Theoretical and Experimental Investigation of a Direct Injection Dual Fuel Diesel-Natural Gas Engine

The compression ignition engine of the dual fuel type has been employed in a wide range of applications to utilize various gaseous fuel resources while minimizing soot and oxides of nitrogen emissions without excessive increase in cost from that of conventional direct injection diesel engines. The use of natural gas as a supplement for liquid diesel fuel could be a solution towards the efforts of an economical and clean burning operation. The high auto-ignition temperature of natural gas is a serious advantage since the compression ratio of most conventional diesel engines can be maintained. In the present work a comparison between experimental and theoretical results is presented under dual fuel operation. For the theoretical investigation a computer simulation model has been developed which simulates the gaseous fuel combustion processes in dual fuel engines. The combustion model is a two-zone one, taking into account details of diesel fuel spray formation and mixing with the surrounding gas, which is a mixture of air and natural gas. The combustion rate of natural gas depends on the entrainment rate of surrounding gas into the fuel jet and on the velocity of the flame front, which is formed around the area of the burning zone and spreads inside the combustion chamber. A soot model has been used to estimate the formation of soot while the Zeldovich mechanism has been used to determine the concentration of oxides of nitrogen. To validate the predictive ability of the model, experiments were conducted on a single cylinder DI diesel test engine, which had been modified to operate under dual fuel conditions using natural gas. The engine is located at the author's laboratory and experiments have been taken at various operating conditions. Under dual fuel operation, liquid fuel is replaced by gaseous one at various percentages, to maintain the power output of the engine the same as for normal diesel operation at the specific operating conditions. In this case the amount of gaseous fuel represents the supplement. The experimental results are found to be in good agreement with the theoretical ones, obtained from the computer simulation program. It is revealed a serious effect of dual combustion on the heat release rate mechanism compared to standard diesel operation. Furthermore both experiment and simulation reveal that dual fuel operation has a positive effect on nitric oxide and soot emissions compared to normal diesel operation. This effect is promoted when increasing the percentage of gaseous fuel. The last reveal that the developed model is capable to predict at least qualitatively the effect of dual fuel operation on the combustion and pollutants formation mechanism.