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Abstract Background The present study presents a 3-D numerical simulation of a direct injection diesel engine powered by base diesel oil and soybean biodiesel fuel at different load conditions. The modeling was performed using commercial computational fluid dynamics (CFD) software linked to a chemical solver. A chemical kinetic reaction mechanism was developed to simulate the combustion and fuel spray processes. Base diesel oil results were verified using a single-cylinder, 4-stroke diesel engine. Results The study showed that the usage of the soybean biodiesel fuel caused a reduction in carbon monoxide (CO) and hydrocarbon (HC) emissions by about 42.38% and 41.35%, compared with base diesel and an increase in nitrogen oxides (NOx) and carbon dioxide (CO2) emissions of about 21.8% and 11.2%, respectively. Exhaust gas temperature (EGT) is reduced by an average value of 9.4%, the brake-specific fuel consumption (BSFC) is increased by an average value of 11.8% and the brake thermal efficiency (BTE) is dropped by an average value of 11.3% for soybean biodiesel fuel. Conclusions The CFD model showed the effect of the unsaturated fatty acid methyl esters present in soybean biodiesel on the spatial distributed values of NOx, oxygen and temperature during the combustion in engine cylinder. It was observed that the combustion of soybean biodiesel began about 3.89 CAD earlier than base diesel, and the in-cylinder peak pressure was dropped by 8.25%. Soybean biodiesel fuel was optimized by performing four starts of injection (SOI) at timings of − 18, − 16, − 15 and − 13,5 bTDC, and it was found that the combustion characteristics of soybean biodiesel are optimum at SOI = − 15 bTDC. These results indicate that the biodiesel fuel can be used as an alternative and environmentally friendly fuel in the engine without any modifications.

Abstract This study aims to investigate the effects of piston bowl shape and biodiesel fuel on combustion and emissions of a direct injection (DI) diesel engine. Three configurations of piston bowl geometry are modeled: hemispherical, toroidal, and rectangular. Biodiesel fuel derived from soybean oil is being considered. An experimental work was conducted on a diesel engine with a hemispherical piston bowl, and a better validity was attained. The study showed that, diesel engine with toroidal piston bowl delivered the largest power than other shapes, where turbulent regions are formed inside the toroidal piston bowl with higher intensity, which resulted in better combustion characteristics and lower emission concentrations. It is observed from the combustion of biodiesel fuel that, the rapid combustion stage began earlier than conventional diesel fuel by about 5o CA, heat release rate (HRR) decreased by 18.88%, emissions of carbon monoxide (CO), and unburnt hydrocarbon (UHC) reduced but nitric oxide (NO) emission increased by about 17.78% at 55o CA, as compared to diesel fuel. During the injection process, the jet core of biodiesel fuel appeared thicker and longer than conventional diesel fuel. For optimization, preheating with exhaust gas recirculation (EGR) of biodiesel fuel contributes to reduce the emissions of nitrogen oxides (NOx) and improve the atomization rate.