Effect of direct injection dimethyl ether on the micro-flame ignited (MFI) hybrid combustion and emission characteristics of a 4-stroke gasoline engine

- Dimethyl ether can help to achieve micro-flame ignited (MFI) combustion in gasoline.
- MFI combustion in gasoline engine is a three-stage heat release.
- The approaches to inject dimethyl ether change the MFI combustion processes.
- Split injection of dimethyl ether is favorable for thermal efficiency improvement.
- Split injection of dimethyl ether can decrease CO and hydrocarbon emissions.

The low temperature combustion is required for an internal combustion gasoline engine to achieve higher thermal efficiency and lower NOx emissions. The gasoline controlled auto-ignition (CAI) has been extensively researched to achieve low temperature combustion but its lack of control in combustion process has limited its applications. To overcome this problem, dimethyl ether (DME) is directly injected into the cylinder to generate multiple micro-flame ignition sites and regulate the entire heat release of premixed gasoline mixture. This micro-flame ignited (MFI) hybrid combustion was achieved in a 4-stroke gasoline engine in this work. The results show that the heat release process consists of three stages: stage I of low temperature oxidation reactions, followed with a short delay of a few crank angles by stage II of high temperature reactions of DME and the main combustion of premixed gasoline mixture in stage III. With the increased amount of DME in the late injection, the start of main combustion advances and the combustion duration reduces for both single and split injections, which results in lower HC and CO emissions. The introduction of split DME injections improves the combustion process and reduces HC and CO emissions as well as the maximum thermal efficiency.