High-efficiency and low-emission combustion in methanol-fueled linear range extender: experimental investigation of injection and ignition timing effects

As a linear range extender, the Free-Piston Engine Generator (FPEG) holds significant promise for promoting low-carbon development in internal combustion engines. However, its operation still faces critical challenges, including high fuel short-circuit losses and difficulties in emission control. Given the strong coupling between in-cylinder combustion and piston motion, fuel injection timing (FIT) and ignition timing (IT) have become key control parameters for achieving efficient and low-emission operation.To address these issues, this study systematically investigates the effects of FIT–IT coordination on combustion and emission characteristics. It is the first to experimentally demonstrate the feasibility of stable ultra-lean combustion in a direct-injection FPEG using a clean, high-reactivity methanol fuel.Results show that optimizing FIT and IT enables stable operation and efficient combustion under high compression ratio conditions. Retarding IT from 20 mm to 24 mm increases the indicated mean effective pressure (IMEP) by 16.3 %, with a higher peak heat release rate and shorter combustion duration. Under FIT = −6 mm, the peak heat release rate is maximized, and compared with FIT = −12 mm, IMEP and combustion stability (COV_IMEP) are improved by 5.2 % and 18.9 %, respectively. Under optimal conditions, NO and HC emissions are reduced to 120 ppm and 890 ppm.