An experimental and numerical study of polyoxymethylene dimethyl ethers on a homogeneous charge compression ignition engine

Polyoxymethylene Dimethyl Ethers (PODEn) are a kind of high-activity and promising soot-reduction additive fuel. The effects of enhanced intake charge temperature (Tin) and low fuel-air equivalence ratio (Phi) on the combustion and emission characteristics of PODE2-8 were captured on a homogeneous charge compression ignition (HCCI) engine. Meanwhile, a numerical study using Chemkin was conducted to investigate the effects of Tin on the combustion characteristics of PODE3. The results indicate that with the increase of Tin, rate of production (ROP) of CH2O and OH radicals and rate of exothermic reactions related to H- and O- species are greatly promoted, which result in a highly reactive atmosphere, advanced ignition timing (IT) and increased heat release rate (HRR). Typical two-stage auto-ignition behavior is also observed in lean PODE2-8 HCCI at high Tin. The enhanced Tin can promote the rate of CO-to-CO2 and decrease combustion duration (CD), which eliminates the third HRR oxidation peak. At Phi = 0.143, the peak in-cylinder pressure has increased by about 40% and the combustion duration (CD) has decreased by nearly 25% when Tin is from 80 degrees C to 140 degrees C. Moreover, for all test cases, NOx, HC, and CO emissions are lower than 15 ppm, 100 ppm, and 1%, respectively. Phi has a slight effect on the ignition timing of low temperature (ITL) but an obvious effect on the ignition timing of high temperature (ITH). With an increase of Phi, CO emissions decrease, HC emissions decrease first and then increase, and NOx emissions slightly increase. This research reveals that the combustion and emission characteristics of PODEn are sensitive to Tin, PODEn with enhanced Tin can form a chemical atmosphere with high-activity radicals.