Auto-Ignition of DME/DMM Fuel Blends. Part I: Minimizing Temperature Dependency by Blend Optimization

Abstract

Combustion concepts based on auto-ignition are prone to detonation formation and premature ignition in the presence of local spots with increased reactivity, which can be caused by unavoidable temperature inhomogeneities in real technical applications. In this study, a fuel blend optimization approach using a fuel component with negative temperature coefficient (NTC) behavior (dimethyl ether, DME) and a fuel component without NTC behavior (dimethoxymethane, DMM) is employed to reduce the temperature sensitivity of ignition delay times and therefore reduce the tendency of premature ignition and detonation development. First, ignition delay times and first stage ignition of a DMM/air mixture are measured behind reflected shock waves in a high-pressure shock tube (20 and 35 bar, stoichiometric conditions) to select a suited chemical kinetic model. Using these data and additional literature data, a mechanism is chosen, which is used to numerically optimize the blending ratio of DME and DMM to minimize the temperature sensitivity of ignition delay times in the temperature range between 800 and 900 K for both considered pressures. The two optimized fuel blends are investigated in the shock tube regarding their ignition characteristics as before (20 and 35 bar, stoichiometric conditions). Finally, the impact of pressure, equivalence ratio, and temperature range on the optimization result is investigated numerically.

Autoignition of DME/DMM Fuel Blends: Part II: Reaction Wave Propagation Modes in the Presence of Temperature Gradients

“…The overall reactivity and radical production are strongly dependent on the chemical kinetic pathways of each fuel component and their interaction. Reaction path analyses and sensitivity analyses of ignition delay times from Burke et al, Curran et al, and Jacobs et al in combination with the sensitivity analysis of ignition delay times conducted in Part I of this study can explain the described phenomena. Figure shows sensitivity analyses of ignition delay times at 800 and 900 K for various fuel blend ratios, which are presented in the Supporting Material of Part I of this study …”

“…Figure 6 shows sensitivity analyses of ignition delay times at 800 and 900 K for various fuel blend ratios, which are presented in the Supporting Material of Part I of this study. 26 At low temperatures after the first H-atom abstraction from fuel molecules by hydroxyl radicals (reaction R22 in Figure 6), the methoxymethyl radical (CH 3 OCH 2 ) adds to molecular oxygen (R18) and continues the alkane-like low-temperature oxidation undergoing isomerization (R16) and second oxygen addition (R24) and second isomerization, eventually leading to chain branching. Therefore, these reactions promote ignition and exhibit negative sensitivity coefficient in Figure 6 for both investigated temperatures.…”

“…To understand those phenomena, kinetic analyses were conducted within Part I of this study . This analysis serves as a basis for further exploration.…”

“…In Part I 26 of this study, it could be shown that a fuel blend of dimethyl ether (DME) and dimethoxymethane (DMM) significantly reduces the temperature sensitivity of ignition delay times in a certain temperature range. The second part of this study (Part II) aims to classify this effect and focuses on two main questions: which effect has fuel blending with DME and DMM on the nondimensional parameters ξ and ε?…”

“…As discussed in Part I, 26 the thermal conditions are of major importance to those questions. In Part I, two pressures 20 and 35 bar and an important temperature range between 800 and 900 K are relevant for the practical application of SEC and HCCI concepts.…”