| Popis: |
Preignition is an ignition event that happens before it is expected to happen and, many times, where it is not expected to happen. Understanding this phenomenon is of great importance as it influences the design and operation of modern downsized boosted internal combustion engines. To gain a fundamental understanding of preignition, homogeneous reactors like shock tubes and rapid compression machines may be used to decipher the influence of fuel chemical structure, temperature, pressure, equivalence ratio and bath gas on preignition. In this thesis, a comprehensive study of the preignition tendency of various chemical systems is presented. Firstly, renewable fuels like ethanol, methanol and a surrogate of conventional fuels, n-hexane, are characterized by traditional shock tube techniques, such as the measurements of ignition delay times and pressure-time histories, to identify thermodynamic conditions which promote non-ideal ignition behavior. Preignition pressure rise and the expedition of measured ignition delay times are identified as the indicators of non-homogeneous combustion. It is shown that preignition effects are more likely to be observed in mixtures containing higher fuel concentration and that preignition energy release is more pronounced at lower temperatures. High-speed imaging was implemented to visualize the combustion process taking place inside the shock tube. End-wall imaging showed that low-temperature ignition may be initiated from an individual hot spot that grows gradually, while high-temperatures ignition starts from many spots simultaneously which consume the reactive mixture almost homogeneously. Simultaneous lateral and endwall imaging was implemented in both low- and high-pressure shock tube facilities. All tested fuels exhibited localized ignition at low temperatures, and methanol showed a higher propensity than ethanol to ignite far from the endwall. Imaging experiments were also performed in a rapid compression machine to understand preignition at lower temperatures. Herein, ethanol showed non-homogeneous ignition while iso-octane and diethyl ether exhibited homogeneous ignition at the low-temperature conditions. Various criteria for the onset of preignition were tested against experimental observations to propose an adequate predictor of non-ideal ignition phenomena in practical applications. A non-dimensional number, relating the ignition delay sensitivity and laminar flame speed of the mixtures, was found to be the best criterion to elucidate ignition regimes. |
