The increasing demand for fossil fuels and the concerns about their harmful emissions have forced the scientific society to search for alternative fuels for internal combustion engines. Therefore, researchers have been developing new alternatives from different sources, biodiesel fuel is one of the best alternative fuels for diesel engines because of its high cetane number, low carbon fuel, non-toxic, biodegradable, lubricity, oxygenation nature, no aromatics and almost no sulfur content, so it is considered a renewable energy source and environmentally friendly [1,2,3,4].
Vegetable oils, animal fats and reused oils are the most common sources of biodiesel. Biodiesel fuel is produced by a common chemical method, namely transesterification; in this method, a chemical reaction occurs between the triglycerides (vegetable oil) and alcohol in the presence of a catalyst. If methanol alcohol is used, the reaction products will be fatty acid methyl esters (FAMEs; biodiesel) and glycerol. FAMEs may be saturated and unsaturated; their proportions in biodiesel fuel directly affect the physical and chemical properties [5,6,7].
Soybean biodiesel fuel is considered in the present study. However, the disadvantages of soybean biodiesel relative to base diesel oil, such as higher viscosity, higher density and lower volatility. So, the usage of biodiesel for a long term may cause engine durability problems such as poor fuel atomization, injector coking, piston ring sticking, a clogged fuel filter and higher pressure inside fuel lines . Therefore, different experimental methods are required to make soybean biodiesel more suitable for diesel engines such as preheating at a wide range of temperatures, blending with base diesel, increasing injection pressure and adding additives . CFD codes have recently gained popularity in the industry as a tool for facilitating and speeding up engine evaluation under various conditions. The purpose is to minimize the cost and prototyping time and have the ability to make several studies by changing individual parameters such as injection pressure, inflow temperature of the fuel and air, injection timing, fuel blends and the engine loads [10, 11].
A compression ignition (CI) engine is considered one of the most complex problems in fluid dynamics to model because the flow inside the cylinder is unsteady, compressible, turbulent and cyclic. All physical processes that take place inside the cylinder from the start of air entry to the end of combustion required a mathematical model to describe them. Turbulence, fuel spray, atomization, breakup, collision, coalescence, droplets vaporization, kinetic reaction mechanism (fuel chemistry), self-ignition, premixed and diffusion combustion, heat losses and emission development (NOx and soot)—all of these processes require subroutines that work together in order to simulate the engine [12, 13].
Many studies conducted on biodiesel fuel and its effect on engine performance, Zhang et al.  studied the performance and emission of a marine diesel engine fueled with natural gas ignited by biodiesel blends at different conditions of engine load and concluded that at all engine conditions, the NOx, CO and soot emissions decreased with increasing the natural gas fraction. According to EL-Seesy et al. , the addition of butanol to the diesel/jojoba oil blend lowers the mixture's viscosity compared to diesel/jojoba oil, increases the in-cylinder peak pressure and heat release rate (HRR), and also results in a reduction in NOx, CO and HC emissions compared to diesel fuel. Zhang et al.  conducted a study on the effect of assisted hydrogen on a diesel engine performance fueled with biodiesel fuel and reported that the in-cylinder pressure and temperature are increased by about 7.42% and 7.14%, respectively. Meanwhile CO and HC decreased but NOx emissions increased. Coughlin et al.  studied the combustion characteristics of soybean oil–butanol blends and found that the addition of alcohol could improve the soybean combustion characteristics. Ng HK et al.  studied the effect of palm, coconut and soybean biodiesel fuels on a diesel engine performance numerically using a CFD code, which was coupled to a chemical kinetic reaction mechanism including 80 species and 303 reactions. According this study, the biodiesel properties influence the formation of emissions; so, the soybean biodiesel fuel provided a greater thermal NOx than others because its higher unsaturation level. Al-Dawody et al.  carried out a 2-D numerical and experimental study on a DI diesel engine fueled with different blends of soybean biodiesel with diesel fuels. The effects of blending on the in-cylinder pressure, heat release rate (HRR), unburned hydrocarbon (UHC), carbon monoxide (CO), nitrogen oxides (NOx) and smoke opacity were calculated. He found that the use ofiesel results in lower smoke opacity up to 48.23% with higher brake-specific fuel consumption (BSFC) by 14.65%, compared to diesel fuel. For the blends of B20 (20% soybean biodiesel + 80% diesel fuel) and B100 (100% soybean biodiesel), the CO emissions reduced by 11.36% and 41.7% respectively, compared to diesel fuel. At blends of B20, B40 and B100, the unburnt hydrocarbon emissions were found to be 15%, 27% and 38.4% lower than that of diesel fuel, respectively. Also, he reported that at all blends the NOx emissions were observed to be higher than that of diesel fuel. The in-cylinder pressure for soybean biodiesel is lower than that of diesel fuel by about 2.98% due to the reduction in the heat supply for the blended fuel and the maximum pressure obtained for biodiesel is near to top dead center (TDC) than diesel fuel.
