Investigations into fuel reactivity using spray ignition experiments

Abstract

Experiments on aromatic compounds comprised of ignition delay measurements on blends of alkylbenzenes in n-alkanes and pure forms of two large alkylbenzenes (n-octylbenzene and n-decylbenzene). The DCN measurements for alkylbenzene and n-alkane blends decreased nonlinearly with increasing alkylbenzene content. A model was proposed to that accounts for this non-linearity by considering an apparent reaction rate order for the blended components. Comparisons of these blends with the large alkylbenzenes having identical proportions of aromatic carbons revealed that there is a synergistic effect of an attached benzyl group on the reactivity of the alkylbenzene. Hence, we conclude that blends of alkylbenznes in n-alkanes are not ideal surrogates to model the low-temperature reactivity of large alkylbenzenes.

Finally, ignition delay measurements were reported for two C10 alkenes (1-decene and trans-5-decene) and compared with data for n-decane. While alkenes exhibited lower low temperature reactivity than the corresponding alkanes, the location of the double bond was found to be crucial to this effect. From review of literature as well as the current data, it is concluded as a thumb rule that the abundance of contiguous CH₂ groups around the double bond increases the alkene’s reactivity. Comparisons of the data on decenes with shock tube measurements from literature showed consistent reactivity trends in the low-temperature regime.

Spray ignition delay measurements for several alkanes (linear, branched and cyclic) revealed a strong dependence on pre-ignition thermodynamic conditions, with decreasing ignition delays for increasing temperature and pressure. The ignition delay in n-alkanes decreased with increasing chain length and correlated as ?? ∝carbon number⁻⁰·⁷). Branched alkanes exhibited a unique two-stage ignition behavior due to weak low-temperature oxidation chemistry and a prolonged transition to hot ignition. This behavior was more pronounced in iso-octane and iso-cetane, which have a high degree of branching. 2,6,10 trimethyl-dodecane, which is a weakly branched alkane, exhibited high reactivity, comparable to n-alkanes, thus corroborating the hypothesis that increased degree of branching causes weak low temperature reactivity. Interestingly, experiments on cyclic alkanes (cyclohexane and methylcyclohexane) showed low reactivity similar to the branched alkanes but ignition in a single stage.

Experimental investigations of spray ignition and combustion were carried out in a constant volume spray combustion chamber (CVSCC), whose design and implementation is also discussed. The focus of these studies was to characterize liquid fuel reactivity through the measurement of spray ignition delay and heat release rate for several important hydrocarbon components of transportation fuels. The CVSCC is an externally heated device that can operate at a range of pre-ignition temperatures (650 – 850 K) and pressures (0.1 – 5 MPa). Electronic fuel injection allows for precise control of injection duration and therefore, injected mass. A reliable and quantitative definition of ignition delay was articulated based on the peaks in the rate of heat release due to combustion of the liquid fuel spray. For a fixed thermodynamic condition (818 K, 2.14 MPa), the ignition delays were correlated with Derived Cetane Number (DCN) thereby allowing for the determination of DCN of several fuel components and blends.