Octanes ignition temperature

Recently Livengood, Toong, Rona, Taylor, and Black used an externally heated hot spot to induce surface ignition in a motored engine (53). Under the conditions of these experiments, benzene required a somewhat higher hot-spot temperature for surface ignition than iso-octane, but the difference in temperature was far less than the difference between the spontaneous ignition temperatures of the two hydrocarbons as determined in the laboratory. 

Octane is an eight-carbon alkane and has a boiling point of 125.6°C, a flash point of 56°F, a flammable range of 1.0 to 6.5%, and an ignition temperature of 428°F. The structure and molecular formula for pentane are shown in Figure 5.31. 

Levedahl (106) noted that aliphatics from acetylene to octane all gave hot flames at 600° C. He suggested that some reaction, such as thermal decomposition, which was common to all aliphatics, became important. Benzene showed a very high, inconsistent hot flame limit, while cyclohexane was low and variable. Levedahl believed the cool flame and subsequent reaction, along with compression, served to raise the mixture temperature to the critical value. Acetylene was believed to play a major role in the ignition reaction. 

Experimentally measured hot-spot temperatures required for surface ignition of iso-octane and benzene in an operating engine are similar in spite of known great differences in actual surface-ignition resistance as measured by other procedures. Alquist and Male (1) and Male and Eward (57) concluded that hot-spot temperatures could not be used to characterize the surface ignition resistance of fuels. 

Suppose we wish to measure the energy of combustion of octane (CgHig), a component of gasoline. A 0.5269-g sample of octane is placed in a bomb calorimeter known to have a heat capacity of 11.3 kJ/°C. This means that 11.3 kJ of energy is required to raise the temperature of the water and other parts of the calorimeter by 1°C. The octane is ignited in the presence of... 

Another factor is fuel. If the gasoline is of a grade that is better able to withstand the temperatures and pressures within the cylinder without spontaneously igniting, then knock is less likely. A fuel with a high octane rating is formulated to resist burning until it is sparked. 

Fig. 6. 21. Ignition delays as a function of compressed gas temperature for the normal alkanes, n-butane to n-heptane, i-octane (2,2,4-trimethylpentane) and toluene in stoichiometric proportions in air. The results were obtained in a rapid compression machine at a compressed gas density of 128 mol m (0.65-0.75 MPa). Ignition in the ntc and lower temperature range was not observed under these experimental conditions [50].

Fig. 7.17. Influence of NO2 on ignition delay-times in a rapid compression machine [141], Fractional ignition delays are shown for various post-compression temperatures. Note that at the higher temperatures NO2 enhances autoignition at small concentrations while hindering it at greater concentrations. Stoichiometric mixture of 90% iso-octane/10% n-hep-tane, total concentration 3.2 x 10 mol cm . ...

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