Combustion Modeling of a Medium-Speed Dual-Fuel Engine Using Double Vibe Function

Abstract

Stringent emission regulations and low price of natural gas make dual-fuel (DF) engines an attractive solution in the large engine market. DF engines have drawn keen attention because of their low emissions in gas mode (CO2, NOx, SOx and particulate matter) which can comply with emission regulations without the use of a costly aftertreatment system, and flexible operation with either natural gas or diesel fuel depending on situations. Nevertheless, DF engines show relatively lower power density and thermal efficiency compared to conventional diesel engines due to their lower compression ratio to prevent abnormal combustion (knocking and pre-ignition) and their premixed combustion governed phenomena, which lead to combustion instabilities. Despite the fact that there have been many efforts to improve DF combustion engines in the past, the dual-fuel combustion process is still not well understood, especially in medium-speed engines. Combustion is initiated with the injection of a small amount of diesel fuel (usually 0.5 to 2 % of total energy at full load) which auto-ignites after a long ignition delay. Once the diesel fuel ignites, it leads to a subsequent lean homogeneous air-fuel mixture combustion. These subsequent combustion processes are highly complex, involving autoignition chemistry, premixed turbulent flame propagation and the transition between the two which is not well understood. Thus, it is necessary to find out the effects of various operation and control parameters on DF combustion, in order to improve the engine performance. The present work focuses on the analysis of DF combustion in Hyundai Heavy Industries' H35DF dual-fuel medium-speed engine. The analysis is performed on engine measurements where operating conditions were varied, in combination with an empirical combustion model. Operating condition variations include micro pilot (MP) injection timing, MP injection duration, MP injection pressure, air chamber conditions and compression ratio. The model is used in order to improve the understanding of changes observed through variations in the measurements. To this end, a double vibe function is employed to express the heat release rate mathematically. Each test data is analyzed individually to have a vibe fitted heat release rate. Vibe parameters obtained here are used as the target values, and engine parameters are used as the input values to find the appropriate correlations. The correlations are developed using the linear method. The correlations predict vibe parameters with sufficient accuracy. The developed combustion model well predicts the dual-fuel combustion phenomenon. The DF combustion model is utilized to understand the effects of each parameter on the heat release rate of DF combustion and can be used for the optimization of engine performance.