Abstract
Fuel injection characteristics play a decisive role in small bore sized DI diesel engines in the performance characteristics, in-cylinder thermal / mechanical stresses and formation of nitric oxide and soot. This paper reports on investigations using Computational Fluid Dynamics (CFD), carried out to evaluate various schemes of pre-injection rate shaping at an idling speed of 900 rpm. The fuel considered for the combustion analysis is n-heptane. The main emphasis is on split injection, typically employed in Pressure Controlled (PC) injectors. The fuel injection schemes comprise a pre-injection pulse before compression Top Dead Centre (TDC) followed by a main injection pulse starting at compression TDC, with both the pulses separated by a dwell period. A nozzle Needle Lift Controlled (NLC) injection rate shaping scheme used in common rail fuel injection systems is also analysed computationally. In-cylinder spatial velocity profiles during combustion, and heat release rate profiles subjected to validation agree reasonably well with experimental data available in open literature. From the pressure based results it is concluded that the scheme with 50% pilot injection quantity with 3 to 6 crank angle degree dwell period showed higher peak pressures and decreased combustion duration. The decreased combustion duration is indicative of enhanced mixing, likely induced by the rapid gas expansion of a larger proportion of premixed burn. Soot - NO_X trade-off analysis revealed the NLC injection scheme to be highly advantageous over other split injection schemes at low load.
