On the impact of using nanoparticles type additives on the altering of diesel fuel jet characteristics

Even though diesel-powered passenger cars and light-duty vehicles may not be sold in the European Union by 2035, possibly in most other countries will be. Truck diesel engines will remain in use after 2035 because electric trucks are not a viable alternative for goods transportation. This involves continuing research and development on diesel engines to reduce their pollutant emissions. Alternative (renewable) fuels with combustion improvers can diminish emissions at the source. Nanoparticle-based additives can enhance the efficiency and emissions of diesel engines through their intense catalytic activity and by improving the fuel injection process. However, their effect on the injection process has not been well tested and published in the dedicated literature. The use of nanoparticles mixed in small fractions with original diesel fuel influences the physicochemical properties of the fuel as well as the formation of fuel jets inside the combustion chamber. This research focuses on the opportunity of using different diesel fuel blends mixed with combustion improver additives such as MWCNT and CeO2 nanoparticles. In this sense, a simulation investigation was conducted using the AVL BOOST Hydsim software. The physicochemical properties of fuels were used to assess the macroscopic characteristics of the fuel spray, such as penetration, spray cone angle, and Sauter mean diameter. When increasing the nanoparticle dosage, the penetration and spray cone angle values are decreasing, while the Sauter mean diameter values are increasing. This tendency is present for both nanoparticle types considered.


Introduction
Since the way of managing the injection process inside the diesel engine cylinder has become increasingly important in terms of preserving power and limiting fuel consumption and emissions, attention has been drawn to the possibility of correlating specific aspects of combustion preparation and development to the physicochemical properties of the injected fuel.Thus, different studies investigated the relevance of using several types of nanoparticles as additives for different diesel fuel blends with varying effects on ignition characteristics, rate and duration of combustion, rate of heat release and transfer, and so on.Because of their surface and thermal characteristics, modest amounts of nanoparticles with metallic or organic structures might provide encouraging outcomes in terms of combustion process operation, lowering emissions concentrations across a broad range of engine testing regimes.
Various carbon-based nanoparticles are used to cause fuel droplet micro-explosions, resulting in a more efficient air-fuel mixture and managing the combustion flame temperature through their high latent absorbing heat [1].Consequently, the oxidation chemical reactions are accelerating, reducing the 1303 (2024) 012027 IOP Publishing doi:10.1088/1757-899X/1303/1/012027 2 amounts of unburned hydrocarbons (UHC) and carbon monoxide (CO) [2].Ardabili et al. [1] studied the characteristics of a natural gas diesel engine powered by water-emulsified (WE) diesel/biodiesel (D/B) blends (5% vol.biodiesel and 3% wt.water) containing 30 and 60 ppm aqueous carbon nanoparticles.At a low addition level, the tested fuel minimized hydrocarbon emissions by 8 to 40% under various engine operating loads.During an experimental one-droplet investigation, Ooi et al. [3] added 1000 mg/L graphite oxide (GO) to diesel and biofuel (B100).According to the data, adding GO to both fuels resulted in a 38% shorter ignition delay, a nearly 8% lower peak temperature, and a 12% shorter combustion duration.A similar experiment [2] was carried out on isolated droplet combustion for the MF15 fuel blend comprising 15% vol.2-methylfuran and 85% vol.neat diesel fuel with 25, 50 and 100 ppm multi-walled carbon nanotubes (MWCNT) amounts.Higher nanoparticle content reduced the occurrence of micro-explosions while increasing their intensity, as explained by the higher viscosity and surface tension of nanofuels compared to basic fuel.The greater thermal conductivity of the MWCNT provides a shorter autoignition delay, as well as a decrease in heat release and peak temperature, resulting in improvements in terms of combustion efficiency and emissions [1][2][3].
Metal nanoparticles form a solid layer structure in fuel.This structure improves the thermal conductivity of the mixture, reducing the autoignition delay, increasing the cetane number of the fuel, and therefore improving the ignition characteristics.For several reasons, cerium oxide (CeO2) is an excellent additive to utilize since it has a superior surface-volume ratio and a high oxygen content, delivering the necessary for the oxidation reactions of hydrocarbons as well as soot.It is also capable of reversing the chemical reaction by converting to cerous oxide (Ce2O3), allowing nitrogen oxide reduction [4].Bao et al. [5] performed an experimental investigation to examine the spray characteristics of DE20 fuel blend (20% vol.ethanol, 80% vol.diesel), DE20Ce25 (containing 25 ppm CeO2), DE20Ce50 (containing 50 ppm CeO2), and DE20Ce100 (containing 100 ppm CeO2) in a common rail direct fuel injection system.When compared to pure diesel, the spray penetration of DE20, DE20Ce25, DE20Ce50, and DE20Ce100 modified by -3%, -1.8%, -0.3%, and 9.5%, the spray cone angle changed by -3%, 4%, 2.8%, and 12.5%, and the spray area varied by -3%, -1.3%, 0.6%, and 31%.However, the benefits of each type of nano-additives stated above might be combined by using the metal-organic framework (MOF) crystalline nanoparticles, containing metal clusters with organic ligands [6].
The purpose of this study was to determine how the use of additives such as CeO2 and MWCNT nanoparticles affects the physicochemical properties of certain fuels, as well as the effects on certain macroscopic characteristics of the fuel jet, such as penetration, spray cone angle, and Sauter mean diameter.

