Abstract
The polyoxymethylene dimethyl ethers (PODE)/methanol reactivity controlled compression ignition (RCCI) mode was realized by direct injection of PODE as high-reactivity fuel to ignite pre-mixed methanol as low-reactivity fuel in a high-pressure common-rail diesel engine. The effects of load extension ratio (LER) on in-cylinder combustion, emission characteristics and fuel economy of RCCI engine were investigated, and the influences of PODE injection strategy on the high-load extension of PODE/methanol RCCI mode were also systematically studied, including the start of main-injection (SOImain), start of pilot-injection (SOIpilot) and pilot-injection quantity (PIQ). The results show that with the increase of LER, the peak values of in-cylinder pressure and heat release rate of RCCI mode increase, the brake thermal efficiency (BTE) and the hydrocarbons (HC), carbon monoxide (CO) and unregulated emissions also increase, but the increase of the ringing intensity (RI), the coefficient of variation of IMEP (COVIMEP) and nitrogen oxides (NOx) limit the further increase of LER. The delay of SOImain can effectively reduce RI, but it will lead to the deterioration of CO and HC emissions and fuel economy. The pilot-injection strategy can weaken the trade-off relationship between NOx and soot emissions and improve combustion stability. With the advance of SOIpilot, the RCCI combustion process evolves from two-stage heat release to single-stage heat release, and the emissions of HC, CO, NOx and soot are reduced. With the increase of PIQ, the combustion phase of PODE/methanol RCCI mode advances, and the RI, COVIMEP and IMEP increase. At 59.5 mg/cycle condition, when PIQ is increased from 2 mg to 10 mg, CO and HC emissions decrease by 19.57% and 6.97% respectively, while NOx emissions increase by 7.93%. With the appropriate LER combined with the optimized PODE injection strategy, the BTE of PODE/methanol RCCI engine reaches 48.68%, and the IMEP is extended from 0.77 MPa to 1.08 MPa.
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