Effect of Dibutyl Ether - Methanol Blend Ratios on Deflagration-Based and Autoignition-Based Knock in Spark-Ignition Engines

Abstract

[EN] Methanol is an attractive fuel that can be produced using renewable sources. However, its low reactivity may present challenges to combustion modes (like Spark-Assisted Compression Ignition) that rely on autoignition. To tweak its reactivity, a highly reactive bio-derived molecule, dibutyl ether (DBE), was blended in varying volume percentages (from 0 to 40% v/v). Experiments were conducted for a) lean operation with air-fuel ratio lambda = 2, b) stoichiometric steady-state operation, and c) stoichiometric transient-state operation. For lean operation, adding increasing fractions of DBE increased the reactivity, as evidenced by a retarded knock-limited combustion phasing. For stoichiometric steady operation, the addition of DBE had very little impact on the knock limits at lower blends up to 20% v/v. A unique knock phenomenon, hereby termed as deflagration-based knock, was observed for these blends, wherein pressure oscillations were observed as the flame-front progressed, without autoignition. The knock mode shifted to conventional end-gas autoignitionbased knock for 30% and 40% DBE blends. For stoichiometric transient conditions, marked by cooler operation, all DBE blends showed only deflagration-based knock. Spectral analysis of the deflagration-based knock suggests higher contribution from the 10-15 kHz range, corresponding to second-mode harmonics. Computational studies conducted to match the stoichiometric operating condition showed similar pressure oscillations. Analysis of the simulated results imply that deflagrationbased knock occurs when the flame reacts to the pressure disturbances such that a feedback loop increases the pressure oscillations in each flame-front passage. The findings of this study highlight the physics behind deflagration-based knock, as observed in DBE-methanol blends, and are critical to other renewable fuels such as ethanol and hydrogen. Novelty and Significance: The manuscript uses fully renewable blends of dibutyl ether and methanol for sparkassisted compression ignition with lean fuel-air mixture, and for conventional stoichiometric spark-ignition combustion. For stoichiometric operation, the authors observe a unique phenomenon where pressure oscillations start right after the spark timing, and increases as flame-front moves forward, hereby termed as deflagration-based-knock. This novel phenomenon is explained on the basis of pressure-dependent reactivity, which provides a feedback mechanism to the pressure oscillations. According to the authors, the observations are significant to fast-burning fuels (or more generally, fast combustion), and observations have been recorded elsewhere for ethanol (by authors) and hydrogen (elsewhere). To the best of our knowledge, this is the first investigation of deflagration-based-knock in small-bore engines, or in conventional spark-ignition combustion. Authors suggest that depending on the operating conditions, deflagration-based-knock may form a bottleneck to engines operating on such renewable fuels. Moreover, conventional knock mitigation mechanism, like retarding spark timing, may not be effective against the novel deflagration-based-knock.