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Review

Improving Biodiesel Atomization Performance in CI Engines: A Review of Spray Behavior, Droplet Impingement, and Advanced Techniques

by
Zehao Feng
1,*,
Junlong Zhang
1,
Jiechong Gu
1,
Xianyin Leng
1,
Zhixia He
1 and
Keiya Nishida
2
1
Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China
2
Department of Mechanical System Engineering, Hiroshima University, 1-4-1, Kagamiyama, Higashi-Hiroshima 739-8527, Japan
*
Author to whom correspondence should be addressed.
Processes 2025, 13(8), 2527; https://doi.org/10.3390/pr13082527
Submission received: 23 July 2025 / Revised: 6 August 2025 / Accepted: 8 August 2025 / Published: 11 August 2025
(This article belongs to the Special Issue Low/Zero Carbon Fuels Production, Transportation, Storage and Utilization)
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Abstract

The escalating challenges of greenhouse gas emissions, coupled with the severe depletion of oil reserves and the surging global energy demand, have emerged as critical concerns requiring urgent attention. Against this backdrop, biodiesel has been recognized as a viable alternative fuel for compression ignition (CI) engines. The primary objective of this research is to review the application of biodiesel in CI engines, with a focus on enhancing fuel properties and improving atomization performance. This article examines the spray and atomization characteristics of biodiesel fuels and conducts a comparative analysis with diesel fuel. The results show that biodiesel has a longer spray tip penetration, smaller spray cone angle, larger Sauter mean diameter (SMD) and faster droplet velocity due to its higher viscosity and surface tension. Blending with other fuels, such as ethanol, butanol, dimethyl ether (DME) and di-n-butyl ether, results in reduced viscosity and surface tension in these mixed fuels, representing a simple and effective approach for improving biodiesel atomization performance. A comprehensive analysis of spray and droplet impingement is also conducted. The findings reveal that biodiesel exhibits a higher probability of fuel–wall impingement, suggesting that future research should focus on two key directions: first, developing combined strategies to enhance impact-induced secondary atomization while minimizing fuel deposition; and second, investigating single-droplet impingement, specifically that of microscale biodiesel droplets and blended fuel droplets under real engine operating conditions. This paper also presents several advanced techniques, including air-assisted atomization, dual-fuel impingement, nano-biodiesel, and water-emulsified biodiesel, aimed at mitigating the atomization limitations of biodiesel, thereby facilitating the broader adoption of biodiesel in compression ignition engines.

