Alex C. Alkidas

On the application of Wiebe functions to simulate normal and knocking spark-ignition combustion

Employing a Wiebe function is a very convenient method to simulate fuel burning in internal combustion engines. The present study examines the various forms of the Wiebe functions and presents a methodology which optimises the Wiebe parameters for simultaneously best fit of the cumulative heat release and the rate of heat release histories. The accuracy of the results of the Wiebe function when solving for two parameters and for four parameters is compared; the former is done in closed form, whereas the latter is done by using an optimisation routine. The latter methodology was applied for the simulation of combustion in spark-ignition engines under normal and knocking conditions. For the knocking cycles, a linear weighted combination of two Wiebe functions, one enabling the modelling of the flame-propagation combustion and the other that of the autoignition combustion was found to give excellent results.

Cylinder-pressure-based methods for sensing spark-ignition engine knock

This study used experimental in-cylinder pressure measurements and heat-release analysis to examine the various methods used to quantify the severity of knock and its onset, for a range of engine speeds of 1000 to 4000 rpm. The methods proposed in the literature for assessing the magnitude of knock are grouped into four major categories. It is shown that most have good correlation with the maximum amplitude of the pressure oscillations, which is the most commonly used method. Based on the heat-release analysis, the auto-ignition behaviour of the end-gases was categorised into three types, depending on the location and magnitude of the highest heat release rates due to flame propagation combustion and explosive combustion. At the higher engine speeds, the onset of auto-ignition could be reliably established from the heat-release analysis and it was found to occur about 2 crank angle degrees advanced of the knock onset.

Transient surface-heat-flux measurements in the exhaust of a SI engine

Surface-heat-flux measurements were performed at two locations in a straight-pipe extension of the exhaust-port of a SI engine. The measured surface-heat-flux history was characterised by a double peak, during the exhaust valve-open period. The largest peak resulted from the action of high-velocity, blowdown gases exiting the combustion chamber at the beginning of the exhaust process. The significantly lower second peak was due to the slower piston-driven gas motion that occurred during the displacement phase. During the closed-valve period, the local surface-heat-flux was found to be relatively low (10-45 kW/m 2 ) in comparison to the peak heat flux (80-288 kW/m 2 ), and to exhibit minimal decay. The predicted heat-flux histories from an engine-simulation analysis, which employed a heat transfer correlation developed by the authors, were in good agreement with the corresponding measured histories during blowdown, however, predictions were relatively poor during the displacement phase, both in magnitude and behaviour.