A relative assessment of emulsification and water injection methods to mitigate higher oxides of nitrogen emissions from biodiesel fueled light-duty diesel engine

Abstract

In-cylinder emission control strategies based on fuel modifications have gained considerable interest in diesel engine applications because of the potential to simultaneously reduce the oxides of nitrogen (NOx) and soot emissions. There is a renewed interest to replace diesel with carbon-neutral biodiesel to mitigate global warming potential from the transport sector. Further, the bi-directional NOx-soot trade-off problem in conventional diesel combustion has been reduced to a unidirectional higher NOx problem with biodiesel. The efficacy of utilizing emulsification and water injection methods to mitigate the higher NOx emissions with biodiesel is investigated in the present work. The water to biodiesel ratio for the emulsion and injection methods was kept similar at 3%, 6%, and 9% by mass of fuel. Experiments were conducted in a light-duty diesel engine at rated speed, varying load conditions with neat biodiesel, biodiesel-water emulsion, and biodiesel water injection. Biodiesel-water emulsions prepared using a combination of non-ionic surfactants, Span 80 and Tween 80, were stable for over four months. The average droplet size measured using the Dynamic Light Scattering (DLS) technique was 305 nm, 334 nm and 443 nm with 3%, 6% and 9% emulsion, respectively. The intake manifold was modified to include a low-pressure port fuel injector (PFI) to enable water injection. The pulse width of the PFI injector was controlled using a National Instruments PFI module to vary the injected water mass per cycle. The results obtained with biodiesel-water emulsion and biodiesel water injection were compared with neat biodiesel as a reference fuel. The ignition delay time increased, and the peak pressure was reduced with biodiesel-water emulsion. The combustion duration was reduced with emulsion indicating faster burning characteristics. The engine brake thermal efficiency increased with emulsion with a maximum increase of 6%, and there was a 10% reduction in the brake-specific fuel consumption. Both emulsion and water injection result in a simultaneous decrease in NOx and smoke emissions. The drop is significant with emulsion, with a maximum reduction of 40% and 34% in the NOx and smoke emissions, respectively. Overall, the present study shows that biodiesel-water emulsion has a higher potential to achieve a simultaneous reduction in NOx and smoke emissions with better engine performance.