The Mg-doped CaO and CaO nano additives were emulsified with Sapindus Trifoliatus biodiesel and investigated in the CI engine. The experiments were conducted using four different samples such as raw Diesel, STBD, STBD25 + 30 ppm CaO and STBD25 + 30ppm Mg doped CaO Nano additives. The engine performance, combustion and emission parameters have arrived at a constant speed of 1500rpm. Experiments were conducted with a bTDC injection timing of 23 degrees, 220 bar injection pressure and 18:1 compression ratio at different load conditions of 0%, 25%, 50%, 75% and 100% using a 4-stroke single-cylinder diesel engine. The engine is experimentally tested with Sapindus Trifoliatus biodiesel, with and without nano-additives, and the results are compared to conventional diesel fuel.
5.1 Effect of Mg-doped CaO Nano Additives on In-Cylinder Pressure
The development of in-cylinder pressure in an internal combustion engine depends only on the amount of fuel burned during the uncontrolled combustion (premixed combustion) stage (Ali et al. 2019). Cylinder pressure is an important parameter when analyzing the combustion characteristics of test fuels. Factors that affect cylinder pressure variation include viscosity, cetane number, ignition delay, and air/fuel mixture ratio (Imdadul et al. 2017; Emiroğlu and Şen 2018; Çeli̇k and Bayindirli 2020). Figure 9 shows changes in cylinder pressure with respect to the crank angle under full load conditions. From the graph, the in-cylinder pressure of STBD25 + 30ppm Mg-doped CaO increased by 3.22%, 6.24%, and 9.02% in percentage variation rate compared to diesel, STBD25-30ppm CaO, and STBD25. Diesel’s low in-cylinder pressure is due to the shortened ignition delay time (Feng et al. 2020; Sahu et al. 2021), which made combustion uniform. Additionally, Table 2 shows the improved calorific value of diesel fuel. This helps to reduce fuel consumption, improve combustion efficiency, and shorten peak pressure (Qi et al. 2009; Saravanan et al. 2020). The addition of 30 ppm Mg-doped CaO nano-additive to STBD25 increased the peak pressure. This was attributed to the increased thermal conductivity of the resulting nano fuel mixtures, leading to increased vaporization rates of fuel droplets and improved combustion processes. The combustion of nanoparticles takes place in two stages.
During the primary combustion stage, nano-additives tend to mix with biodiesel fuel as a binding mechanism, breaking down its chemical structure and accelerating the reaction. During the secondary combustion stage, molecular oxygen present in the Mg-doped CaO structure is released (Elkelawy et al. 2021b). This released oxygen, along with higher heat, reacts with unburned hydrocarbons to increase the combustion rate. Due to the high thermal conductivity of nanofuel, fuel combustion starts earlier, resulting in higher peak pressures in the cylinder (Russell et al. 2000; Dhar et al. 2012; Soudagar et al. 2020). Figure 11, reveals that at full load conditions, the BTE of diesel, STBD25, STBD25 + 30ppm CaO, and STBD25 + 30ppm Mg doped CaO are 62.22 bar, 58.49 bar, 60.28 bar and 64.29 bar.
5.2 Effect of Mg-doped CaO Nano Additives on Heat Release Rate
The engine combustion characteristics depend on the fuel type, cylinder design, and fuel injection type which influences the heat release ate (Lei et al. 2016; Lee et al. 2019). The maximum heat release is achieved due to a lower ignition delay during combustion in a CI engine (Das et al. 2018; Alloune et al. 2018). In a CI engine, two types of combustion phases occur, the premixed combustion phase (PCP) and the diffusion phase (DP). In PCP, the overall burning rate can be determined during ignition delay period. In DP, combustion rate can be limited (Shelke et al. 2016). Figure 10 shows that heat release rate variation on Diesel, STBD25, STBD25 + 30ppm CaO and STBD25 + 30 ppm Mg-doped CaO with crank angles at ful load conditions. The heat release rate for Diesel is 54.43 \(\frac{\text{J}}{^\circ \text{C}\text{A}}\), STBD25 is 37.37\(\frac{\text{J}}{^\circ \text{C}\text{A}}\), STBD25 + 30ppm CaO is 46.12\(\frac{\text{J}}{^\circ \text{C}\text{A}}\) and STBD25 + 30 ppm Mg-doped Cao is 55.32\(\frac{\text{J}}{^\circ \text{C}\text{A}}\). From the graph, it is revealed that HRR increases by 1.68%, 16.69% and 32.5%, compared with diesel, STBD25 + 30ppm CaO and STBD25.
