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Chemical interaction of dual-fuel mixtures in low-temperature oxidation,
comparing n-pentane/dimethyl ether and n-pentane/ethanol
Hanfeng Jin*, Eike Bräuer, Julia Pieper, Lena Ruwe, Christian Hemken, Luc Sy Tran,
Katharina Kohse-Höinghaus
Department of Chemistry, Bielefeld University, Germany
* Corresponding author. E-mail address: [email protected]
Research on the combustion of individual fuel components has been performed extensively
to enhance the basic knowledge on low-temperature combustion (LTC) chemistry.
Comparatively less information is available regarding potential interactions between different
fuel components in the LTC regime, in spite of its importance for practical applications in
engines. In this work, the low-temperature oxidation of two prototypical dual-fuel mixtures
(ϕ = 0.7), i.e. of n-pentane (C5H12)/dimethyl ether (DME) and n-pentane/ethanol (EtOH), was
investigated in a flow reactor at 970 mbar and in the temperature range of 450-930 K. The
behavior of the mixtures was compared to that of the individual components. Experimental
measurements of the mixtures were performed with two different C2H6O (DME or EtOH)
doping ratios, namely 75% C5H12/25% C2H6O and 50% C5H12/50% C2H6O. Mole fractions of a
significant number of oxidation species (CnH2(n+x)Oy, n = 1 to 5, x = -1 to 1, y = 0 to 3) of
n-pentane, dimethyl ether and ethanol were determined by electron ionization
molecular-beam mass spectrometry. As a less active fuel additive, ethanol decreases the
mixture's reactivity compared to n-pentane, while it is promoted by dimethyl ether. An
interesting observation in this experiment is the significant synergistic interaction between
n-pentane and dimethyl ether. The dual mixtures show a stronger negative temperature
coefficient (NTC) effect than each of the individual components, and it was attempted to
investigate the reasons for this behavior. In short, formic acid, formaldehyde, acetaldehyde,
and hydroperoxymethane are typical intermediates that reflect the interaction between
n-pentane and dimethyl ether. Hydroperoxymethane formation is improved due to the
promotion of n-pentane reactions by dimethyl ether. It in turn enhances its contribution on
the OH radical production, thus increasing the maximum conversion of n-pentane and
dimethyl ether. Ethanol is activated by the addition of n-pentane. It is an OH consumer,
undergoes one O2-addition step, and then produces acetaldehyde or formaldehyde. Recently
published models were adopted to reproduce these experimental observations in this work.
However, significant deviations are observed in both fuel consumption and intermediate
production. The inhibition effect of ethanol is not predicted fully, whilst the synergistic effect
of dimethyl ether is poorly captured. From an analysis of the current findings, the
temperature region can be identified in which deeper knowledge of the reaction chemistry
would be useful. Such extended analysis can rely on the experimental data in this work and
potentially contribute to the further improvement of the fuel-mixture models.