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Environmental improvement by reducing emissions as well as alternative energy issues will become an important in the future. Diesel engines have a higher efficiency level compared to gasoline engines, but the problem is that their emissions are higher than other types of engines. That's one of the main reasons why Natural Gas engines became more important over the last few years. This research work was aimed at studying these effects using computational fluid dynamic techniques. For this, it is essential to use suitable mathematical models for the processes that take place in the engines. In the present study, this validated model was used to study the effects of engine operating parameters and combustion chamber geometry on engine performance and emissions thus providing a suitable analytical background for the designing of dedicated engines optimised for natural gas fuel. When designing and optimizing an internal combustion oftentimes key parameters are missing. The purpose of this dissertation to prove that a simple and accurate model can generate excellent results. The results of the model were verified using two different engine configurations and found to give accurate results for pressure and total duration combustion, thermal efficiency and NOx. These theoretical results helped to better understand each engine configuration. The engine configurations were all tested using natural gas as a fuel. The "FORTRAN" code, ZINOX-1, has been used in this study on the combustion and emissions of a natural gas direct-injection spark ignition engine under different compression ratios was carried out. The two engines configurations were based on the same engine with different compression ratio and engine speed. The engines was based on a 1.6L four cylinder engine. Fuel-Air equivalence ratio ϕ where used ranged from 1.112 to 0.58. The engines configuration had compression ratio from 8:1 to 12:1, an engine speed from 1200 to 2000 rpm and ignition timing from 20º to 34º BTDC. The model was able to capture good agreement results between the experimental and theoretical model. The results show that the compression ratio has a large influence on the engine performance, combustion, and emissions. Increased of maximum pressure the penetration distance of the natural gas jet is decreased and relatively strong mixture stratification is formed as the compression ratio is increased, giving a fast burning rate and a high thermal efficiency, especially at low and medium engine loads. Experiments showed that a compression ratio of 12:1 is a reasonable value for a compressed natural gas direct injection engine to obtain a better thermal efficiency without a large penalty of emissions.
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