Dimethyl ether (DME) is of interest as an alternative fuel for compression ignition (CI) engines due to its ease of production and advantageous properties as a diesel alternative. As an example of that interest, Volvo Trucks has just announced its intent to begin production of DME-fueled heavy-duty trucks in North America in 2015. (Earlier post.)
DME offers a high cetane number of 55 and low auto-ignition temperature (making it desirable for diesel engine operation), and has been shown to reduce significantly diesel engine particulate emissions as well as NOx, SOx, and CO emissions. However, the characteristics of DME autoignition at CI engine-relevant conditions have not been widely explored at this point. Partly addressing that lack is a new study by researchers at Shanghai Jiao Tong University and Rensselaer Polytechnic Institute; their paper is published in the ACS journal Energy & Fuels.
In addition to studies aimed at ascertaining the performance of compression−ignition internal combustion engines fuel with DME, there have been a number of studies focused on experimental characterization of fundamental combustion properties for DME. DME laminar flame speed measurements have been reported by a number of authors. Premixed and non-premixed flame extinction has been investigated by Wang et al., and diffusive flame ignition has been investigated by Zheng et al. Several groups have reported speciation measurements made during both DME oxidation and pyrolysis in jet-stirred reactors (JSRs), flow reactors, and premixed flames.
DME autoignition is of particular interest, owing to the potential for DME use in compression−ignition engines; hence, previous ignition delay measurements...have been carried out in shock tubes and rapid compression machines (RCMs)...most have considered dilute reactant mixtures at either high-temperature shock tube conditions (Dagaut et al., Cook et al., and Tang et al.) or at low-temperature RCM conditions (Mittal et al.).
Only Pfahl et al. have previously investigated DME autoignition at conditions consistent with those found in compression−ignition engines: DME/air mixtures at high pressures and a range of temperatures spanning the low-temperature, negative-temperature-coefficient (NTC), and high-temperature reactivity regimes. Here, we report DME autoignition measurements at engine-relevant conditions that substantially extend the conditions investigated in the Pfahl et al. study.—Li et al.
The researchers measured ignition delay times in reflected shock experiments at temperatures from 697 to 1239 K (424 to 966 °C) and at a nominal pressure of 22−23 bar for DME/air/N2 mixtures at equivalence ratios of 0.5, 1.0, and 1.5 and with 0−40% N2 dilution to simulate exhaust gas recirculation (EGR).
The observed that DME ignition delay times display three regimes of reactivity (high-temperature, negative-temperature-coefficient (NTC), and low-temperature) characteristic of paraffinic hydrocarbons. Delay times decreased with increasing equivalence ratio and increased with increasing dilution at the conditions studied.
At high-temperatures (∼1200 K), H abstraction from DME and reactions involving methyl radicals and formaldehyde—two important intermediates in DME oxidation—were sensitive in controlling DME autoignition.
Examination of DME oxidation kinetics at NTC to low-temperature conditions illustrated the importance of competition between β-scission of and molecular oxygen addition to the methoxymethyl (CH3OCH2) radical, formed by H abstraction from DME, and competition between β-scission of and oxygen addition to the hydroperoxy-methoxymethyl (CH2OCH2O2H) radical, formed by isomerization of methoxymethyl-peroxy radical (CH3OCH2O2).
Zhenhua Li, Weijing Wang, Zhen Huang, and Matthew A. Oehlschlaeger (2013) Dimethyl Ether Autoignition at Engine-Relevant Conditions. Energy & Fuels 27 (5), 2811-2817 doi: http://pubs.acs.org/doi/abs/10.1021/ef400293z
Pfahl, U.; Fieweger, K.; Adomeit, G. Proc. Combust. Inst. 1996, 26, 781−789.