A Simulation Model for the Combustion Process of Natural Gas Engines with Pilot Diesel Fuel as an Ignition Source 2001-01-1245

During the last years a great deal of efforts have been made to reduce pollutant emissions from Direct Injection Diesel Engines. The use of gaseous fuel as a supplement for liquid diesel fuel seems to be one solution towards these efforts. One of the fuels used is natural gas, which has a relatively high auto - ignition temperature and moreover it is an economical and clean burning fuel. The high auto - ignition temperature of natural gas is a serious advantage against other gaseous fuels since the compression ratio of most conventional diesel engines can be maintained. The main aspiration from the usage of dual fuel (liquid and gaseous one) combustion systems, is the reduction of particulate emissions. In the present work are given results of a theoretical investigation using a model developed for the simulation of gaseous fuel combustion processes in Dual Fuel Engines. The model is a two - zone combustion 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 natural gas burning initiates after the ignition of the diesel fuel. The combustion rate of natural gas depends on the rate of entrainment of surrounding gas into the fuel jet formed and on the velocity of the flame front, which is formed around the area of the burning zone and spreads inside the combustion chamber. The flame front, which is used in the present work, takes into account the history of pressure and temperature values inside the chamber and the local composition to estimate the flame velocity. The model is applied on a single cylinder test engine located at the author's laboratory at various operating conditions. The engine performance under normal diesel operation is used as basis for evaluating engine performance and emissions when using gaseous fuel. The engine load is controlled by changing the amount of the primary gaseous fuel added to the intake air, while the quantity of the pilot liquid fuel per injection stroke is fixed for the given engine regardless of engine operating conditions. It is examined the effect of gaseous fuel quantity (load) on engine performance and emissions at various operating conditions. From the results it is revealed a change of the heat release mechanism when compared to standard diesel operation, an increase of maximum combustion pressure and an increase of NO emissions especially at high engine load. On the other hand soot emissions are considerably reduced and become negligible.