Effects of Negative Valve Overlap on the Auto-Ignition Process of Lean Ethanol/Air Mixture in HCCI-Engines

This paper presents a computational study of the effects of
fuel and thermal stratifications on homogenous charge
compression ignition (HCCI) combustion process in a
personal car sized internal combustion engine. Stratified
HCCI conditions are generated using a negative valve overlap
(NVO) technique. The aims of this study are to improve the
understanding of the flow dynamics, the heat and mass
transfer process and the onset of auto-ignition in stratified
charges under different internal EGR rate and NVO
conditions. The fuel is ethanol supplied through port-fuel
injection; the fuel/air mixture is assumed to be homogenous
before discharging to the cylinder. Large eddy simulation
(LES) is used to resolve in detailed level the flow structures,
and the mixing and heat transfer between the residual gas and
fresh fuel/air mixtures in the intake and compression strokes.
Multi-Zone model based on a detailed chemical kinetic
mechanism is then used to simulate the onset of auto-ignition
in the combustion stroke near TDC, based on the mixtures
predicted in LES. It is found that for low and moderate EGR
rates (low and moderate NVO) the onset of ignition is more
sensitive to the temperature of the mixture than to the fuel
concentration. For the high EGR rate and large NVO case,
there is a preferred mixture and temperature at which the first
ignition occurs. Under similar operating conditions the
moderate NVO and EGR rate case is found to have the
earliest ignition, whereas the longest combustion duration is
found in the lowest EGR rate and the lowest NVO case.