An increase in spark-ignition engine efficiency can be gained by increasing the engine compression ratio, which requires fuels with higher knock resistance. Oxygenated fuel components, such as methanol, ethanol, isopropanol, or iso-butanol, all have a Research Octane Number (RON) higher than 100. The octane numbers (ON) of fuels are rated on the CFR F1/F2 engine by comparing the knock intensity of a sample fuel relative to that of bracketing primary reference fuels (PRF). The PRFs are a binary blend of iso-octane, which is defined to an ON of 100, and n-heptane, which represents an ON of 0. Above 100 ON, the PRF scale continues by adding diluted tetraethyl lead (TEL) to iso-octane. However, TEL is banned from use in commercial gasoline because of its toxicity. The ASTM octane number test methods have a “Fit for Use” test that validate the CFR engine’s compliance with the octane testing method by verifying the defined ON of toluene standardization fuels (TSF). The RON test method defines TSFs in the range of 65.1 RON to 113.0 RON with blends of toluene, n-heptane, and iso-octane. Since TSFs do not contain TEL, they could potentially be used as bracketing reference fuels instead of leaded PRFs beyond RON 100. In this work, multiple CFR engines performed “Fit for Use” tests per the RON test method (ASTM D2699) and the TSF ratings closely correlated to their defined RON values without the need of intake air temperature tuning. In the next step, TSFs were used as non-leaded reference fuels to rate the RON of neat methanol, ethanol, iso-propanol, iso-butanol, ethyl acetate, and diisobutylene, all of which have a RON exceeding 100. These same fuels were tested on a separate CFR engine per the official ASTM D2699 RON test method with leaded PRFs. Their TSF-based RON ratings were found to be within the variations of RON values reported in the literature and closely matched with their standard RON rating using leaded reference fuels. Therefore, octane ratings of fuels beyond RON 100 with TSFs as reference fuels proved to be one viable pathway to rate fuels >100 RON without the need for leaded reference fuels.
