Paraffins 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)). 

Fig. 3.4. Autoignition temperatures of paraffin hydrocarbons at 1 atm. (Data from Bodurtha 1980P)...

When higher hydrocarbons are desired for use as reaction solvents, mineral oil (bp = 260 to 330°C) is recommended because the autoignition temperature of mineral oil (260 to 370°C) is much higher than those of the n-paraffins. Since it is produced in large quantity, it is quite cheap, and is a mixture of various branched aliphatic hydrocarbons. 

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. 

Synthetic FT diesel fuels can have excellent autoignition characteristics. The FT diesel is composed of only straight-chain hydrocarbons and has no aromatics or sulfur. Reaction parameters are temperature, pressure and H/CO ratio. FT product composition is strongly influenced by catalyst composition the yield of paraffins is higher with cobalt catalytic ran and the yield of olefins and oxygenates is higher with ironcatalytic ran. 

Attempts have been made to estimate the volumetric heat release rate during autoignition, from pressure records and CARS temperature measurements [106]. It was estimated that the activation temperature for this was in the region of 20,000 K under knocking conditions. Paraffinic fuels gave a higher volumetric heat release rate than aromatic fuels and were more prone to knock severely. 

---