Abstract:

To improve combustion stability and shorten combustion duration in highly dilution conditions, the auto-ignition of a small amount of direct injection dimethyl ether (DME) only through compression formed multi-point micro-flame, aiming at regulating the combustion processes of highly diluted gasoline mixture formed by port fuel injection, which is known as micro-flame ignited (MFI) hybrid combustion. The effects of hot exhaust gases and air dilution on the combustion and emissions characteristics of MFI combustion were experimentally investigated at part loads in a single-cylinder four-stroke thermodynamic engine, and increasing intake pressure increased the excess air coefficient (λ) and lowered internal exhaust gases rate. The results show that ignition timing is slightly affected by λ, while combustion duration elongates at higher λ in the case of low amount of fuel. In the case of higher amount of fuel, ignition timing advances and combustion duration gradually increases at higher λ. Net indicated mean effective pressure (p_IMEPn) and indicated thermal efficiency increase firstly and then reduce with increased λ, and the indicated thermal efficiency can be reached at 35.4% at 0.32 MPa p_IMEPn and 42.3% at 0.56 MPa p_IMEPn, respectively. With λ increasing, unburned hydrocarbon emissions increase in the cose of low amount of fuel, while reduce in the cose of higher amount of fuel. In the meantime, CO emissions increase, and NO_x emissions which are lower than 15×10^-6 reduce.

Key words: gasoline engine, emissions characteristics, hybrid combustion, exhaust gases dilution, dimethyl ether(DME), air dilution