Numerical and Experimental Fuel Flow Analysis of Small Engine Carburetor Idle Circuits

Abstract

Small engine carburetors have been used for over 100 years and their continual development has produced a mechanical device that although simple in principle is difficult to control in practice. This study aims to develop a mathematical model of the air and fuel delivery process that occurs in the idle path that can predict the air-to-fuel ratio in small engines. A one-dimensional model was created that incorporates single phase air and fuel flow effects using an electrical circuit analog. The model incorporates dynamic fuel flow effects due to inertia, and was coupled to the engine airflow via input data provided by commercial engine simulation software. The methodology developed for the idle path creates a foundation on which to build a transition system and the boundary conditions that control it. A simplified model of the main path circuit was also created that incorporates single phase inertial effects and was used to provide boundary conditions to the idle path model. Finally, a dynamic sensitivity analysis of the idle path was conducted to analyze which elements of the idle path control the fuel flow response. The second part of this study addressed the results of the sensitivity analysis. The idle discharge metering orifice has the largest effect on the air-to-fuel ratio delivered to the engine by the idle path. An experiment was created to characterize two-phase orifices representative of the idle discharge metering orifice. The data produces an empirical correlation that can be used to predict the pressure loss across the metering orifice. The correlation displays results that are inconsistent with the homogeneous assumption and highlights the need for further research in this area.