GM Powertrain Europe and Istituto Motori CNR have undergone a research project aimed at studying the effects on engine performance, emissions and fuel consumption of alternative diesel fuels, from both first (FAME) and second (GTL) generation. The present paper reports some of the results achieved studying the impact on injection and spray behavior of rapeseed and soybean methyl-esters, as well as of GTL diesel blends. The test were performed on a Bosch second generation common rail solenoid-driven fuel injection system capable of 1600bar maximum injection pressure, fitted on GM 1.9L Euro4 diesel engine for passenger cars.
The characterization of the injection process has been carried out in terms both of fuel injection rate, as well as of spatial and temporal fuel distribution in a quiescent non-evaporative optically accessible chamber.
The injection schedules that have been analyzed, as well as the gas density in the spray bomb, have been chosen as representative of different engine working conditions for data correlation, both at partial and full load.
Digital processing of the spray images, captured at different instant from the start of injection and for the diverse operating conditions, enabled the spatial and temporal characterization of the fuel in terms of tip penetration and spray-cone angle.
Experimental results support the consideration that, at ambient temperature, alternative fuels induce small variations in the injection pattern, in particular the pilot event which is mostly impacted by variations in fuel viscosity and density that affect the needle dynamics. Also spray pattern exhibit very similar behaviour among the different fuels, with deviations over the standard dispersion that are noticeable at low injected quantities. As a consequence, the variations in performance and emissions that were measured on the multi-cylinder engine with the same fuels can be primarily traced back to the chemical composition of the fuel itself and to the drift in engine working point it triggers in torque-based diesel engine controllers.