Conventional compression ignition engines can easily be converted to a dual fuel mode of operation using natural gas as main fuel and diesel oil injection as pilot to initiate the combustion. At the same time, it is possible to increase the output power by increasing the diesel oil percentage. A detailed performance and combustion characteristic analysis of a heavy duty diesel engine has been studied in dual fuel mode of operation where natural gas is used as the main fuel and diesel oil as pilot. The influence of intake pressure and temperature on knock occurrence and the effects of initial swirl ratio on heat release rate, temperature-pressure and emission levels have been investigated in this study. It is shown that an increase in the initial swirl ratio lengthens the delay period for auto-ignition and extends the combustion period while it reduces NOx. There is an optimum value of the initial swirl ratio for a certain mixture intake temperature and pressure conditions that can achieve high thermal efficiency and low NOx emissions while decreases the tendency to knock. Simultaneous increase of intake pressure and initial swirl ratio could be the solution to power loss and knock in dual fuel engine. 1. Introduction Natural gas, because of cleaner nature of its combustion and also lower price compared with conventional liquid fuels, is the most attractive and widely accepted among alternative gaseous fuels. Diesel engines could easily be designed or modified to use natural gas in power production. Due to relative high compression ratio, high thermal efficiency is achievable from these engines. The objective is to increase the percentage of natural gas used as fuel by improving the engine design and combustion process with least exhaust emissions, together with thermal efficiencies compatible with the base engine [1]. There are some problems associated with the diesel engines conversion to dual fuel mode of operation. One of the problems is the knock occurrence at high load operation that the maximum power output of a dual fuel engine is limited by knock and another is the power loss due to lower power density of gaseous fuels. A great deal of numerical research into knock phenomena and power loss in dual fuel engines has been carried out. The combustion models, which are used for investigation, could be divided into two groups: multizone models and multi-dimensional models. Many combustion simulations of dual fuel engines have used multi-zone model to analyze the combustion. Karim simulated autoignition and knock in dual fuel engine using a two-zone
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