TED-AJ03-565 Two-position Measurement of Preflame Reactions at Autoignition Under Knocking Operation

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There are demands to improve the thermal efficiency of spark ignition engines in order to reduce carbon dioxide (CO_2) emissions and conserve energy in an effort to prevent global warming. One of the major factors impeding further improvement of the thermal efficiency of spark ignition engines is the phenomenon of knocking. Knocking is widely recognized as autoignition of the unburned end gas accompanied by propagation of high-frequency pressure oscillations. However, the combustion process from preflame reactions to the occurrence of autoignition is still not clearly understood owing to the complex mechanisms involved. With the aim of shedding some light on that process, this study focused on light emission behavior in the preflame reaction (Fig. A-1) interval before hot flame reactions. Light emission behavior was measured at wavelengths corresponding to the spectra of formaldehyde (HCHO; characteristic spectra of 395.2 nm), which shows cool flame reactions, the OH radical (306.4 nm) and Vaidya's hydrocarbon flame band (HCO; 329.8 nm), which exhibits blue flame reactions, during a progression from normal combustion to knocking operation. A four-cycle, air-cooled, single-cylinder gasoline engine with a side valve arrangement was used as the test engine in this study, as it allowed easy modification of the cylinder head for measurement purposes. Light emission behavior was simultaneously observed at two positions (the end zone and the center zone) in the combustion chamber, Light emission behavior measured for the three radicals by light emission spectroscopy was obtained by introducing combustion light into a polychromator. The test fuel used in this study was n-heptane (0 RON). The test engine was operated at three speed levels (1400,1800 and 2200 rpm) under a constant boost pressure. A progression from normal combustion to knocking operation was induced by reducing the supply of cooling air to the cylinder head, causing it to overheat. The light emission intensity waveforms recorded during engine operation at each operating speed indicate the passage of a cool flame and the occurrence of a blue flame in the preflame reaction interval. In the end zone of the combustion chamber, behavior corresponding to the passage and degeneracy of a cool flame was observed. It was also observed that the tendencies of the preflame reactions varied depending on the engine speed. In this paper, those tendencies are discussed on the basis of the experimental results.[figure]

TED-AJ03-565 Two-position Measurement of Preflame Reactions at Autoignition Under Knocking Operation

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