Producing and using renewable fuels for transportation is one approach to a sustainable energy future for the world. A renewable fuel contributes to a lesser global climate change. Vegetable oil derivatives, namely, biodiesel, is being used in smaller volumes in some of the present day diesel engines. High smoke emission and lower thermal efficiency are the main problems associated with the use of neat vegetable oils in diesel engines. In the present paper we report on the utilization of rubber seed oil (RSO) as a primary fuel with eiethyl ether (DEE) as a combustion enhancer in a diesel engine. DEE was recently reported as a renewable fuel and a low-emission high-quality combustion improver. A single-cylinder, four-stroke, air-cooled direct injection diesel engine having a rated output of 4.4 kW was used for the experiments. DEE was injected into the intake port during the suction stroke, while RSO was injected directly inside the cylinder at the end of the compression stroke [23 degrees BTDC (before top dead center)]. Parameters such as the injection timing, duration, and quantity of DEE were optimized at different loads. The brake thermal efficiency improves from 26.5% with neat RSO to a maximum of 28.5% with DEE injected at a rate of 200 g/h. The results also indicate a reduction in emissions except the NOx level at all loads. Smoke is reduced significantly from 6.1 to 4 BSU (Bosch smoke units) with DEE injection. The combustion duration decreases with DEE injection, which will contribute to a higher heat release rate in the initial stages of combustion. On the whole it is concluded that DEE can be injected into the inlet port to reduce emissions and improve the thermal efficiency of vegetable-oil-fueled diesel.