The light-medium load operating regime (4-8 bar net IMEP) presents many challenges for advanced low temperature combustion strategies (e.g. HCCI, PPC) in light-duty, high speed engines. In this operating regime, lean global equivalence ratios (Φ<0.4) present challenges with respect to autoignition of gasoline-like fuels. Considering this intake temperature sensitivity, the objective of this work was to investigate, both experimentally and computationally, gasoline compression ignition (GCI) combustion operating sensitivity to inlet swirl ratio (Rs) variations when using a single fuel (87-octane gasoline) in a 0.475-liter single-cylinder engine based on a production GM 1.9-liter high speed diesel engine.
For the first part of this investigation, an experimental matrix was developed to determine how changing inlet swirl affected GCI operation at various fixed load and engine speed operating conditions (4 and 8 bar net IMEP; 1300 and 2000 RPM). Here, experimental results showed significant changes in CA50 due to changes in inlet swirl ratio. For example, at the 4 bar net IMEP operating condition at 1300 RPM, a reduction in swirl ratio (from 2.2 to 1.5) caused a 6 CAD advancement of CA50, while increasing swirl ratio (from 2.2 to 3.5) resulted in a 2 CAD retard of CA50. This advancement in CA50 at the 1.5 swirl ratio operating point was accompanied with significant increases in NOx emissions (from 0.2 to 1.6 g/kg-fuel). Minor adjustments in injection strategy could be made to maintain NOx emissions less than 1 g/kg-fuel.
In subsequent experiments at 4 bar net IMEP, first equivalence ratio, then CA50 were matched in an effort to further isolate the effects of changing swirl ratio. In these later cases conditions allowed for a 25C reduction in the required inlet temperature at the lower swirl condition (from 77C to 52C when reducing swirl from 2.2 to 1.5). Experimental measurements were numerically simulated to help analyze the combustion behavior and emissions characteristics using a 3D-CFD code coupled with detailed chemistry. This numerical investigation quantified the thermal and mixing effects of swirl ratio variation on mixture conditions before ignition and subsequent influence on ignition timing, in-cylinder pressure profile, and emissions.