Abstract Ethanol is becoming more popular as a fuel component for spark- ignited engines. Ethanol can be used either as an octane enhancer of low RON gasoline or splash-blended with gasoline if a single injector is used for fuel injection. If two separate injectors are used, it is possible to inject gasoline and ethanol separately and the addition of ethanol can be varied on demand. In this study, the effect of the ethanol injection strategy on knock suppression was observed using a single cylinder engine equipped with two port fuel injectors dedicated to each side of the intake port and one direct injector. If the fuel is injected to only one side of the intake port, it is possible to form a stratified charge. The experiment was conducted under a compression ratio of 12.2 for various injection strategies. From the experimental results, it was found that injecting ethanol to the left side only of the intake port while both intake valves were open required approximately 52.1% (at 1500rpm) or 60.6% (at 2000rpm) less ethanol compared to the case in which ethanol is injected to both sides of intake port while intake valves are closed under a similar level of knock frequency. Furthermore, the engine load was maintained to the same level. Introduction Ethanol is becoming more widely used for spark-ignited engines and has the potential to replace gasoline as a fuel due to energy security issues. Ethanol fuel use in the U.S. increased dramatically from approximately 1.7 billion gallons in 2001 to approximately 13.2 billion in 2013 [ 1]. There are many advantages to using ethanol as a fuel in spark-ignited engines. Ethanol has an intrinsically high octane number, a higher latent heat of vaporization and faster laminar flame speed, which can suppress knock behavior during combustion [ 2, 3, 4]. Additionally, the presence of oxygen molecules in ethanol enhances the oxidation rate of fuel, and thus CO and THC emissions can be decreased [ 5, 6]. However, due to ethanol having a higher latent heat of vaporization and boiling point it is unfavorable for the cold start condition in a spark-ignited engine [ 7, 8, 9, 10]. Furthermore, ethanol has a lower energy density, so when it is used in real cars, the cruising distance becomes shorter due to restricted fuel tank size [ 11, 12]. Therefore, ethanol is widely used currently in blended form with gasoline rather than its pure form. However, fueling ethanol in a gasoline-blended form has a limit of impossibility for real-time variation of the ethanol-gasoline ratio according to various engine operating conditions. Thus, to educe the potential merits and overcome the demerits of ethanol as a fuel, on-demand blending ratio change of gasoline and ethanol is needed. The concept was first introduced by Cohn et el. [ 13], In this study, demonstrating the direct injection of ethanol in a conventional PFI type engine, the authors proposed the addition of ethanol during knock occurrences during engine operation. Stein et al. [ 11], using a 3.5 L turbocharged engine, observed that an E85 DI and a gasoline PFI system facilitate the maintenance of spark timing at MBT until the engine load is raised up to 18 bar of BMEP. Zhuang et al. [ 14] investigated the effect of ethanol and gasoline on engine load, BSFC, volumetric efficiency and emission characteristics with an ethanol direct injection (EDI) and gasoline port injection (GPI) system. As the amount of ethanol was increased, the charge cooling effect led to an increase in volumetric efficiency. The authors also studied the effects of injection pressure and timing [ 15], and the results showed that further knock mitigation effect is observed when ethanol is injected after the intake valves are closed. Kim et al. [ 16], used a dual fuel system of ethanol port injection (EPI) and gasoline direct injection (GDI) on a single cylinder gasoline engine to exploit the charge cooling effect of gasoline, investigated the knock