The heat release rate dx = dw can be calculated by the following formula.
$$\frac{dx}{d\phi }=p\frac{{C}_{P}}{R}\frac{dV}{d\phi }+V\frac{{C}_{v}}{R}\frac{dp}{d\phi }+mT\frac{d{C}_{v}}{d\phi }+\frac{d{Q}_{w}}{d\phi }$$
$$\frac{d{Q}_{w}}{d\phi }={h}_{c}A\left(T-{T}_{w}\right)$$
Here, p – Cylinder Pressure, Cp - Specific Heat @ Constant Pressure, Cv - Specific Heat @ Constant Volume, R - Gas Constant, hc – Coefficient of Heat Transfer, A - Wall Area, and Tw - Wall Temperature.
The higher HRR of diesel fuel can be attributed to its higher calorific value leading to higher heat production during combustion. Also, due to short burn time and short ignition delay (Chandrasekaran et al. 2016; Ağbulut et al. 2022). As the concentration of Mg-doped CaO nanoadditives increases, the HRR further increases due to the enhanced ignitability. This enhanced catalytic reactivity of the resulting fuel mixture accelerates the combustion rate of the fuel precursor, thereby releasing the maximum rate of heat release (Saxena et al. 2017; Devarajan et al. 2018). This work is in good agreement with previous studies that improved HRR by attaching nanomaterials to base fuels. The reactive surface area of nanoparticles can enhance their catalytic reactivity due to their size and also increases fuel droplet spreading and fuel injection dispersion, improving combustion characteristics and fuel-air mixing (Vijay Kumar et al. 2018). At high load and high speed, the peak pressure is decreased, which could be due to the increase in friction losses resulting in reduced heat release rate.
5.3 Effect of Mg-doped CaO Nano Additives on Brake Thermal Efficiency
Brake thermal efficiency is defined as how well the heat energy of the fuel is converted into mechanical energy. It is seen from experiment that increasing engine load increases fuel consumption and increases BTE (Asokan et al. 2019). Figure 11, reveals that at full load conditions, the BTE of diesel, STBD25, STBD25 + 30ppm CaO, and STBD25 + 30ppm Mg doped CaO are 32.23%, 29.79%, 30.99% and 33.77%. BTE of STBD25 + 30ppm Mg doped CaO is found to be increased in percentage variation by 4.56%, 8.23% and 11.79% compared with diesel, STBD25 + 30ppm CaO and STBD25. The BTE of STBD25 is lower than other blends due to its higher density, higher viscosity and lower CV. The BTE of the fuel blended with 30ppm CaO is higher than that of STBD25. The use of nanoparticles converts fuels into usable energy more efficiently and improves combustion (Pandian et al. 2017; Kumar et al. 2019a, b). Therefore, the BTE of STBD25 + 30ppm Mg-doped CaO is higher than other fuel blends. The main reason for the improved performance is the additional energy generated inside the cylinder by increasing the surface-to-volume ratio of the nanoparticles (Soudagar et al. 2020). Also, the increased HRR is due to a shorter ignition delay time, and the superior ignition properties of the nanoparticle additive improve premixed combustion (Chandra Sekhar et al. 2018; Chinnasamy et al. 2019). The fuel's excellent catalytic and ignition properties result in higher peak pressures and higher instantaneous heat release during the combustion process. When nanoparticles are added, the fuel droplets become smaller, the fuel becomes less viscous, and more effective fuel surface area is exposed (Vijay Kumar et al. 2018). The improved combustion parameters of nanoparticles are due to their ability to release oxygen atoms from the lattice during combustion (Barzegar et al. 2019; Çeli̇k and Bayindirli 2020).
