Location of the First Auto-Ignition Sites for Two HCCI Systems in a Direct Injection Engine

To elucidate the processes controlling the auto-ignition timing and overall combustion duration in homogeneous charge compression ignition (HCCI) engines, the distribution of the auto-ignition sites, in both space and time, was studied. The auto-ignition locations were investigated using optical diagnosis of HCCI combustion, based on laser induced fluorescence (LIF) measurements of formaldehyde in an optical engine with fully variable valve actuation. This engine was operated in two different modes of HCCI. In the first, auto-ignition temperatures were reached by heating the inlet air, while in the second, residual mass from the previous combustion cycle was trapped using a negative valve overlap. The fuel was introduced directly into the combustion chamber in both approaches. To complement these experiments, 3-D numerical modeling of the gas exchange and compression stroke events was done for both HCCI-generating approaches. The STAR CD code was applied to predict the distributions of the local mixture temperature, residual gas fraction (RGF) and equivalence ratio throughout the cylinder at the end of the compression stroke. Knowledge of these distributions is important as they are strongly affect the ignition timing and burn duration of the HCCI combustion. Selected points from the calculated distribution maps were used as initial conditions for 0-D detailed-chemistry calculations, using the SENKIN code to identify combinations of properties that lead to first auto-ignition. Tracing corresponding points back to the STAR CD mesh gave information about the location of the auto-ignition sites. Results obtained in this way corresponded well to the auto-ignition locations derived from optical measurements.