In contrast to conventional combustion, Ganesan et al.  investigated a light-duty diesel engine using the reactivity-controlled compression ignition technique by introducing cottonseed oil biodiesel as a high-reactivity fuel directly into the combustion chamber and n-pentanol as a low-reactivity fuel. In comparison with the neat cottonseed oil biodiesel, this method led to lower concentrations of NOx and smoke emissions by roughly 44.2% and 35.0%, as well as a higher thermal efficiency. Nayak et al.  investigated the performance, combustion and emission characteristics of a direct injection diesel engine running on biodiesel fuel produced from waste palm cooking oil and fish oil and reported that the biodiesel fuel resulted in a lower thermal efficiency and higher specific fuel consumption compared to diesel fuel. An experimental study carried out by Singh et al.  on a diesel engine running on Jatropha biodiesel at varied compression ratios. Jatropha biodiesel diesel blend (B30) and neat diesel fuel were used to power the diesel engine. According to the experimental study, using the B30 blend reduces HC and CO emissions from diesel by about 16.7% and 24%, respectively. However, using the B30 blend resulted in a notable increase in NOx and CO2 emissions. Gritsenko et al.  investigated how disconnecting some of the engine cylinders in low-load and idling modes increased fuel efficiency in cars and tractors. Experiment results indicate that when some engine cylinders are disconnected, the performance parameters of the car (tractor) are interdependent. It has also been established that the maximum reduction in hourly fuel consumption occurs when the engine is idling, and that it decreases as the load increases.
Due to its higher viscosity and lower calorific value, biodiesel causes CI engines to operate poorly and consume a lot of fuel. This issue can be solved by raising the injection temperature of biodiesel to a specific temperature. The results of an experimental study conducted by Kodate et al.  on a DI diesel engine running on preheated biodiesel fuel (at 95 °C) showed that the viscosity of the fuel decreases at higher temperatures, improving combustion characteristics and fuel atomization as well as fuel vaporization in a diesel engine, resulting in higher engine performance and lower CO and HC emissions with a slight increase in NOx and CO2 emissions when compared to unheated biodiesel fuel.
Biswas et al.  compared the performance, emissions and engine noise of a quadruple (early–pilot–main–after) injection strategy to three different triple [early–pilot–main and pilot–main–after] injection strategies in a heavy-duty BS-IV diesel engine with a 45% EGR fraction. According to this study, the quadruple injection strategy outperforms the promising triple injection strategy, which provides optimal results in terms of both performance and emissions. Using Fisher's primary breakup model, Beutler et al.  carried out numerical modeling on a spray of diesel and polyoxymethylene dimethyl ether in a high-pressure chamber at several variable parameters (ambient pressure, temperature and injection pressure) and compared it with the results of the experiments. Baek et al.  analyzed the spray behavior and the performance of a diesel engine powered by biodiesel and jet propellant-5 (JP-5) at multiple split injection strategies and reported that for all injection timings, biodiesel showed disadvantages as compared to JP-5 in terms of combustion and emission characteristics. Kuti et al.  used the Reynolds averaged Navier–Stokes model to simulate the spray combustion characteristics of waste bio-oils and traditional diesel fuel. They used a combination of n-heptane and n-tetradecane as surrogates for diesel fuel and a combination of methyl decanoate, methyl-9-decenoate and n-heptane as surrogates for the waste bio-oils. The results of the spray liquid length, ignition delay period, soot formation and flame lift-off length were compared to the experimental results, and good validity was found.
Although biodiesel derived from rice bran oil is seen to be a viable green fuel substitute for traditional diesel fuel, when utilized as fuel in diesel engines. Particularly in cold areas, the high free fatty acid concentration of rice bran biodiesel fuel is viewed as a significant disadvantage. More specifically, this undesirable characteristic causes noticeably higher viscosity, surface tension and higher density, which may then cause problems with fuel pumping, poor fuel spray and atomization, the formation of heterogeneous air–fuel mixtures and ultimately incomplete combustion . The main factors causing wear, corrosion and abrasion in engines may be deposits created during the burning of biodiesel. In comparison with neat diesel, the use of biodiesel and plant oil-based fuels typically results in cylinder head carbon deposits, deposits on the injector tip and on the piston crown, degradation of lubricating oil and the formation of more deposits. This is especially true for long-term applications .
Fuel stability is a main procedural problem regarded in biodiesel manufacturing and is related to several issues like phase separation, polymerization, fuel degradation and oxidation. There are three types of biodiesel stability: oxidation, storage and thermal stability. When biodiesel is kept in storage for a long time, oxidation occurs. During the storage time, the free radicals attack the unsaturated lipids in biodiesel and produce lipid peroxide products. The oxidation of biodiesel fuel is brought on by the double and triple bonds that are present in the chains of fatty acid methyl esters. Some of the extrinsic elements that impact the oxidative stability of biodiesel include the presence of air, light, antioxidants, metal traces and storage container material .
Compared to the previous studies, the diesel engine simulation process saves the time and effort of practical experiments. It also enables us to conduct several parametric studies for engine development. From this perspective, this study was chosen to evaluate the performance of the diesel engine, where the numerical results of conventional diesel fuel were compared with the experimental results, and a good convergence was found. A suitable kinetic reaction mechanism was chosen to simulate soybean-derived biodiesel, and the combustion and emission results were compared with previous works, and good validity was attained. In order to accurately simulate the in-cylinder turbulence, fuel atomization, droplet vaporization, kinetic reaction mechanism, premixed and diffusion combustion, heat losses and exhaust emission generation, it is necessary for many subroutines to work together. The objective of the current work is to create a 3-D computational fluid dynamics (CFD) model to simulate the combustion and emissions of a direct injection (DI) diesel engine running on diesel and soybean biodiesel fuels at different load conditions.