Numerous studies have demonstrated the benefits of ethanol in increasing the thermal efficiency of gasoline-fueled spark ignition engines via the higher enthalpy of vaporization and higher knock resistance of ethanol compared with gasoline. This study expands on previous work by considering a split fuel injection strategy with a boosted direct injection spark ignition engine fueled with E0 (100% by volume reference grade gasoline; with research octane number = 91 and motor octane number = 83), E100 (100% by volume anhydrous ethanol), and various splash-blends of the two fuels. Experiments were performed using a production 3-cylinder Ford Ecoboost engine where two cylinders were de-activated to create a single-cylinder engine with a displacement of 0.33 L. The engine was operated over a range of loads with boosted intake manifold absolute pressure (MAP) from 1 bar to 1.5 bar. The fuel injection timing of single fuel injection events was varied at MAP = 1 bar using different blend ratios (E0, E30, E50, E85 and E100) to identify the range of injection timing corresponding to maximum thermal efficiency for each fuel blend.The results indicated knock limited operation for E0 at MAP higher than 1 bar (boosted), whereas none of the ethanol blends was knock-limited even at the highest MAP tested. A split fuel injection strategy with 50% of the fuel mass in each of two injection events was investigated for the range of MAP conditions studied. The different fuel blends showed little sensitivity to the split injection strategy, which indicated fuel air mixing did not significantly affect combustion at the conditions studied. The highest gross indicated thermal efficiencies (GITE) of 38.4% were achieved with E85 and E100 at 1.1 and 1.2 bar MAP for an absolute improvement of 4% compared with baseline gasoline for the same intake pressures. The improvement in GITE scaled with the fraction of ethanol in the fuel blend. GT-Power simulations were used to evaluate the contributions of the enthalpy of vaporization and cooling effects on GITE. Comparison of the simulation results with the experimental data indicates the benefit of increasing GITE with increasing ethanol in the fuel blend is due to enthalpy of vaporization accounting (e.g. of liquid versus gas-phase fuel) and cooling effects on thermodynamic properties such as the ratio of specific heats.