Development of a compressed natural gas (CNG) mixer for a two stroke internal combustion engine

Abstract

Compressed Natural Gas (CNG) has been accepted widely as an alternative to gasoline. More importantly the use of CNG in two stroke engines will drastically reduce the high emission output from these engines as these engines are widely used around the world. A conversion kit is used to apply the fuel in engines. A bi-fuel conversion system converts engines without much modification to other systems. They are normally produced for four stroke application. This kit has to be studied to be modified for two stroke application. The part that connects the engine to the kit is called a gaseous fuel mixer. This part mixes the air and fuel due to its venturi shape. A mixer provides fuel suction at different engine speeds due to pressure difference at the throat. The optimisation of the throat is important as a small throat will cause poor performance at high speeds while a large throat will reduce fuel suction. The smaller throat size creates higher velocity and lower pressure. This low pressure creates fuel suction into the mixer. The mixer was designed for a two stroke engine air flow. Computer aided design (CAD) and computational fluid dynamic (CFD) software were used as a tool for the design. The design is optimised for inlet and outlet angles, number and size of the hole at the throat circumference and also the throat size. The prototype design was manufactured based on optimised dimensions of the mixer that were obtained from CFD analysis. The mixer was validated to show that the CFD analysis was correct. Testing apparatus were used to do the validation. The apparatus consists of a laminar flow element (LFE), a smoke generator, a digital manometer and a gaseous flow meter. It was used to validate the flow pattern, pressure drop from the mixer and the air fuel ratio given by the mixer