Direct injection of natural gas for diesel engine fueling

Transient turbulent underexpanded jets are formed when natural gas is directly injected, late in the compression stroke, in the combustion chamber of diesel engines. An analysis of gaseous jets, entering a large chamber at high Reynolds number and at sonic nozzle conditions, defines the similarity conditions pertaining to the jet penetration and mixing with the surrounding air. It is shown that the jet penetration varies with (Μ[sub n]/Ρ[sub ch])[sup ¼] t[sup ½], where M[sub n] is the momentum injection rate at the nozzle, Ρ[sub ch] the chamber gas density and t the time from beginning of injection. This dependency is supported by jet theory and by experimental data. The experimental data, which are also used to confirm the validity of multidimensional simulations, were obtained by schlieren visualization of underexpanded transient methane jets. Autoignition or ignition through pilot fuel combustion of the gaseous jets leads to transient flames whose configuration is affected by the length of the ignition delay. The ignition and combustion of the gaseous jets were studied using a multidimensional model which includes a new ignition scheme reproducing well the temperature dependency of methane autoignition data, a two-step high-temperature kinetic, and an eddy-dissipation turbulent combustion model. The simulations indicate that transient flames penetrate at much the same rate as cold jets, which can be formally shown for autoigniting flames with little or no premixed combustion (early ignition). The simulations demonstrate that the ignition delay and the nozzle injection rate of the gaseous jets affects the proportion of premixed and mixing-limited burning. In the case of pilot-ignited gaseous jets, it is shown that increasing the interaction between the pilot fuel sprays and the gaseous jets promotes early ignition of the gaseous jets. Results also indicate that injecting the natural gas later than the pilot fuel spray promotes early ignition and complete combustion of the gaseous fuel jet. The simulations, of both cold and igniting jets, permitted to highlight consequences and practical considerations regarding the choice of injection pressure, nozzle diameter, number of holes, and regarding the ignition of methane jets through pilot fuel combustion.Applied Science, Faculty ofMechanical Engineering, Department ofGraduat