In spark ignition engines fuelled by alcohol gasoline blends, the proportion of the two fuels has to be varied according to the operating condition. Further high amounts of alcohol cannot be blended with gasoline because the two phases can separate under certain conditions. In this work a dual injection system, wherein n-butanol and gasoline can be injected separately in any ratio has been employed in a spark ignition engine. The objective is to determine the most suitable amounts of n-butanol and gasoline to be used at different operating conditions of a four stroke spark ignition engine when these fuels are simultaneously injected into the intake manifold using this dual injection system. Experiments are conducted at different fuel ratios and throttle positions at an equivalence ratio of 1. The system results in good vaporization of the fuels even at low load conditions because the fuel jets are aligned to hit different portions of the intake valve. Results indicate that with proper selection of the fuel ratio significant reduction in HC emissions can be achieved as compared to operation on neat gasoline. Up to 60% of n-butanol could be used at 15% throttle while up to 80% could be used at 25% throttle. These proportions are higher than what have been achieved by pre-blending these fuels. The possibility of using high amounts of n-butanol reduced the tendency to knock. Hence, with the dual injection system n-butanol can be effectively used along with gasoline. © 2013 Elsevier Ltd. All rights reserved.