Study of Fuel Stratification on Spark Assisted Compression Ignition (SACI) Combustion with Ethanol Using High Speed Fuel PLIF

An engine can be run in Homogenous Charge Compression Ignition (HCCI) mode by applying a negative valve overlap, thus trapping hot residuals so as to achieve an auto-ignition temperature. By employing spark assistance, the engine can be operated in what is here called Spark Assisted Compression Ignition (SACI) with ethanol as fuel. The influence of fuel stratification by means of port fuel injection as well as in combination with direct injection was investigated.

A high-speed multi-YAG laser system and a framing camera were utilized to capture planar laser-induced fluorescence (PLIF) images of the fuel distribution. The charge homogeneity in terms of fuel distribution was evaluated using a homogeneity index calculated from the PLIF images. The homogeneity index showed a higher stratification for increased proportions of direct-injected fuel.

It was found that charge stratification could be achieved through port fuel injection in a swirling combustion system. For the combined port fuel and direct injection strategy, a strong stratification was demonstrated. Although combustion phasing remained constant, a slight increase in the combustion duration was observed. For increased stratification with direct injection in a base of port-injected fuel, an increased amount of ignition sites could be observed. However, the most fuel-rich regions did not necessarily ignite first. Rather, ignition occurred in the mixing zone between the rich and the leaner regions.