The effect of swirl on spark assisted compression ignition (SACI)

Auto ignition with SI compression ratio can be achieved by retaining hot residuals, replacing some of the fresh charge. In this experimental work it is achieved by running with a negative valve overlap (NVO) trapping hot residuals. The experimental engine is equipped with a pneumatic valve train making it possible to change valve lift, phasing and duration, as well as running with valve deactivation. This makes it possible to start in SI mode, and then by increasing the NVO, thus raising the initial charge temperature it is possible to investigate the intermediate domain between SI and HCCI. The engine is then running in spark assisted HCCI mode, or spark assisted compression ignition (SACI) mode that is an acronym that describes the combustion on the borderline between SI and HCCI. In this study the effect of changing the in-cylinder flow pattern by increased swirl is studied. This is achieved by deactivating one of the two intake valves. The effect of the increased turbulence is studied both on the initial slow heat release originating from the spark plug and on the following HCCI combustion. The early SI flame development is highly dependent on the flow field so by increasing the turbulence the flame expansion speed is affected, also at high residual rates. Also, HCCI combustion rate has been shown to slow down as turbulence is increased. As high reaction rate is an issue for HCCI combustion this means that it could be possible to reduce the reaction rate and simultaneously increase the possible usage of SACI combustion by increasing the turbulence. Synchronized simultaneous pressure and high speed chemiluminescence measurements are conducted making it possible to reproduce fully resolved cycles from the onset of the spark throughout the entire combustion event. From the chemiluminescence images it is possible to calculate a flame expansion speed. The effect on combustion in terms of auto ignition timing, combustion duration and the amount of heat released in the different combustion modes is investigated using heat release analysis. LDV measurements are conducted to support the turbulence effects on SACI combustion.