Large Eddy Simulation of Turbulent Reactive Flows under HCCI Engine Conditions

Abstract: Large Eddy Simulation (LES) modeling is developed to study homogeneous charge compression ignition (HCCI) combustion under different engine operation and mixture conditions, including real engine configurations and generic test cases. A previously developed HCCI model has been validated by comparing the simulation results with engine experiments. A new spark-assisted HCCI (SACI) combustion model has been developed and tested. The tumble flow dynamics and turbulence eddies under an experimental low speed engine condition are simulated to improve the understanding of the engine flow dynamics. The onset of temperature inhomgeneity is systematically investigated and compared with laser diagnostic data. The ignition kernel/eddy interaction is simulated to gain insight into the HCCI ignition process in different turbulence conditions. It is found that HCCI combustion is primarily dictated by the in-cylinder temperature stratification. For a given combustion phasing, e.g. a given crank angle at which 10% of heat has been released (CA10), with large temperature stratification the combustion process is slower and the pressure-rise-rate is slower. This can offer an opportunity to run the engine at high load. LES results show that turbulence can affect the HCCI combustion process under certain conditions. Due to rapid propagation of the ignition front, under typical engine conditions turbulence cannot directly affect the reaction zones, e.g., by wrinkling the reaction front or by differential diffusion to adjust the radical levels. Turbulence affects HCCI combustion mainly through modifying the temperature field. Turbulence has two effects on the process: one is to generate temperature stratification by heat transfer between the wall and the in-cylinder gas; another is to smear out the temperature stratification in the gas mixture by turbulence eddy action. If the size of hot zone or the cold zone in the mixture is large, e.g. larger than the integral length scale, turbulence heat transfer may not smoothen the temperature stratification quickly. As a result, the effect of turbulence on the ignition process is less significant. On the other hand if the scales of the hot/cold zones are small, turbulence can play an important effect on the HCCI combustion process. It is shown that the stratification of temperature in an engine is generated through three main mechanisms: mixing of the cold intake gas with the hot residual gas, wall heat transfer, and compression of the mixture. The last mechanism is less well known. By having large amount of residual gas or exhaust gas recirculation (EGR) in the cylinder the temperature stratification can be enhanced and thereby affect the HCCI combustion process. SACI combustion is shown to be sensitive to both turbulence and temperature stratification. The operation window for SACI is narrow: if the temperature is low the process is mainly SI, and if the temperature is high, the process is HCCI. As a result using SACI to control HCCI engine may not be easy.