Ignition regions of hydrocarbons

Researches into the combustion of complex hydrocarbons, designed to throw light on the problem of knock in internal-combustion engines, have revealed generally that the mechanisms involved are far from simple. Much new light has recently been thrown on the subject by systematic investigation of the influence of pressure on the spontaneous ignition points of these materials.

Inflammable mixtures with air of the paraffins containing three or more carbon atoms, while not spontaneously ignitible at low pressures below about 500°C, give rise abruptly to ignition at higher pressures in a temperature range between about 310° and 370°C, where normally only “cool flames” are initiated; and although neither methane-air nor ethane-air mixtures appear to develop cool flames, the latter are ultimately ignitible in a lower temperature system less complex than that characteristic of the higher members. There is general agreement between ease of ignition in the lower temperature range and the knock ratings of the materials concerned. This holds good for side-chain paraffins, olefins, naphthenes, and aromatic fuels.

All olefins higher than ethylene behave in a similar manner; they differ from the paraffins in that the cool flames are less intense and the preflame time-lags are not only greater but decrease less rapidly with increase of pressure.

The influence of higher aldehydes, nitrogen dioxide, and diethyl ether as promoters of ignition is also discussed.