5.4 Effect of Mg-doped CaO Nano Additives on HC Emission
There are various possible sources for unburned hydrocarbon formation in CI engines. Fuel trapped in nozzle, crevice areas and cylinder piston interface accounts for the majority of HC emission (Anchupogu et al. 2018; Tang et al. 2021). Other factors include incomplete evaporation of fuel mixture, local over rich/over lean mixture and liquid wall film interface with more spray impingement, high viscosity of the fuel (Koegl et al. 2018). It is observed from the experiments, It can be seen that HC emissions increase with increasing loads owing to more fuel entering into the combustion chamber thus resulting in several rich mixture zones, poor combustion, thus emitting higher unburnt fuel fractions, i.e. higher HC emissions (Raheman and Kumari 2014; Yesilyurt et al. 2020). The presence of inbuilt O2 in biodiesel blend helps in sustaining the combustion thus resulting in lowered HC emissions for CaO and Mg-doped CaO Nano-additives blended STBD25. CaO and Mg-doped CaO Nano-additives improved the fuel injection and leads to better atomization of fuel due to lower viscosity, increasing the surface to the volume ratio of nanoparticles, and enhancing the heat-transfer rate.
Figure 12, reveals that at full load conditions, the HC emission of diesel, STBD25, STBD25 + 30ppm CaO, and STBD25 + 30ppm Mg doped CaO are 25 ppm, 23 ppm, 21.89 ppm and 20.64 ppm. On comparison, it is revealed that HC emission for STBD25 + Mg doped CaO decreased by 21.12%, 6.06% and 11.43% compared with diesel, STBD25 + 30ppm CaO and STBD25. Lower HC emissions were also observed due to higher catalytic activity and higher surface-to-volume ratio resulting in improved combustion (Vijay Kumar et al. 2018). The addition of nanoparticles lowers the activation temperature of carbon combustion and improves fuel oxidation (Vairamuthu et al. 2016). The nanoparticles act as an oxygen buffer, providing supplemental oxygen that facilitates the formation of a stoichiometric mixture within the combustion chamber.
5.5 Effect of Mg-doped CaO Nano Additives on CO Emission
HC emission and CO are affected by high viscosity, poor fuel-air mixture, less oxygen content, poor vaporization and incomplete combustion (Raheman and Kumari 2014; Yesilyurt et al. 2020). At partial load, CO emissions are high. However, CO emissions increase with increasing engine load and it will be highest at full load conditions. CO emissions are influenced by engine load, fuel consumption and oxygen availability (Sendzikiene et al. 2006). In nature, biodiesel has higher oxygen content compared to raw diesel. Hence CO emission is lower for biodiesel compared to fossil fuels (Arbab et al. 2015). The presence of nanoparticles in biodiesel results in improvement of fuel air mixing, improved ignition characteristics and higher surface-to-volume which improve fuel droplets size that leads to complete combustion and reduction in CO. Figure 13 reveals that at full load conditions, the CO emission of diesel, STBD25, STBD25 + 30ppm CaO, and STBD25 + 30ppm Mg doped CaO are 0.06%, 0.03%, 0.03% and 0.02%. Lower viscosity improves fuel atomization and plays an important role in reducing CO emissions. The reactive surface of the nanoparticles supports reactivity as a potential catalyst and improves fuel atomization. Nanoparticles reduce the homogeneity of the base fuel composition and cause more decomposition during fuel injection. STBD25 + 30 ppm Mg-doped CaO biodiesel reduces CO emissions from 33–67% compared to diesel and other blended fuels.
