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 degrees 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 O-2 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.

Automated summary

This study extends the previous research on the oxidation of di-n-butyl ether (DBE) to include its oxidation in a motored homogeneous charge compression ignition engine. The study reveals that despite the different experimental conditions, the oxidation mechanism of DBE remains similar and leads to the formation of the same chemical products.