Due to its considerable advantages over lower alcohols such as ethanol, in particular with regard to its physical properties like volatility, corrosivity, and hygroscopicity, butanol has attracted considerable interest as a potential biofuel candidate. It has therefore been a target of a series of experimental studies probing its combustion characteristics. Nevertheless, its ignition behaviour at elevated pressures still remains widely unexplored. The present study investigates the oxidation of n-butanol at pressures near 80 bar. Ignition delays were determined experimentally in the temperature range of 795-1200 K between 61 and 92 bar.The time of ignition was determined by recording pressure and CH-emission time histories throughout the course of the experiments. The results display the first evidence of the influence of negative temperature coefficient (NTC) behaviour which was not observed in earlier ignition studies. The high pressure measurements show that NTC behaviour is enhanced as pressures are increased. The experimental results were modelled using an improved chemical kinetic mechanism which includes a simplified submechanism for butylperoxy formation and isomerisation reactions currently absent in n-butanol kinetic models.The detailed mechanism validated with the high pressure ignition results for realistic engine in-cylinder conditions can have significant impact on future advanced low temperature combustion engines.
Auto-ignition characteristics of ethanol were experimentally investigated using two Shock Tube (ST) facilities and a Rapid Compression Machine (RCM). Ignition delay times for stoichiometric ethanol-air mixtures were measured for temperatures between 775–1300 K in a High Pressure Shock Tube (HPST) at pressures close to 80 bar by probing pressure time histories and CH* emission. In some experiments the HPST was additionally employed for schlieren imaging to visualize ignition behavior by probing density gradients during ignition for ethanol-air mixtures. The ignition delay experiments in HPST were complemented by RCM measurements for extending the temperature regime to the Low Temperature Combustion (LTC) regime, down to 705 K, providing kinetic model validation data over a very wide temperature and pressure range. The current results also extend the earlier shock tube measurements performed in the same laboratory for pressures around 40 bar for temperatures down to 800 K [Heufer et al., Shock Waves 20 (2010) 307]. Furthermore, a Rectangular Shock Tube (RST) was solely used for additional schlieren imaging experiments to acquire information on ignition modes in stoichiometric ethanol-air mixtures around 10 bar. An improved chemical kinetic model was developed based on the Li et al. mechanism [Li et al., “Ethanol Model v1.0”, Princeton University, 2009] which was updated with evaluated rate parameters from the literature and validated through results obtained from the aforementioned facilities. The model predictions were compared to previously published low-pressure, premixed flat flame molecular beam mass spectrometry speciation data [Kasper et al., Combust. Flame 150 (2007) 220; Wang et al., J. Phys. Chem. A 112 (2008) 9255] where reasonable agreement is obtained considering the uncertainties in experiments and model. However, the model provides excellent agreement for the auto-ignition results obtained in the RCM and the high temperature shock induced ignition delays. Significant disparities with the model predictions are obtained for the shock tube results at temperatures below 1000 K as it transitions from the intermediate to the low temperature regime. The reasons for these deviations are assigned to strong fuel specific “pre-ignition” effects observed in ethanol auto-ignition, in contrast to other investigated fuels, which was satisfactorily explained through schlieren experimental results. To our knowledge this work is first of its kind that combines results from complementary experimental methods from three different facilities providing a holistic description on the auto-ignition behavior of ethanol. Furthermore, this paper reports ignition delay measurements for ethanol in air, at the highest pressures applicable to practical combustors.
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