The efficiency of spark ignition (SI) engines is usually limited by the occurrence of knock, which is linked to fuel octane number. If running the engine at its optimal efficiency requires a high octane number at high load, a lower octane number can be used at low load.Saudi Aramco, along with its long-term partner IFP Energies nouvelles, has been developing a synergistic fuel engine system where the engine is fed by fuel with an octane number adjusted in real time, on an as needed basis, while running at its optimal efficiency.Two major steps are identified to develop this “Octane on Demand” (OOD) concept: First, characterize the octane requirement needed to run the engine at its optimal efficiency over the entire map.Then, select the best dual fuel combination, including a base fuel and an octane booster to fit this concept.For this purpose, the behavior of different octane boosters, including ethanol, reformate and a blend of butanol isomers (SuperButolTM), are studied on a CFR engine when blended with a very low RON naphtha-based fuel (RON 71). It is shown that the fuel combination [naphtha; ethanol] offers the most promising boosting effect.Dedicated tests on an up-to-date gasoline direct injection multicylinder engine reveal the opportunity to use a naphtha-based fuel of RON 71 over a significant area of the engine map. The advantage of using ethanol as an octane booster is then clearly demonstrated, linked both to its very high RON and its high latent heat of vaporization and favorable sensitivity. Finally, around two-thirds of the engine map can be run using a moderate ethanol rate within the range 0% to 40%, making this OOD concept compatible within the E10-E20 context. Finally, deeper analyses are also made to correlate the engine octane requirement with RON, MON, Octane Index, and heat of vaporization.