Understanding octane number evolution for enabling alternative low RON refinery streams and octane boosters as transportation fuels

Most of the time, spark ignition (SI) engine performance is limited by knock phenomena (especially for turbocharged engines), which are linked to fuel resistance to auto-ignition, quantified by its octane number (Research Octane Number - RON and Motor Octane Number - MON). If high octane numbers are crucial for efficient high load operating points, they are less necessary at low load. Thus, if the octane number of the fuel could be tuned as any other engine setting parameter, the engine efficiency and CO2 emissions could be improved, leading to an Octane on Demand concept, using for instance a dual fuel strategy. This requires understanding the behavior of dual fuel combustions with lower/higher octane fuels, and more particularly the evolution of RON when blending high RON fuels with low RON ones. Developing an Octane on Demand concept requires to choose appropriate octane enhancers and understand their blending behavior. For this purpose, RON measurements were performed on a CFR engine using a wide range of mixtures of low-octane base fuels with various boosters capable of increasing the antiknock resistance of the blends. The chemical composition of booster streams was chosen to assess the potential of using alternative refinery products for improving fuel resistant auto-ignition properties when added to a whole-range naphtha and RON 91 gasoline. The study covers five octane boosters: ethanol, reformate, di-isobutylene, 2-butanol, and a mixture of butanols. The experimental results show a non-linear behavior of RON values with respect to volumetric incorporation rates of octane boosters. In the cases when the booster is an alcohol (C2 or C4), linear by-mole blending rules can be applied with an acceptable prediction error. For boosters rich in olefins and aromatics, molar blending becomes less accurate. Ethanol shows the strongest boosting effect among all the octane boosters on the one hand, and on the other hand, the octane enhancing effect is stronger for the base fuel of lower starting RON value. Experimental results of the current study represent a comprehensive database for tailoring fuel RON properties aimed to explore combustion behavior of low-octane fuels enhanced through an addition of an external booster. (C) 2015 Elsevier Ltd. All rights reserved.