Self-ignition of S.I. engine model fuels: A shock tube investigation at high pressure

• Published in: Combustion and flame 1997-06, Vol.109 (4), p.599-619
• Main Authors: Fieweger, K., Blumenthal, R., Adomeit, G.
• Format: Article
• Language: English
• Subjects: Applied sciences; Energy; Energy. Thermal use of fuels; Engines and turbines; Equipments for energy generation and conversion: thermal, electrical, mechanical energy, etc; Exact sciences and technology; Gases; Photography; Q1; Temperature
• ISSN: 0010-2180; 1556-2921
• DOI: 10.1016/S0010-2180(97)00049-7

The self-ignition of several spark-ignition (SI) engine fuels ( iso-octane, methanol, methyl tert-butyl ether and three different mixtures of iso-octane and n-heptane), mixed with air, was investigated experimentally under relevant engine conditions by the shock tube technique. Typical modes of the self-ignition process were registered cinematographically. For temperatures relevant to piston engine combustion, the self-ignition process always starts as an inhomogeneous, deflagrative mild ignition. This instant is defined by the ignition delay time, τ de fl . The deflagration process in most cases is followed by a secondary explosion (DDT). This transition defines a second ignition delay time, τ DDT , which is a suitable approximation for the chemical ignition delay time, if the change of the thermodynamic conditions of the unburned test gas due to deflagration is taken into account. For iso-octane at p = 40 bar, a NTC (negative temperature coefficient), behaviour connected with a two step (cool flame) self-ignition at low temperatures was observed. This process was very pronounced for rich and less pronounced for stoichiometric mixtures. The results of the τ DDT delays of the stoichiometric mixtures were shortened by the primary deflagration process in the temperature range between 800 and 1000 K. Various mixtures of iso-octane and n-heptane were investigated. The results show a strong influence of the n-heptane fraction in the mixture, both on the ignition delay time and on the mode of self-ignition. The self-ignition of methanol and MTBE (methyl tert-butyl ether) is characterized by a very pronounced initial deflagration. For temperatures below 900 K (methanol: 800 K), no secondary explosion occurs. Taking into account the pressure increase due to deflagration, the measured delays τ DDT of the secondary explosion are shortened by up to one order of magnitude.