Low-temperature oxidation of a gasoline surrogate: Experimental investigation in JSR and RCM using high-resolution mass spectrometry

Abstract

The oxidation of a gasoline model-fuel (2500 ppm of n-heptane and 2500 ppm of isooctane), called RON 50, was studied in a jet-stirred reactor (JSR) over the temperature range 560 ‒ 700 K, at a total pressure of 10 atm, at a residence time of 1.5 s, and an equivalence ratio of 0.5. Gas samples were collected as a function temperature. Ignition of RON 50/air mixtures was also studied in a rapid compression machine (RCM) under the same fuel-lean conditions, 20 bar, 640 K. Gas samples were collected at variable reaction time. Products of low-T oxidation formed in a jet-stirred reactor and a rapid compression machine were dissolved in acetonitrile and analyzed by high-resolution mass spectrometry. Flow injection analyses and ultrahigh-pressure liquid chromatography coupled to an OrbitrapⓇ were used to characterize a wide range of species such as hydroperoxides, diols, ketohydroperoxides, carboxylic acids, diketones, cyclic ethers (CnH2nO) formed by decomposition of alkyl hydroperoxy radicals, and highly oxidized species formed via up to six O2 additions on alkyl radicals (CnH2nO11). Mass spectrometry analyses were conducted using atmospheric pressure chemical ionization running in negative and positive ionization modes. For confirming the presence of –OH or –OOH groups in the products, we performed H/D exchange by addition of D2O to samples. Under JSR conditions, we observed a wide range of n-heptane oxidation products: C7H14Ox (x = 1‒11), C7H12Ox (x = 1‒11), C7H10Ox (x = 1‒9), C7H8Ox (x = 1‒9), C7H6Ox (x = 1‒8), and C7H4Ox (x = 1‒6). Similarly, the following products of isooctane oxidation were observed: C8H16Ox (x = 1‒12), C8H14Ox (x = 1‒11), C8H12Ox (x = 1‒12), C8H10Ox (x = 2‒10), C8H8Ox (x = 2‒8), C8H6Ox (x = 1‒7). Finally, CnH2n (n = 4‒8), CnH2n-2 (n = 4‒8), CnH2nO (n = 3‒8), CnH2n-2O (n = 3‒8), CnH2n-4O (n = 3‒8), CnH2n+2O2 (n = 3‒8), CnH2nO2 (n = 2‒8), CnH2n-2O2 (n = 3‒8), CnH2n-4O2 (n = 3‒8), and CnH2nO3 (n = 2‒8) were also observed. Most of these products were also detected in RCM samples. Products measurements indicated that RON 50 oxidation routes are similar under RCM and JSR experimental conditions. A kinetic reaction mechanism was used to compare the formation of products versus temperature in a JSR. New pathways need to be introduced in existing reaction schemes for predicting newly detected cool-flame products.