Simulation of Spark Assisted Compression Ignition Combustion Under EGR Dilute Engine Operating Conditions.

Abstract

Spark Assisted Compression Ignition (SACI) combustion has been shown to provide highly efficient, potentially low NOx operation similar to Homogeneous Charge Compression Ignition (HCCI) combustion. Direct control over ignition timing and burn rate through SACI operation has the ability to overcome shortcomings of HCCI operation allowing an increase in power density. Detailed SACI models capable of capturing the charge preparation process and impact of dilution method on combustion are currently limited. The current work addresses this need by developing such a model and investigating SACI combustion in an engine simulation.

Modeling requires valid predictions of laminar flame speeds under SACI conditions which are not available in the literature. To address this need under highly EGR dilute, high preheat temperature SACI conditions, laminar reaction front simulations were conducted. Moderate burning velocities were observed for EGR dilutions typical SACI operation, provided that preheat temperatures were elevated and burned gas temperatures exceeded 1450K. For a given preheat and burned gas temperature, EGR dilution suppressed burning velocities relative to air dilution, behavior attributed to decreases in mixture oxygen. Correlations of laminar burning velocity and thickness were developed from these data.

An existing model for HCCI, SI, and SACI combustion in KIVA-3V was extended to capture engine breathing and charge preparation by direct injection under conditions utilizing EGR dilution. The model was capable of predicting trend-wise agreement with metal engine cylinder pressure measurements for HCCI, SI, and SACI combustion.

Analysis showed that during SACI operation, compression heating from reaction front heat release increased the end-gas temperature to initiate end-gas auto-ignition, providing control over the combustion process. Manipulation of the flame heat release by varying intake temperature, spark timing, and dilution composition allowed control over heat release rates independent of combustion phasing, reducing peak heat release rates while increasing load and efficiency. The influences on end-gas heat release rate were the total end-gas mass and the temperature stratification prior to auto-ignition, which evolved significantly during the flame propagation phase. Insights from this work can be used to guide SACI operating strategies to enable high efficiency engine operation at higher power density than with HCCI combustion.