5.6 Effect of Mg-doped CaO Nano Additives on NOx Emission
NOx production is driven by peak cylinder temperatures, oxygen availability, and ignition delay (Panneerselvam et al. 2016). Due to the high oxygen content of biodiesel, NOx emissions are high. Increasing the proportion of nanoparticles decreased NOx emissions (Karthikeyan and Prathima 2017; EL-Seesy et al. 2018). This is due to the presence of oxygen-containing additives, which improved combustion. The addition of CaO metal oxide nanoparticles leads to complete combustion as it acts as an oxygen catalyst. In such cases, maximum heat release rates and high peak pressures during combustion are to be expected. EGT is directly proprtional to NOx content. This indicates that increased EGT plays an important role in NO production (Yadav et al. 2018; Elkelawy et al. 2021a). Lower ignition delay of Mg-doped CaO nanoparticles reduced premixed combustion rate which in turn decreased incylinder temperature during combustion and reduced NOx emissions. Mg will retreat CaO from decomposition and retain its original form for longer period. Hence oxygen taking part in combustion is stable and so the combustion temperature, hence NOx emission is reduced. Addition of Mg-doped CaO nanoparticles to STBD25 biodiesel reduced NOx emissions compared to the STBD25 biodiesel blend due to the catalytic charactistics of the nanoparticles.
$$\text{M}\text{g}.\text{C}\text{a}\text{O}+2{NO}_{2}\to {Mg\left({NO}_{2}\right)}_{2}+\text{C}\text{a}\text{O}$$
Figure 14, reveals that variation of the NOx emission of diesel, STBD25, STBD25 + 30ppm CaO, and STBD25 + 30ppm Mg doped CaO at full load conditions. As engine load increases, NOx emissions also increases due to increase in combustion characteristics and adiabatic flame temperature. The NOx emission of diesel, STBD25, STBD25 + 30ppm CaO, and STBD25 + 30ppm Mg doped CaO are 1010 ppm, 1224 ppm, 1311.53 ppm and 1105.47 ppm. The NOx emissions of the STBD25 + 30ppm CaO is higher than all other fuel belnds. NOx released while using STBD25 + Mg-CaO in CI engine was observed to decrease compared with STBD25 and STBD25 + CaO (by 10.72% and 18.64%) and increase compared with diesel (by 8.64%). Whereas an insignificant drop in NOx was observed while using STBD25 + Mg doped CaO compared with STBD25 + CaO and STBD25 despite a significant increase in HRR and BTE which might be due to the capture of excess oxygen by Mg during the combustion diffusion phase.
5.7 Effect of Mg-doped CaO Nano Additives on Smoke
Figure 15 shows the evolution of smoke emissions for diesel, STBD25, STBD25 + 30CaO and STBD25 + 30Mg-doped CaO under full load conditions and constant engine speed. For diesel and all other biodiesel blends, smoke emissions increase with engine speed. The effects of CI engine exhaust smoke are due to incomplete combustion, low oxygen utilization, fuel carbon content (compared to diesel), and rich mixing zones. Delayed ignition results in a large accumulation of fuel in the cylinder, slowing the oxidation of CO to CO2 and increasing soot concentrations. The premixed combustion stage burns a rich mixture for a short time, resulting in higher CO emissions than the diffusion stage (Mishra et al. 2017; Venu et al. 2019). Smoke emissions from biodiesel are generally lower than emissions from fossil fuels due to the built-in oxygen. Furthermore, the addition of nano-additives to biodiesel reduces smoke generation due to its catalytic activity. The effects of nano-additive reduce smoke due to additive surface area, good automation of fuel, complete combustion, and higher chemical activity (Annamalai et al. 2016). Nanoparticles increase the catalytic activity (combustion reaction) during the combustion process, resulting in less smoke. Figure 15 reveals that at full load conditions, the smoke emission of diesel, STBD25, STBD25 + 30ppm CaO, and STBD25 + 30ppm Mg doped CaO are 5.13 FSN, 3.97 FSN, 3.63 FSN and 3.32 FSN. The smoke was observed to decrease by 19.58%, 9.34% and 54.52% compared with STBD25 and STBD25 + CaO and diesel.