Operating Characteristics of DME-Gasoline Dual-fuel in a Compression Ignition Engine at the Low Load Condition

Combustion and emission characteristics were investigated in a compression ignition engine with dual-fuel strategy using dimethyl ether (DME) and gasoline. Experiments were performed at the low load condition corresponding to indicated mean effective pressure of 0.45 MPa. DME was directly injected into the cylinder and gasoline was injected into the intake manifold during the intake stroke. The proportion of DME in the total input energy was adjusted from 10% to 100%. DME DME injection timing was widely varied to investigate the effect of injection timing on the combustion phase. Injection pressure of DME was varied from 20 MPa to 60 MPa. Exhaust gas recirculation (EGR) was controlled from 0% to 60% to explore the effect of EGR on the combustion and emission characteristics.

As DME proportion was decreased with the increased portion of gasoline, the combustion efficiency was decreased but thermal efficiency was increased. Injection timing was controlled to ensure maximum thermal efficiency for all experimental conditions. Unburned hydrocarbon resulted from over-lean mixture and crevice region was increased. Combustion was dominated by the premixed combustion when the gasoline portion increased so the combustion duration was shortened. It affected the quasi-constant volume combustion and the reduced heat transfer loss. Nitric oxides (NO_x) emissions were decreased with increased gasoline proportion due to low combustion temperature from lean premixed combustion even with decreased combustion duration. Soot emission was nearly zero regardless of DME proportion and injection timing.

When the DME injection pressure increased, NO_x emission was monotonically increased with improved thermal efficiency. Ignition delay was significantly decreased since the fuel atomization and mixing process were enhanced due to the high spray momentum. Combustion duration was also shortened with intense heat release.

With increased EGR rate, the thermal efficiency of dual-fuel combustion was not decreased much but NO_x emission was significantly reduced. This was attributed to the increased heat capacity of the in-cylinder charge resulting in decreased combustion temperature.

High thermal efficiency and low NO_x emissions were realized with the optimized fuel proportion, injection timing, injection pressure and EGR rate. Compared to only DME fuel concept (single fuel concept), lower fuel consumption and improved emission characteristics were achieved with optimized gasoline proportion.