Downsized direct-injected boosted gasoline engines with high specific power and torque output are leading the way to reduce fuel consumption in passenger car vehicles while maintaining the same performance when compared to applications with larger naturally aspirated engines. These downsized engines reach brake mean effective pressure levels which are in excess of 20 bar. When targeting high output levels at low engine speeds, undesired combustion events called pre-ignition can occur. These pre-ignition events are typically accompanied by very high cylinder peak pressures which can lead to severe damage if the engine is not designed to withstand these high cylinder pressures. Although these pre-ignition events have been reported by numerous other authors, it seems that their occurrence is rather erratic which makes it difficult to investigate or reliably exclude them. This paper describes a systematic engine dyno testing approach to force the engine into pre-ignition in order to study and characterize these events. A sensitivity study of various parameters shows that pre-ignition can occur repeatedly at the same load levels if boundary conditions are controlled sufficiently, meaning pre-ignition occurrence is less erratic than previously thought. Several hundred pre-ignition events have been recorded and analyzed in this study. A post-processing tool was developed and applied to analyze and characterize all recorded pre-ignition events. The knowledge gained out of these investigations will help to better understand the pre-ignition phenomena and what combustion development activities need to be applied in order to avoid or counteract pre-ignition during an engine development program or afterwards during customer usage in a passenger car.
<div class="section abstract"><div class="htmlview paragraph">In this multi-phase study, fuel and lubricant effects on stochastic preignition (SPI) were examined. First, the behavior of fuels for which SPI data had previously been collected were characterized in terms of their combustion and emissions behavior, and correlations between these characteristics and their SPI behavior were examined. Second, new SPI data was collected for a matrix of fuels that was constructed to test and confirm hypotheses that resulted from interpretation of the earlier data in the study and from data in open literature. Specifically, the extent to which the presence of heavy components in the fuel affected SPI propensity, and the extent to which flame initiation propensity affected SPI propensity, were examined. Finally, the interaction of fuels with lubricants expected to exhibit a range of SPI propensities was examined. Although this final dataset did not yield conclusive results, it suggests that additional factors such as engine condition can have a very significant effect on SPI propensity.</div><div class="htmlview paragraph">The main findings of the study are that lower volatility fuel components appear to affect the propensity of the fuel to create initiation events (which could be fuel-oil droplets or deposit breakoff) that can lead to SPI, and further that the ease by which a flame can be established in the bulk mixture correlates to SPI tendency when the initiation event tendency is fixed. The study also showed that neither soot-forming tendency of a fuel nor the fuel’s antiknock quality necessarily correlate to SPI tendency.</div></div>
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