame reactions and thus, their tendency to knock. It is difficult to find any precise relationship between chemical structure and antiknock performance in an engine. Members of the same hydrocarbon series may show very different antiknock effects. For example, normal heptane and normal pentane, both paraffins, have antiknock ratings (octane numbers) of 0 and 61.9, respectively (Table 5.5). Very generally, aromatic hydrocarbons (e.g., benzene and toluene), highly branched iso-paraffins (e.g., iso-octane), and... 

Preflame reactions occur in the engine cylinders when portions of the fuel self-initiate combustion prior to the advancing flame from the spark ping. This additional combustion canses an excessive rate of energy release, which is knock. The tendency of a fuel to engage in preflame reactions is dependent npon the stmcture of its component molecnles (see Eigure 1) ... 

---