Numerical and Experimental Study of Small Engine Carburetors

Abstract

Due to price constraints, small engines rely on inexpensive and mechanically-simple de- vices for air-fuel mixture formation: carburetors. A typical carburetor consists of a complex set of internal passages designed to deliver to the engine the correct air-fuel mixture ac- cording to speed and load. This goal is achieved through several physical processes: flows through passages of short length and complex geometry; flows that transition from laminar to turbulent; high-frequency pulsating flow; two-phase flow of various forms, i.e., bubbles, sprays and thin liquid films; and flows with changing fuel and air properties due to rapid changes in temperature and pressure. This study addressed the theoretical modeling of the phenomena inside the main circuit typically found in small engine carburetors. This theo- retical study was done in two steps: first, a general one-dimensional model of the fuel flow as a flow network was derived. This model contains the characterization of the dynamic flow in single-phase elements and the appropriate two-phase flow model for the small diameter pipes. The model was implemented into a one-dimensional engine simulation software, and used to perform sensitivity analyses of carburetor performance as function of carburetor parts. The second part of the study contains the CFD analysis of the most important parts found in the sensitivity analysis: the main fuel orifice and the carburetor venturi. The CFD studies allowed to gain a better understanding of the flow characteristics in these elements,and their results were used to develop engineering correlations that may be implemented in the one-dimensional model. It was found that the flow in the small metering orifices behaves like a small pipe, which may be characterized with an inlet pressure loss coefficient and a Darcy friction factor. The analysis of the carburetor venturi showed that the flow may be considered isentropic from the inlet of the venturi to the venturi throat. Besides the throttle plate, the fuel tube is the most important part controlling the flow field and the pressure losses downstream of the venturi throat.