Significance of RON, MON, and LTHR for Knock Limits of Compositionally Dissimilar Gasoline Fuels in a DISI Engine

Spark-ignition (SI) engine efficiency is typically limited by fuel auto-ignition resistance, which is described in practice by the Research Octane Number (RON) and the Motor Octane Number (MON). The goal of this work is to assess whether fuel properties (i.e. RON, MON, and heat of vaporization) are sufficient to describe the antiknock behavior of varying gasoline formulations in modern engines. To this end, the auto-ignition resistance of three compositionally dissimilar gasoline-like fuels with identical RON values and varying or non-varying MON values were evaluated in a modern, prototype, 12: 1 compression ratio, high-swirl (by nature of intake valve deactivation), directly injected spark ignition (DISI) engine at 1400 RPM. The three gasolines are an alkylate blend (RON=98, MON=97), a blend with high aromatic content (RON=98, MON=88), and a blend of 30% ethanol by volume with a gasoline BOB (RON=98, MON=87; see Table 2 for details). The preliminary findings of this work are that RON and MON, when coupled with latent heat of vaporization information, are sufficient to describe the auto-ignition resistance of a fuel to a degree such that knock-limited combustion phasing shows no measurable differences. While the tested fuels yielded no inconsistencies between their ratings (RON and MON) and properties (latent heat of vaporization) and their performance in a DISI engine, measurable differences were found among the three tested fuels. Specifically, the manner in which the fuels obtained knock-resistance varied, be it through thermal tolerance, charge cooling, or lack of charge-heating Low-Temperature Heat Release (LTHR). In addition, the fuels' knock-limited combustion phasing responses to variations in intake pressure and intake temperature varied with their thermal tolerance and tendency towards LTHR. Yet these dissimilar behaviors combine to produce similar anti-knock qualities and engine performance for naturally-aspirated operation.