Numerical Modeling of Charge Stratification for the Combustion Control of HCCI Engines

Numerical modeling is used to estimate the realizable potential of combustion control and burn duration extension in homogeneous charge compression ignition (HCCI) engines using in-cylinder charge stratification. Parametric analyses of several fuels using a range of operating conditions illustrate the potential physical variables have on ignition timing and burn duration. The detailed stratification model utilizes a one-dimensional CFD gas dynamics representation of radial gas motion in an engine cylinder with time-varying chamber volume and detailed combustion chemistry of n-pentane fuel. This approach provides the ability to incorporate the effects of in-cylinder temperature and species concentration non-uniformities while remaining sufficiently tractable to include a detailed kinetic model of combustion chemistry.

A parametric set of conditions was studied to determine the effect of stratified temperature, equivalence ratio, and EGR on the ignition timing and burn duration of a representative HCCI engine. The assumption of adiabatic engine operation was made to assess the potential of combustion control mechanisms separate from heat transfer effects. The results demonstrate that moderate and potentially achievable amounts of in-cylinder charge stratification result in extended heat release durations relative to homogeneous conditions. The use of combined temperature, equivalence ratio, and EGR variation in a conceptionally achievable configuration is shown to provide the largest benefit. A series of path forward research directions are also discussed.