Simulation Details
Following a literature review, we decided to compare the effects on the macroscopic characteristics of the fuel jet produced by adding CeO2 nanoparticles to the DE20 fuel blend (80% diesel and 20% ethanol) and MWCNT nanoparticles to the MF15 fuel blend (85% vol.diesel fuel and 15% vol.2-methylfuran).
The AVL Hydsim tool is designed to simulate fuel injection.Some fuel jet characteristics, such as penetration, spray cone angle, and Sauter mean diameter, may be computed using this tool, which are key factors related to the evolution of the air-fuel mixture formation [8].The software provides the option of selecting a set of relations to be used for calculating these parameters.Varde-Popa formulas were employed for penetration and Sauter mean diameter, whereas the Sitkei formula was utilized for spray cone angle.In work [9], the authors of this paper have provided a thorough explanation of them.
Using the AVL Hydsim tool it was successfully implemented a simulation model for the Delphi injection system of the UTB 2404055 tractor diesel engine as in prior research [9][10][11].The model was developed to examine the impacts of DE20 and MF15 fuels containing CeO2 and MWCNT nanoparticles, respectively.
The AVL Hydsim simulation tool necessitates the physicochemical properties of the fuel, such as bulk modulus, density, kinematic viscosity, and surface tension.Their values may be determined by utilizing the software's database or by introducing these properties as variable or constant values.The authors of this study have considered constant fluid properties for this simulation investigation.The information presented by the authors of the studies [2,5] on the physicochemical properties of pure DE20 and MF15 fuels or those containing 25, 50, and 100 ppm of CeO2 and MWCNT nanoparticles was used in the simulation model.In comparison to the reference fuels (DE20 and MF15), six additional nanofuels with varying CeO2 or MWCNT fractions were considered and labelled as DE20CeXX and MF15CXX where XX denotes the CeO2 or MWCNT nanoparticle dosage in ppm.These data are presented in Table 1.The pressure and temperature values obtained for the engine's maximum power operating condition, characterized by 2400 rpm speed, are shown in Figure 1.These values are required by the AVL Hydsim software to establish the in-cylinder conditions during fuel injection.The in-cylinder pressure was measured experimentally for DE20 fuel, and the associated global cylinder charge temperature was computed by simulation using an engine model created in the AVL Boost v2021.1 program, considering the same DE20 fuel.First, simulation tests were performed for DE20 fuel to assure model calibration for the engine's maximum power operating condition, which is characterized by a 2400 rpm speed.Figures 3 and 4 show a proper match between the simulated and experimental shapes of the injection characteristic of DE20 in terms of needle lift and line pressure, using a single injection pulse starting at 5.5 CA° before TDC.Both injection-defining parameter variations, maximum needle lift, and maximum rail-injection pressure are quite reproducible with our experimental data, with 0.35 mm needle lift and nearly 700 bar injection pressure.Because MF15 is a denser fuel than DE20, with a greater kinematic viscosity, the start of injection occurs earlier by 0.5 degrees.When utilizing biodiesel instead of diesel, a similar pattern is observed.The maximum needle lift and line pressure values are unchanged when using the MF15 fuel.

