Chemical kinetic study on ignition and flame characteristic of polyoxymethylene dimethyl ether 3 (PODE3)

Abstract

Polyoxymethylene dimethyl ether 3 (PODE3) is a promising diesel additive with high cetane number (78) and oxygen mass content (48%), but its ignition and flame characteristics are rarely reported. This work provides a new fundamental understanding of the ignition delay times, adiabatic flame temperature and laminar flame speed of PODE3 by detail chemical kinetic mechanism. The ignition delay time of PODE3 is lower than n-heptane (reference fuel) due to a higher global reaction rate. The ignition delay profile of PODE3 does not exhibit negative temperature coefficient (NTC) behavior and thus sensitive to temperature variation which benefits low-temperature combustion in compression ignition engines. This study discovers that the high cetane number of PODE3 guarantees a high global reaction rate, but it lacks sufficient temperature rise and active radical accumulation to act as the chemical ignition source. The oxidation reaction pathways between PODE3 and n-heptane are similar, both compose of (i) 1st O2 addition, RO2 isomerization to produce QOOH and (ii) 2nd O2 addition, (iii) ketohydroperoxide decomposition. PODE3 produces a large amount of formaldehyde due to lack of carbon-carbon bond while the n-heptane takes place β-scission to produce olefins. PODE3 has higher premixed laminar flame speed than n-heptane due to a higher global reaction rate and flame temperature. PODE3 benefits the flame sustaining at engine low load because its laminar flame speed suffers less temperature and pressure dependence. A machine learning regression model is proposed to reproduce the non-linear relationship between laminar flame speed and equivalence ratio, temperature and pressure.