Effect of the Multiple Injection on Stratified Combustion Characteristics in a Spray-Guided DISI Engine

In this study, the single-cylinder engine experiment was carried out to investigate the effect of multiple injections on stratified combustion characteristics in a spray-guided direct injection spark ignition engine. The engine was operated at 1200 rpm. The total injection quantity applied was 11 mg/stroke to represent a low-load condition. Single injection and multiple injection were tested. Split ratio of each multiple strategies were 1:1 for double injection and 1:1:1 for the triple injection respectively. Dwell time between each injection was set to 200 μs. In the result of engine experiment with the single injection, indicated mean effective pressure was increased as injection timing was retarded to top dead center due to the increased effective work. However, the retardation of the injection timing was limited by the misfire occurrence resulted from the locally rich mixture generation under the high ambient pressure. Moreover, at the injection timing where the maximum indicated mean effective pressure was shown, hydrocarbon and smoke emissions were increased due to the locally rich area of the stratified mixture. The multiple injection strategy was applied to enhance the local homogeneity of the stratified mixture. In the result of engine experiment with the double injection, indicated mean effective pressure and combustion efficiency were increased. As an index of combustion instability, coefficient variation of indicated mean effective pressure was decreased significantly with the double injection. This result implies that the local homogeneity of the stratified mixture was enhanced by the double injection. Hydrocarbon and carbon monoxide emissions were decreased significantly with the double also. Nitrogen oxides emissions were increased due to the increased in-cylinder temperature resulted by more activated combustion. With the triple injection, the flammable injection timing window became narrower. Furthermore, indicated mean effective pressure and combustion efficiency were decreased. This is likely because the stratified mixture was formed on the upper position of the spark plug due to then shortened spray penetration.