Prenol as bio-fuel additive: Mechanistic insights into knock and soot reduction in n-heptane combustion

3-Methyl-2-butenol (prenol), a promising bio-derived fuel, possesses both a C = C double bond and a hydroxyl (-OH) group. This unique functional group combination confers strong anti-knock properties, effectively integrating the advantages of alkenes and alcohols while mitigating their key drawbacks: the high soot propensity of the former and the high hygroscopicity of the latter. Because anti-knock performance correlates with fundamental ignition behavior, we measured the ignition delay times (IDTs) of n-heptane/air mixtures blended with prenol in a shock tube under end-gas-relevant conditions (700 – 1250 K, 20 bar). Results show that prenol significantly prolongs IDTs below 900 K, demonstrating its pronounced anti-knock efficacy at low temperatures. Kinetic analysis indicates that this effect arises from the inefficient low-temperature chain-branching chemistry of prenol. To bridge fundamental IDT measurements with practical fuel ratings, we developed a statistical model for predicting the Research and Motor Octane Numbers (RON and MON) from computed IDTs. The model was validated against both Toluene Reference Fuels (TRFs) and Primary Reference Fuels (PRFs). RON predictions reveal that prenol exerts a non-monotonic dual effect on n-heptane, transitioning from a pro-knock to an anti-knock agent only at sufficiently high blending ratios. Beyond anti-knock enhancement, reaction flux analysis reveals that prenol suppresses the formation of benzene, a key soot precursor, through a radical competition effect. Specifically, prenol effectively competes with the base fuel for the available ȮH radical pool, thereby reducing the reaction flux toward aromatic rings. This study demonstrates that prenol concurrently mitigates engine knock and reduces soot precursor emissions in spark-ignition engines, providing a quantitative framework for its implementation in advanced fuel formulations.