This chapter studied the effect of ethanol fumigation on engine performance using a modern compression ignition engine. Performance-related parameters were investigated at ethanol substitutions of 0, 10, 20, 30, and 40% (by energy) under 25, 50, 75, and 100% load at 1500 and 2000 rpm. Using E10 and E20 in some of the operating modes decreased FMEP and BSFC; while using E40 increased FMEP and BSFC. The mechanical efficiency improved with the use of E10 in half of the operating modes; however, in general there was a decreasing trend associated with increasing ethanol substitution. While ethanol improved the thermal efficiency, lower substitutions performed better. At lower loads, thermal efficiency decreased with higher substitutions, while at higher loads, it increased with higher substitutions. Increasing the ethanol substitution increased the maximum in-cylinder pressure. The maximum rate of pressure rise was minimally impacted at low substitutions, although it increased significantly at high substitutions (>20%). At 1500 rpm, increasing the ethanol substitution decreased the CoV of IMEP, especially with E30 and E40. However, at 2000 rpm, using higher substitutions slightly increased the CoV of IMEP (~2%) at higher loads. Under 25% load, increasing the ethanol substitution increased the maximum apparent heat release rate. Under 50 and 75% loads, by increasing the ethanol substitution there was a tendency toward having double peaks in the heat release diagram. Also, increasing the substitution rate increased the peak values. Under full load, the first peak values increased and the second peak diminished as the ethanol rate increased.