Insights into stoichiometric and lean combustion phenomena of gasoline–butanol, gasoline–ethanol, iso-octane–butanol and iso-octane–ethanol blends in an optical SI engine

Introduction of novel fuels, such as mixtures of ethanol or butanol with hydrocarbons, requires new fundamental understanding of in-cylinder combustion properties in modern direct-injection spark-ignition engines since those can be quite sensitive to fuel properties. Gasoline and its blends with 25% ethanol and butanol at 25% and 16% per volume (the latter equivalent to 10% ethanol blending ratio in terms of oxygen content) were studied in comparison to gasoline, ethanol, and butanol combustion. The same alcohol blending ratios were also employed with iso-octane as the base component for direct comparison. Testing was performed at 1500 RPM with 0.5 bar intake plenum pressure using 20°C or 80°C engine coolant temperature. Thermodynamic parameters were derived using in-cylinder pressure analysis for stoichiometric (ϕ = 1.0) and lean (ϕ = 0.83) fueling over a range of spark advances. Additionally, high speed color and greyscale chemiluminescence imaging was conducted at gasoline’s maximum break torque spark timing, calculating flame growth speeds, flame roundness, and centroid motion. Laminar burning velocity data from the literature and in-cylinder flow measurements from the same engine were used for interpretation. Overall, the analysis showed small differences between gasoline and the blends in general, but showed changes for the pure alcohols with typically much faster flame progression for ethanol and issues with the combustion stability of butanol at low engine temperatures. Alcohol blending, particularly with iso-octane, showed some benefits at lean conditions