The need for alternative fuels which are sustainable and renewable has led to increased attention on application of biodiesel as a substitute to pure diesel fuel (PDF) in diesel engines. Even though diesel engines are mostly preferred for both on road and off road use due to their engine efficiency, they lead to high emission of pollutants notably NOx and PM. Consequently alternative fuels for use in diesel engines, apart from being compatible with diesel engines, should exhibit low pollutant emission in line with current regulations.

The study focused on three main areas Soybean biodiesel production, its combustion characteristics in a diesel IC engine and quantification of pollutants from the diesel engines exhaust. Production of biodiesel was optimized using homogenous transesterification process and the influence of factors of reactants ratio, time and catalyst were investigated. The pollutants emissions of NOx, PM and PAHs in soybean biodiesel-diesel blends were compared with reference pure diesel fuels.
The optimized production conditions for biodiesel was found to be 6:1 methanol to oil ratio in the presence of 0.7% w/w of oil catalyst concentration and at a breakthrough time of 35-40 minutes.
Testing fuels in diesel engine were carried out at partial loading of 120 Nm torque or 60 % of the engine full torque and at three engine speed regimes (1000, 1500 and 2000 rpm). BSFC increased with the blending of soybean biodiesel into diesel as well as with speed, ranging from 130.1 g kWh-1 for PDF at 1000 rpm to 150 g kWh-1 for BD 30 at 2000 rpm. Energy efficiency of various fuels reduced with increase of engine speed. At individual speeds of 1000 and 1500 rpm, energy efficiency did not change significantly with increase of blending of biodiesel into diesel fuel. At 1000 rpm, Energy Efficiency ranged from 29.8% to 30.4% for PDF and BD 30 respectively while at 1500 rpm it ranged from 29.3% to 29.5% for PDF and BD 30 respectively. However at 2000 rpm, EE reduced with increased blending of Soy bean biodiesel into diesel. It ranged from 27.5% for PDF and 26.8% for BD 30, representing a significant drop when compared with other speed regimes for all the fuels tested.

PM and PAH concentration from diesel engine exhaust reduced with increased blending of soybean biodiesel in diesel fuel and also with increasing Engine Speed. Specific PM Emission ranged from 604 g kWh-1 for PDF at 1000 rpm to 161 g kWh-1 for BD 30 at 2000 rpm. Total PAH concentration reduced with increased speed as well as with increased blending of Soybean biodiesel into diesel fuel. Total PAH concentration ranged from 190 g M-3 for PDF at 1000 rpm to 85 g M-3 for BD 30 at 2000 rpm.

NOx Concentration increased with increased blending of soybean biodiesel in diesel fuel for all the engine speeds tested. The highest specific NOx emission was 36.8 g kWh-1 in BD 30 at 1000 rpm engine speed, while PDF fuel recorded lowest specific NOx emission of 14.2 g kWh-1 at 2000 rpm.

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