Effect of Multiple Injection Strategies on Emission and Combustion Characteristics in a Single Cylinder Direct-Injection Optical Engine

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Abstract
The effect of multiple injections in a heavy-duty diesel engine was investigated by focusing on single-pilot injection and double-pilot injection strategies with a wide injection timing range, various injection quantity ratios, and various dwell times. Combustion characteristics were studied through flame visualization and heat release analyses as well as emissions tests. Single-pilot injection resulted in a dramatic reduction in nitrogen oxide and smoke emissions when the injection timing was advanced over 40° CA before the start of injection (BSOI) due to combustion with partially premixed charge compression ignition. A brown-colored flame area, which indicates a very fuel-rich mixture region, was rarely detected when more fuel was injected during single-pilot injection. However, hydrocarbon emission increased up to intolerable levels because fuel wetting on the cylinder wall increased. When double-pilot injection was applied, nitrogen oxide and smoke emissions were lowered by up to 73% and 84%, respectively, compared with single-injection. This result implies that double-pilot injection induced a better premixed mixture due to an improved turbulent effect between the two pilot injections. Furthermore, hydrocarbon emission was reduced by 50% compared with single-pilot injection because the spray tip penetration of double pilot injection was shortened, which resulted in a reduced amount of wall-wetted fuel. Images from the double-pilot injection experiment revealed that the diffusion burn portion was decreased and the luminosity intensity was the weakest among all the other experimental conditions.

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