Improvement in Thermal Efficiency of a Diesel Engine by Homogenized Flame Distribution 2019-24-0166

The heterogeneous character of diesel engine combustion is well-known. However, in the thermodynamic efficiency calculations, a homogenous combustion is generally assumed. This results in poor accuracy of specific heat ratio estimations. Therefore, this study aims to evaluate how the real diesel engine specific heat ratio behaves by means of a two-zone model calculation. Efficiency improvement from a higher burned zone specific heat ratio was investigated. This was achieved by better air entrainment and a highly dispersed flame in the cylinder. To investigate into the local phenomena, combustion homogeneity was estimated by utilizing the two-zone model where the in-cylinder volume was divided into unburned zones and burned zones. To numerically confirm the effect of a highly dispersed flame on the specific heat ratio, a single-cylinder diesel engine equipped with three injectors (located at the cylinder center as well as at the rim of the piston bowl) was utilized. Three types of side injector nozzle orifice directions were prepared to compare the difference in the temporal and spatial flame distribution and specific heat ratio. When spraying both along and against the swirl, the time-varied specific heat ratio in the burned zone was increased compared to spraying only along or against the swirl. This increase occurred due to the higher combustion homogeneity. Therefore, the in-cylinder pressure was increased resulting in thermal efficiency improvement. In addition, it was confirmed that the cooling loss to the liner and the cylinder head was reduced due to reduction of spray/wall interaction. Furthermore, the late combustion period was shortened. Thus, it was concluded that homogenized flame distribution is effective for improving thermal efficiency.