Startability and Warm-Up Performance of a Spark Ignition Engine Fueled with Ethanol–Gasoline Blends: An Experimental Study

Decarbonizing road transport requires fuels with low life-cycle carbon footprints and compatibility with existing Spark Ignition (SI) engines. Bioethanol is a promising candidate, but the cold-start and warm-up phases of ethanol-fueled engines, especially at high ethanol fractions, remain underexplored. This study analyzes the warm-up and startability performance of a naturally aspirated, four-cylinder, port-fuel-injected SI engine fueled with four gasoline-ethanol blends (E5, E30, E60, and E100). Warm-up tests were performed at idle for all fuels and at 20 Nm brake torque and 2000 rpm for E5, E30, and E60, while dedicated startability tests were conducted on E60 and E100 from progressively higher initial engine-wall temperatures. The results show that warm-up duration was only weakly affected by ethanol content, remaining approximately 1650 s at idle and 560–580 s under partial load, while structural temperatures showed only minor variations across the tested blends. Startability is highly sensitive to ethanol content and initial wall temperature. E5 and E30 achieve stable operation on the first cranking attempt at an initial wall temperature of 15 °C, whereas E60 and E100 exhibit progressively greater starting difficulties. Reliable startability thresholds were estimated at approximately 30–35 °C for E60 and 45–50 °C for E100. These results highlight the dominant role of engine thermal state in the startability behavior of high-ethanol fuels.