99 Abstract — Gas mixer is a device used to determine the amount of natural gas and air before entering the engine. It injects the gas into the intake air stream of an internal combustion chamber using a combination of radial holes and radial tubes located around the perimeter of an air flow passage. The size of intake air passage, the number and size of the radial holes and tubes varies to achieve adequate gas or air mixing for the particular engine. Measurement of air flow necessary for engine operation is the selection criteria of mixers. The power obtained from engine is dependent on the size of the mixer. It can be seen from the experimental result that a smaller mixer caused higher pressure at the throat in compare to the analytical result. High pressure caused low velocity at the throat. Both smaller and larger mixer was able to promote the methane into the outlet from the inlet tube. Smaller size throat mixer at the throat restricted caused undesirable results. Keywords: Compress natural gas (CNG), internal combustion chamber, air flow. I. INTRODUCTION Compressed natural gas is a growing alternative fuel which was introduced to reduce the dependent on conventional fuel. It has been found that natural gas vehicle (NGV) gives good performance and can be use for long travel. Unlike gasoline or petrol, natural gas does not contain lead because of its high octane rating, 120-130. As combustion is more complete, the contaminating effects of exhaust emission are reduced. In Malaysia, the number of natural gas vehicle has increases and are projected to be more in the future based on the government policies to encourage the use of alternative fuel in the transportation sector [1]. With the increasing concern on the use of alternative Author is with the Faculty of Mechanical Engineering, Universiti Malaysia Pahang, Kuantan, 25000 Pahang, Malaysia. (e-mail: muhamad@ump.edu.my ). fuel for engine application, various studies have been done concerning the performance of the engine. Mixing of the compressed natural gas (CNG) with air is necessary to achieve adequate mixing in order for combustion. Too lean or too rich mixture will cause poor performance of the engine or combustion will not occur at all. The mixer should provide an air-fuel ratio in accordance with the engine operating requirements and this ratio must be within the combustible range. Therefore the research for a proper mixing device is essential in order to achieve this objective [1]. II. MIXER DESIGN The conceptual mixer design of the proposed mixer is based on the venturi effect, whereas Bernoulli’s principle is applied [2]. Figure 1 shows the overview of the venturi. A venturi consist of a tube with a constricted throat which produces an increased velocity accompanied by a reduction in pressure followed by gradually diverging portion in which the velocity is transformed into pressure with slight friction loss[2]. Figure 1: Basic venturi overview g V z p g V z p 2 2 2 2 2 2 2 1 1 1 + + = + + γ γ (1) Figure 2 shows the conceptual mixer design based on the venturi. Higher pressure at the mixer throat will cause lower velocity. This will results in poor suction of compressed natural gas from the tube and air from the inlet. Poor mixing will degrade the engine performance because of the air fuel Design and Simulate Mixing of Compressed Natural Gas with Air in a mixing device K.Kadirgama, M.M.Noor, A.R.N.A. Rahim, R.Devarajan, M.R.M.Rejab, N.M.Zuki N.M. Proceedings of MUCET2008 Malaysian Technical Universities Conference on Engineering and Technology March 8-10, 2008, Putra Palace, Perlis, Malaysia ISBN 978-983-42358-4-0