This study compares the role of the in-cylinder flow field for spray-guided stratified-charge combustion and for traditional well-mixed stoichiometric operation, both using E70 fuel. The in-cylinder flow field is altered by changing the engine speed between 1000 and 2000 rpm. The stratified operation with the ethanol blend enabled “head ignition” of the fuel sprays, thus minimizing the available fuel/air-mixing time prior to combustion, creating a highly stratified combustion event.
For well-mixed stoichiometric operation, the heat-release rate (HRR) scales proportionally with engine speed due to increased in-cylinder turbulence, as is well-known from literature. In contrast, increasing the engine speed influences the stratified combustion process very differently. Ensemble-averaged over 500 cycles, the time-based HRR in kW remains comparatively unchanged as the engine speed increases. However, cyclic variability of the stratified combustion increases substantially with engine speed.
These observations lead to the development of two hypotheses; a) For highly stratified spray-guided combustion, the heat-release rate of the main combustion phase is primarily controlled by mixing rates and turbulence level associated with fuel-jet penetration. b) During the main combustion phase, the role of the in-cylinder flow field generated by the intake and compression strokes is primarily its stochastic disturbance of the mixing and flow associated with the fuel jets, thereby causing cycle-to-cycle variations of the spray-guided stratified combustion.
These hypotheses are supported by cycle-resolved heat-release analysis, and also by PIV measurements in a companion paper [1], which show that the magnitude of the flow variations increases with engine speed. This increased variability may explain the occurrence of partial-burn cycles at high engine speed. In particular, the in-cylinder flow frequently becomes sufficiently strong to disturb both fuel/air-mixing and flame spread, both being critically important for the heat-release rate of the highly stratified charge.
