Alkyl End Group Effects on the Thermal Decomposition of Oxymethylene Ether Fuel Additives

Cano Botero, Andres Felipe

Graduate Thesis Or Dissertation

Alkyl End Group Effects on the Thermal Decomposition of Oxymethylene Ether Fuel Additives Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/0z708z013

Abstract

Efforts to improve the efficiency of internal combustion engines continue, and with them so do the investigations of fuel additives with favorable properties that reduce harmful emissions. One potential additive is dimethoxymethane (DMM), the simplest oxymethylene ether (OME), its high oxygen to carbon ratio is ideal for reducing engine soot formation. However, major disadvantages like low cetane number, low lower heating value (LHV), and high water solubility may impede DMM from being widely adopted. Although OMEs with more oxymethylene units and higher cetane numbers have been explored as alternatives, drawbacks regarding their low energy density, oxidative stability, and poor sealing material compatibility remain.Recent studies have shown that OMEs with longer alkyl end groups like diethoxymethane (DEM) and dipropoxymethane (DPM) have better diesel compatibility, higher energy densities, and remain environmentally friendly. This work seeks to elucidate the initiation chemistry of DEM and DPM by quantifying the primary reactions that occur during the onset of the combustion. To do this, a chemical kinetic investigation was carried out to study the pyrolysis of DEM, DPM, and the most relevant radicals formed from their thermal decomposition. High-level electronic structure calculations at the (CCSD(T)/cc-pV∞Z//M06-2X/cc-pVTZ) level of theory were leveraged in conjunction with master equation theory to quantify the rates of reaction. The predicted temperature and pressure-dependent rate constants showed that single-step soot precursor-forming reactions in DEM and DPM have low importance at ignition temperatures, instead these soot precursors likely form through radical decomposition reactions. In addition, it was found that H-atom abstraction radicals from DEM favor the production of ethyl formate and acetaldehyde. Although similar compounds were produced by equivalent radicals for DPM, the results suggest that these radicals are also prone to generate abundant amounts of formaldehyde at high temperatures. This is the first comparison of the pyrolysis of different OME end groups and is the first high-level ab initio study of DPM. Lastly, python codes were developed and integrated with existing software to automate the computational process. The improved computational pipeline was leveraged to expedite the application of quantum mechanical methods to compute rates for DEM and DPM.

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