An investigation of iso -octane auto-ignition using a rapid compression facility.

Abstract

Homogeneous charge compression ignition (HCCI) is a category of low temperature combustion (LTC), which represents a potential major advancement in high-efficiency, low-emissions engines. Although HCCI engines have been demonstrated to operate well under certain conditions, obstacles have been encountered at high-load, cold-start and during transient operating conditions. Thus, an extensive understanding of HCCI combustion is required to address technology barriers and develop valuable control strategies for HCCI engines. The study of iso-octane auto-ignition is of particular interest because iso-octane is one of the primary reference fuels that determine octane numbers. In this work, auto-ignition of iso-octane was examined using a rapid-compression facility (RCF) with iso-octane, oxygen, nitrogen and argon mixtures. The effects of HCCI conditions on the iso-octane ignition characteristics, including ignition delay time, time-resolved OH time histories, and detailed speciation at select times, were studied. Experimental results for iso-octane ignition delay times, tau_ign, were obtained over a range of equivalence ratios (&phis; = 0.25-1.0), pressures (P = 5.12-23 atm), temperatures (T = 943-1027 K), oxygen mole fractions (chi_O2 = 9-21%), and with the addition of trace amounts of combustion product gases (CO₂ and H₂O). The ignition delay times were well represented by the expression:<display-math> <fd> tign=1.3x10-4&dot;P -1.05&dot;f-0.77&dot;cO 2-1.41 &dot;exp33700/R cal/mol/k</fen>T</fen> . </fd> </display-math> Absolute quantitative OH mole fraction time histories were obtained using differential narrow-line laser absorption of the R₁(5) line of the A<super>2</super>&Sgr;<super>+</super>&larr;X<super>2</super>product_i(0,0) band of the OH spectrum (nu_o = 32606.56 cm<super> -1</super>). For these experiments, diluted iso-octane/argon/nitrogen/oxygen mixtures were used with fuel/oxygen equivalence ratios from &phgr; = 0.25 to 0.35. The pressures and temperatures after compression ranged from 8.5 to 15 atm and from 945 to 1020 K, respectively. Intermediate species were quantified by chemically quenching the test gases in the core region at various times during the combustion process via rapid expansion into a vacuum chamber. The composition of the contents in the sampling chamber was analyzed using gas chromatography for stable species, hydrocarbons (C1-C8), and oxygenates. The speciation experiments were conducted at fuel lean (&phgr; = 0.4) and fuel rich (&phgr; = 1.2) conditions. The experimental results of this work provide data critical for the development of LTC engine technologies, particularly kinetically controlled HCCI engines. The data also have direct relevance to other LTC applications where the reaction kinetics affects combustion performance. Quantitative understanding of iso-octane ignition characteristics at lean, low temperature, and moderate pressure conditions can now applied with confidence.