Systems and methods are provided for performing on-board separation of a fuel into a higher octane fuel fraction and a lower octane fuel fraction using a membrane under osmosis conditions. By performing the separation under osmosis conditions, the feed for separation can be exposed to the membrane without requiring prior heating. This can avoid the need for having a separate heat exchanger system for heating the feed to the membrane to a desired temperature range. Additionally or alternately, the permeate from the membrane separation can be at a pressure of roughly 100 kPa-a or higher. This can avoid the need for having an eductor to provide a pressure below 100 kPa-a for the permeate side of the membrane. The fuel fractions produced during the membrane separation can then be used as fuel in a dual-fuel spark ignition engine.