Octane autoignition temperature

For straight paraffinic hydrocarbons (i.e., methane, ethane, propane, etc.) the commonly accepted autoignition temperatures decrease as the paraffinic carbon atoms increase (e g., methane 540 °C (1004 °F) and octane 220 °C (428 °F)). 

MTBE and related ethers are used to add octane to gasoline. MTBE also adds oxygen to the gasoline, which allows for more efficient combustion, and therefore less carbon monoxide and unbumed hydrocarbon in the exhaust emissions A relined grade of MTBE is used in the solvents and pharmaceutical industries. Its higher autoignition temperature and narrower flammability range also make it relatively safer to use compared to other ethers. 

There is a good correlation between autoignition temperatures and octane or cetane numbers in motor fuels. Thus, toluene and diisopropyl ether make attractive additives for petrol while the low autoignition temperature of glycol ethers, diethyl ethers and some normal paraffins show them to have high cetane numbers and to be useful as cold start improvers for diesel compression/ignition engines. 

Unlike its homologue, diethyl ether, DIPE has a comparatively high autoignition temperature. Indeed, it can be blended into motor gasoline as an octane improver. Hence precautions are not required to avoid a vapour explosion due to contact with hot surfaces. 

Griffiths [85] and Schrieber et al. [86] have shown how the Muller scheme can be modified to incorporate these features by slightly expanding the low temperature part of the mechanisms and adding a further intermediate. While mass and energy balances were intrinsic in the formulation, an empirical approach was adopted both for the form of the additional reaction and for all the rate constants. The latter, for instance, included pressure dependent terms. Good fits were obtained to rapid compression machine and shock-tube autoignition delay-times for heptane, iso-octane, and their mixtures. 

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 . ...

In 1985 Leppard [156] reported engine measurements, for stoichiometric ethane-air, of pressure and end gas temperature, the latter derived from the energy equation. The occurrence of autoignition agreed closely with prediction based on an earlier chemical model of Westbrook and Dryer [52]. From their engine experiments, Cowart et al. [59] also compared, for iso-octane and -pentane, the predictions of the simplified models of Hu and Keck [75] and Chun et al. [157], and the more detailed kinetic predictions of Westbrook et al. [158]. These were found to simulate the time of knock occurrence if the kinetic data were re-calibrated. This, and the subsequent work of Brussovansky et al. [76], showed the need for accurate allowances for heat transfer and piston blow-by, because of their important effect on the derived end gas temperature. Where end gas temperature can be measured directly this problem is circumvented. 

Bradley et al. [80] report elevations of CARS temperatures, with a 90% iso-octane/10% heptane fuel prior to autoignition, of about 100 K above such values, with mean values of about 900 K in non-knocking cycles. The greater the end gas pressure, the greater was the CARS temperature and the knock intensity. The temperature elevations were in line with the computed predictions of the simplified five-reaction parrot scheme, de-... 

Fig. 7.19. Use of five-step parrot model, in conjunction with engine model, in post-processing of end gas reactions to show the influence of spark advance on autoignition. Values of pressure, temperature, mass fraction burned, and chain carrier mole fraction, C, computed from engine cycle model without end gas chemistry (broken curves) and with end gas chemistry (full curves). Spark advance before top centre with 90 octane number fuel (a) 10°, non-autoigniting cycle, (b) 20°, autoignition indicated by asterisk.

In Figure (9), temperature and species profile plots obtained in an investigation of autoignition in the end-gas of an Sl-engine, known as knock, as presented by Soyhan et al. ( yhan et al., 2000), are shown. The calculations are obtained in employing a two-zone model (burnt and unburnt zones), the detailed mechanism for iso-octane and n-heptane mixtures... 

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