Experimental investigation of the evaporation characteristics of the fuel in a direct-injection spark ignition engine subjected to a direct-start process

Because stopping an engine in the idling state and quickly restarting the engine can improve the engine’s fuel consumption, this paper studied the direct start of a direct-injection spark ignition engine that does not require a larger electrical starter for the conventional start–stop system. The formation of the fuel–air mixture before the first combustion determines the success of the direct start because the explosion energy acquired from first combustion to move the crankshaft is relatively small. The purpose of this study is to explore the evaporation characteristics of the gasoline in a spray chamber that was designed to simulate the cylinder of a direct-injection spark ignition engine. Fundamental studies were conducted to investigate the evaporation process under direct-start conditions. A numerical study was also conducted using evaporation and wall interaction models. The measurements obtained using a fast-response flame ionization detector hydrocarbon detection system (HFR500) indicated that the evaporation ratio of the gasoline spray increased with increasing temperature and achieved an upper limit of 90% at an ambient temperature of 140 °C. The lower limit of the equivalence ratio for a successful direct start should be 1.4–1.7 when the temperature is less than 100 °C. Suitable ignition timing of the first combustion should be 25 ms after the start of injection. Methanol–gasoline and ethanol–gasoline were tested in this study, and a poor evaporation performance was observed owing to the higher evaporation heat values of these fuels. Consequently, gasoline is preferable to oxygenated fuels for the direct start of direct-injection spark ignition engines.