Spark Assisted Compression Ignition, SACI
Sammanfattning: The strong focus on decreasing carbon dioxide emissions due to limited natural resources of fossil fuel as well as alarming climate changes drives the research and development of our prime mover, the combustion engine, faster then ever. The minimum requirement is a power source with increased efficiency while emitting ultra low levels of hazardous local and regional emissions. The concept of Homogeneous Charge Compression Ignition (HCCI) promises increased efficiency and low levels of NOx, insignificant smoke but increased levels of CO and HC when utilized in the spark ignition (SI) engine. Since HCCI in the SI engine can only be utilized at part load and does not cover the entire operating range of the engine mode shifts are necessary. This is where spark assisted compression ignition (SACI) comes in. Using SACI combustion a controlled mode shift from SI to HCCI and vice versa can be achieved under certain conditions. This thesis is based on experimental investigations of HCCI combustion mainly addressing the SI engine environment. Here HCCI combustion is achieved by trapping hot residuals through a negative valve overlap and thereby raising charge temperature during compression to auto ignition. When combined with spark assistance it is addressed as SACI combustion. SACI is shown to increase the possible operating region without switching to SI thus increasing the gain of HCCI combustion further. Further it is shown that HCCI combustion timing is affected using spark assistance under proper conditions. This enables a means of direct control of combustion timing. The usage of SACI combustion at low load can affect cycle to cycle variations related to residual gas status. By decreasing the cycle to cycle dependence a lower load can be achieved without misfire. The effect of spark assistance in SACI combustion is investigated using high speed chemiluminescence, laser Doppler velocimetry (LDV) and heat release analysis for understanding the interaction between the heat release origin from the spark and the subsequent HCCI combustion. It is found to be turbulent flame propagation also at low load from low to high residual dilution that raises the temperature and initiates auto ignition. From LDV measurements a positive effect of increased turbulence is seen on the growing flame. The results are confirmed by experiments with intake valve deactivation changing the tumble flow to include swirl. The flame expansion speed increases with turbulence while the effect on the HCCI part is more modest. The effect on auto ignition is found to be more related to increased mixing. In a combustion boundary layer investigation only small deviations are seen on HCCI combustion when increasing the swirl. On the other hand it is concluded that a thicker boundary layer with more thermal stratification is related to slower combustion. Effects of fuel stratification in combination with SACI and residual dilution are investigated using Planar Laser Induced Fluorescence (PLIF) Charge homogeneity is affected both by different strategies of port injection as well as by combining port injection and direct injection. For increased stratification effects from fuel heat of vaporisation and reactions during the negative valve overlap are seen to counteract. Increased reactivity due to richer zones seems not as strong as reported for fuels exhibiting low temperature reactions.
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