Experimental and Theoretical Analysis of the Combustion and Pollutants Formation Mechanisms in Dual Fuel DI Diesel Engines

With the increasing public interest in energy supply and the environment, attention has focused on the development of ecological and efficient combustion technologies. One of these technologies could be the use of natural gas as supplement fuel for diesel fuel in DI diesel engines. The great availability at attractive prices and the clean nature of combustion are the most important advantages of natural gas compared to conventional diesel fuel. In the present work are given theoretical and experimental results for the combustion mechanism of natural gas in a compression ignition environment, with special emphasis on the combined heat release rate of natural gas and diesel fuel, the duration of combustion and the ignition delay period. Results are also provided for the formation history of pollutants inside the combustion chamber of a DI diesel engine operating in dual fuel mode (with natural gas fuelling). The model used is a two - zone phenomenological one describing the combustion mechanisms of natural gas and diesel fuel. Natural gas is ignited from the diesel fuel and the existence of a flame front is considered to describe its combustion rate. The experimental investigation was conducted on a single cylinder DI diesel engine properly modified to operate under dual fuel conditions using natural gas as supplementary fuel. The experimental results seem to be in good agreement with the theoretical ones, obtained from the simulation model. Comparing the results under normal diesel and dual fuel operation a serious effect of the presence of natural gas on exhaust emissions and main combustion characteristics is observed. As far as the exhaust emissions are concerned, the presence of gaseous fuel affects positively (reduction) the values of NO and Soot. On the contrary dual fuel operation has a negative effect on CO emissions. Concerning the combustion analysis of dual fuel operation, heat release rate is affected seriously by the presence of natural gas in the combustion chamber.Compared to normal diesel operation, the increase of natural gas quantity results to a decrease of the percentage of heat released during premixed combustion period while ignition delay increases. Combustion duration is higher under dual fuel operation at low load, but with the increase of load the difference is decreased and it becomes even lower compared to normal diesel operation at high load.