Theoretical and experimental investigation of physicochemical properties and combustion performance of PODE/methanol blends

Abstract

In this study, oxygenated fuels methanol and polyoxymethylene dimethyl ethers (PODE) were selected as alternative energy for compression ignition engines. The physicochemical properties of PODE and methanol were calculated by group contribution method and corresponding state principle, and the contact angle test was used to analyze the mutual solubility of PODE/methanol blends. Moreover, the oxidation characteristics of PODE/methanol blends were studied by thermogravimetric analysis, and then in-cylinder combustion and emission characteristics of the blends with various methanol ratios were investigated on a common-rail diesel engine. Results showed that with the increase of methanol ratio, the volatility and evaporation of PODE/methanol blends increased obviously. The surface tension of PODE increased with the increase of polymerization degree, and was greater than that of methanol, while the viscosity and density of PODE decreased with the increase of temperature. There was an obvious contact angle between the methanol and diesel surfaces at stable condition, which was about 31.5°, and methanol was easily soluble in PODE and insoluble in diesel fuel. With the increase of methanol ratio, the peak values of combustion pressure and heat release rate (HRR) decreased at low load, and both the ignition delay and combustion duration were prolonged. However, at high load, the peak values of combustion pressure and HRR increased with the increase of methanol ratio, and the combustion duration was shortened. With the addition of methanol, both the NOx and soot emissions of the blends decreased, while CO and HC emissions increased, especially at low load. Compared with pure PODE (P100M0), the NOx emissions of the blends with 30 % methanol ratio (P70M30) were reduced by 20.7 % and 16.1 % respectively at 25 % and 75 % loads, and the soot emissions were reduced by 90.3 % and 43.7 % respectively. Thus, the addition of methanol in PODE could alleviate the trade-off relationship between NOx and soot emissions to a certain extent.