Simulation Results
Figure 4 depicts the penetration values for DE20, MF15, and six nanofuels with varying doses of CeO2 or MWCNT type nanoparticles depending on the crankshaft position between the start of injection (SOI) and start of combustion (SOC) moments.Penetration values increase with time between the SOI and SOC moments, thus the maximum values are reached at SOC.As the dosage of nanoparticles rises, there is a trend for maximum penetration to decrease.Thus, the minimum values were obtained for DE20Ce100 and MF15C100 fuels with decreases of 8.1% and 11.5%, respectively, compared to the reference fuels (DE20 and MF15).At -5 CA°, the penetration values for all fuels investigated are similar; for DE20, the value is 0.5% lower than the value obtained for MF15, while the value obtained for DE20Ce100 is 2.7% greater than the value obtained for MF15C100.The maximum penetration values obtained for DE20 and DE20Ce100 are 41.7% and 47.2% higher, respectively, than the values obtained for MF15 and MF15C100.Figure 5 illustrates the spray cone angle values for DE20, MF15, and six nanofuels with varying concentrations of CeO2 or MWCNT type nanoparticles depending on the crankshaft position between the SOI and SOC moments.Spray angle values increase with time between the SOI and SOC moments, thus the maximum values are reached at SOC.The spray cone angle values decrease as the nanoparticle concentration increases.Thus, the minimum values were reached for DE20Ce100 and MF15C100 fuels, with average reductions of 9.5% and 13%, respectively, as compared to the reference fuels (DE20 and MF15).The spray cone angle values found for DE20 are on average 67.2% greater than the values obtained for MF15.At -5 CA°, the values obtained for DE20 and DE20Ce100 are 58.9% and 64.8% higher than those obtained for MF15 and MF15C100, while at SOC moment, the differences are greater, more precisely 67% and 74.2%.Thus, the highest values were achieved for DE20Ce100 and MF15C100 fuels, with average increases of 6.1% and 6.4%, respectively, above the reference fuels (DE20 and MF15).The Sauter mean diameter values obtained for MF15C100 are on average 30.2%greater than those achieved for DE20Ce100.At -5 CA°, the values obtained for DE20 and DE20Ce100 are 17.9% and 17.8% higher than those obtained for MF15 and MF15C100, while at SOC moment, the differences are greater, more precisely 22.7% and 23.1%.

Conclusions
The findings of this study, which describes the evolution in time of the macroscopic spray characteristics of two different types of fuels, namely diesel-ethanol and its blends with nano-CeO2, and dieselmethylfuran and its blends with multi-walled carbon nanotubes at ppm levels used in a high-pressure injection system equipped with a multi-holes injector, are as follows:  The addition of nanoparticles to the original reference fuels in small fractions has no significant effect on the hydraulic behaviour of the injection system. Essential for the formation of the air-fuel mixture, the macroscopic spray characteristics, such as penetration, spray cone angle, and Sauter mean diameter, reveal almost similar timedependent variations regardless of the original fuels and the presence of nanoparticles. The presence of nanoparticles at a maximum concentration of 100 ppm in both original reference fuels reduces penetration and spray cone angle while increasing Sauter mean diameter. The Sauter mean diameter, which is essential for the vaporization process, decreases in time in a quasi-linear trend, with the greatest values recorded for the fuels dopped with the nanoparticles chosen in the highest fractions.Although nanoparticles influence fuel's physiochemical properties, they have a higher impact on the fuel jet because of the secondary atomization effect and the heat sink effect, which stimulate accelerated vaporization.Even though the study only considered the impact of constant fuel physicochemical properties into account, some interesting findings were obtained.
This work emphasized that, if constant fuel physicochemical properties are taken into account, the development of the fuel jet within the combustion chamber during the auto-ignition delay may be reasonably determined using fuel jet parameters.
An experimental investigation will be conducted in the future to better understand how the inclusion of nanoparticles in fuel affects spray development.

Figure 1 .
Figure 1.The cylinder pressure and temperature variations relative to the crankshaft position for DE20 fuel.

Figure 2 .
Figure 2. The variation of needle lift vs crankshaft position for DE20 fuel (experimental and simulation) and MF15 fuel (simulation).

Figure 3 .
Figure 3.The variation of line pressure vs crankshaft position for DE20 fuel (experimental and simulation) and MF15 fuel (simulation).

Figure 4 .
Figure 4. Spray penetration variation vs the crankshaft position.

Figure 5 .
Figure 5. Spray cone angle variation vs the crankshaft position.

Figure 6
Figure6presents the Sauter mean diameter values for DE20, MF15, and six nanofuels with different doses of CeO2 or MWCNT type nanoparticles depending on the crankshaft position between the SOI and SOC moments.Sauter mean diameter values decrease with time between the SOI and SOC moments, thus the minimum values are reached at SOC.The Sauter mean diameter values rise as the nanoparticle dose increases.Thus, the highest values were achieved for DE20Ce100 and MF15C100 fuels, with average increases of 6.1% and 6.4%, respectively, above the reference fuels (DE20 and MF15).The Sauter mean diameter values obtained for MF15C100 are on average 30.2%greater than those achieved for DE20Ce100.At -5 CA°, the values obtained for DE20 and DE20Ce100 are 17.9% and 17.8% higher than those obtained for MF15 and MF15C100, while at SOC moment, the differences are greater, more precisely 22.7% and 23.1%.

Figure 6 .
Figure 6.Sauter mean diameter variation vs the crankshaft position.