Alcohols are gaining interest as an alternate biofuel for compression ignition engines because they contain oxygen and are produced using biomass. Since they have lower cetane number, they are suitable for premixed combustion applications. In this investigation, the authors have tried to improve a single cylinder diesel engine's performance by injecting ethanol into the intake port during the suction stroke. Rubber seed oil (RSO), rubber seed oil methyl ester (RSOME) and diesel are the primary fuels injected directly into the combustion chamber. The injection timing and duration of ethanol injection were optimized for dual fuel operation. The results indicate that increasing ethanol quantity with RSO and RSOME lead to an increase in brake thermal efficiency and reduction in smoke emissions. The maximum brake thermal efficiency achieved at full load is 31%, 29.9% and 29.3% with diesel, RSOME and RSO at ethanol energy shares of 35.2%, 33.5% and 31.6%, respectively. Smoke reduces by 44.26% with RSO, 43.63% with RSOME and 26.47% with diesel at maximum thermal efficiency point. However, HC, CO, and NOx emissions increases with increase in ethanol energy share at all loads. Peak pressure and maximum rate of pressure rise increases with increase in ethanol injection. Combustion duration reduces with ethanol injection, which in turn contributes to a higher heat release rate.