Low-Temperature Oxidation of Di‑n‑Butyl Ether in a Motored Homogeneous Charge Compression Ignition (HCCI) Engine: Comparison of Characteristic Products with RCM and JSR Speciation by Orbitrap

Previously, the oxidation of di-n-butyl ether (DBE) carried out in a jet-stirred reactor (JSR) and in a rapid compression machine (RCM) revealed that it proceeds similarly under both conditions (Belhadj et al, Combust. Flame 2020, 222, 133–144). Here, we extend that study to DBE oxidation in a motored homogeneous charge compression ignition engine, conditions under which this fuel has never been studied. Samples of exhaust gas were obtained by bubbling in acetonitrile maintained at 0 °C. The samples were analyzed using atmospheric pressure chemical ionization in positive and negative modes, high-resolution mass spectrometry (Orbitrap), and ultrahigh-pressure liquid chromatography. Flow injection analyses of samples before and after H/D exchange using D₂O were also performed to verify the presence of isomeric products containing OH or OOH groups. Carbonyls were identified through derivatization with 2,4-dinitrophenylhydrazine. A large set of chemical products of the DBE cool flame were detected in the engine exhausts. They include hydroperoxides and diols (C₈H₁₈O₃), unsaturated diols or unsaturated hydroperoxides (C₈H₁₆O₃), keto hydroperoxides (C₄H₈O₃ and C₈H₁₆O₄), diketo ethers (C₈H₁₄O₃), olefinic diketo ethers (C₈H₁₂O₃), cyclic and keto ethers (C₈H₁₆O₂), and olefinic cyclic and keto ethers (C₈H₁₄O₂). Also, highly oxygenated chemicals, i.e., keto dihydroperoxides (C₈H₁₆O₆) resulting from three O₂ additions on radicals from the fuel, diketo hydroperoxides (C₈H₁₄O₅) resulting from decomposition of keto dihydroperoxides (C₈H₁₆O₆), in addition to other oxygenated intermediates i.e., hydroxy-DBE (C₈H₁₈O₂) and organic peroxides ROOR′ (C₁₆H₃₄O₄, C₁₁H₂₄O₃, C₁₁H₂₂O₃, and C₁₀H₂₂O₃), were observed in the engine exhausts. The present speciation results of the engine exhausts were compared to those obtained for samples of the oxidation of DBE in an RCM and a JSR. Despite the significant differences in physical experimental conditions, the present study indicates thata common oxidation mechanism proceeds in JSR, RCM, and a motored engine, leading to the formation of products having the same chemical formulas and retention times.