Optical investigations on ignition and combustion characteristics of polyoxymethylene dimethyl ethers/methanol blends

Abstract

ABSTRACT Internal combustion engines using carbon-neutral fuels are effective ways to achieve low CO2 emissions. Polyoxymethylene dimethyl ethers (PODE) and methanol are carbon-neutral fuels, both good alternative fuels for engine combustion. Dual-fuel combustion modes have been widely studied, but investigations on the ignition and combustion characteristics of PODE/methanol at blending mode are still lacking, especially in optical engines. In this study, the effects of methanol blending ratios (0, 10, 20%), intake temperatures (45, 85, 120°C), and injection timings (−60, −45, −30, −15 °CA aTDC) on the ignition and combustion behavior of PODE/methanol blends were optically studied. Synchronization measurements of in-cylinder pressure and high-speed photography were performed for combustion analysis. The results show that a reduced methanol blending ratio, increased intake temperature, and intermediate injection timing exhibit positive influences on flame initiation and combustion evolutions. Specifically, an optimized methanol blending ratio, intake temperature, and injection timing can shorten the ignition delay time by 11.2, 6.1, and 39.2 crank angles, respectively. Meanwhile, unlike methanol blending ratio and intake temperature, injection timing can significantly affect early-stage rapid combustion duration. Optical images show that multiple flame kernels are initially induced at local rich regions with shorter ignition delay time, while when ignition delay time becomes longer, flame kernels start to be discretely distributed throughout the combustion chamber. These observations demonstrate the significance of the competition between chemical reactivity and fuel stratification in ignition and combustion characteristics of PODE/methanol blends.