1. Introduction

The escalating global energy crisis, driven by the rapid depletion of finite fossil fuel reserves and fluctuating oil markets, has underscored the urgent need for sustainable energy alternatives [1,2,3]. Concurrently, growing environmental concerns, particularly climate change driven by greenhouse gas emissions and deteriorating air quality caused by particulate matter and NOx pollutants from conventional diesel combustion, have significantly threatened public health and agricultural productivity [4,5,6,7,8]. These pressing challenges have accelerated global research efforts toward sustainable renewable fuel alternatives. Wind, solar, zero-carbon fuels (ammonia [9], hydrogen [10]), carbon-neutral fuels (biomass and biofuel [11,12,13], methanol [14,15]), and low-carbon fuels (natural gas [16]) have attracted significant research attention due to their potential to address energy and environmental challenges.
Biodiesel, derived from renewable feedstocks such as vegetable oils, waste cooking oil, and algae, offers a carbon-neutral solution by reducing lifecycle CO2 emissions by 50~80% compared to traditional diesel. Figure 1 illustrates that following solar and wind, biofuels are anticipated to contribute 10% of global renewable energy by 2050 [17]. Compression ignition (CI) engines are major energy consumers and significant emitters of pollutants. Among the diverse strategies for reducing fossil energy consumption in diesel engines, biodiesel stands out as an effective means. CI engines running on biodiesel exhibit improved overall lubricity without the need for engine modifications. Extensive research has been carried out to examine the performance of diesel engines fueled with biodiesel and its blends [18,19,20]. Compared with diesel fuel, biodiesel has a lower aromatic hydrocarbon content, contains no sulfur, and its ignition delay remains essentially unchanged. Adding a small amount of biodiesel to diesel engines can effectively reduce exhaust pollutants, while the original engine’s power performance remains almost unchanged [21]. Allami et al. [22] investigated the diesel engine performance by using biodiesel and biodiesel-blended fuels. They reported that when the blending ratio of biodiesel ranges from 11% to 20%, it can significantly improve engine performance and reduce emissions of HC, CO, and NOx. Uyumaz et al. [23] investigated the combustion performance and exhaust emission characteristics of poppy oil in a CI engine, and found that compared with diesel fuel, poppy oil biodiesel not only exhibits considerable engine performance but also reduces exhaust pollutant emissions.
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Figure 1. A forecast map showing the use of renewable energy in 2050 [17].
Figure 1. A forecast map showing the use of renewable energy in 2050 [17].
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However, from the perspective of liquid physical properties, fuel injection and atomization are significantly influenced by viscosity, density, and surface tension [24,25,26,27]. For biodiesel, these properties are higher than those of diesel fuel, resulting in lower gas entrainment mass during the injection process, poor mixing with ambient gas, and inferior atomization [28,29]. Poor fuel atomization leads to degraded combustion and emission performance, making it highly necessary to conduct research on the in-cylinder fuel injection and atomization processes of biodiesel in CI engines [30,31,32]. The macroscopic characteristics of a fuel jet include the jet breakup length and spray morphology evolution [33,34,35], while the atomization quality is typically characterized by key parameters such as the droplet size distribution and velocity profile [36,37,38,39]. Mohan et al. [40] investigated the relationship between the physical properties and spray characteristics of biodiesel and diesel fuel. The results showed that due to the higher density and kinematic viscosity of biodiesel, the density of the liquid core in its spray is greater than that of diesel fuel. Additionally, the volume occupied by the liquid phase of biodiesel is reduced, and the ratio of the liquid core area to the spray projected area of biodiesel is also smaller than that of diesel fuel. Lee et al. [30] examined the atomization and combustion behaviors of biodiesel and biodiesel-blended fuels. They noted that the atomization performance of biodiesel blends is inferior to that of diesel fuel, attributed to their higher viscosity and surface tension.
Impingement between the liquid fuel spray and solid wall is commonly observed in direct-injection CI engines. When a spray impinges on a wall, secondary breakup occurs, significantly influencing the spray morphology, liquid-phase particle size, spatial concentration distribution, and subsequent combustion processes [41,42]. Tripathi et al. [43] found that the probability of wall impingement is higher with palm acid oil (PAO) biodiesel than base diesel due to higher spray penetration of biodiesel. The process of spray impingement is the macroscopic manifestation of numerous droplets colliding with the wall surface [44,45]. Therefore, it is necessary to conduct in-depth studies on the deformation and phase change mechanisms of biodiesel droplets impinging on solid walls under the actual operating conditions of high-pressure in-cylinder direct injection CI engines, summarize the behavioral laws and thermodynamic mechanisms of droplet wall impingement, and statistically analyze the distribution characteristics of secondary droplets. This is of great significance for improving the secondary atomization performance [46,47].
The main purpose of this paper is to provide a comprehensive review of the application of biodiesel in CI engines, mainly focusing on exploring the technical pathways to improve the atomization performance of biodiesel. First, this paper introduces different types of biodiesel fuel and their feedstock, comparing their physical properties with those of diesel and explaining the reasons for the poor atomization performance of biodiesel in Section 2. Second, the spray and atomization characteristics of biodiesel fuels are examined in Section 3. Then, the spray and droplet impingement of biodiesel fuels are analyzed, and the future research directions are identified in Section 4. Finally, some advanced techniques for improving biodiesel atomization are presented in Section 5. The aim is to provide preliminary information to support the work of industrial practitioners, researchers, and engineers working on or interested in biodiesel.

2. Biodiesel Production and Fuel Properties

2.1. Biodiesel Production Technologies

Biodiesel production primarily relies on renewable biomass resources, including oil-bearing plants, algae, fats, CO2, and advanced solar energy-derived materials [3,13,48]. These feedstocks can be categorized into four groups based on their origin. The first generation uses edible crop oils (such as soybean oil, palm oil and rapeseed oil) as raw materials to produce fatty acid methyl esters (FAME) through transesterification [49,50,51]. Bioethanol and butanol are also first-generation biofuels, derived from fermentable carbohydrates and sugars in food-based feedstocks such as rice, wheat, barley, potatoes, corn, sugarcane, and vegetable oil. Though the technology is mature, it faces the issue of “competing with food crops for land”. The second generation shifts to non-edible crops including non-food crops, wheat straw, corn, wood waste, energy crops, non-edible vegetable oil and waste oils. It improves the transesterification process, and develops hydro-processing (HVO), thereby enhancing sustainability [52]. The third generation utilizes microalgae and microbial oils, achieving high yields without occupying arable land, but the cost remains high [53]. The fourth generation explores cutting-edge technologies such as artificial photosynthesis and electro-synthesis, directly producing fuels from CO2. Fourth-generation biofuels hold promising prospects for improved production efficiency, thanks to the abundance, cost-effectiveness, and accessibility of their raw materials [54]. The characteristics of different generations of biodiesel are summarized in Table 1. The future development directions will focus on feedstock diversification, low-carbon processes, and policy support [55].
Table 1. Different generations of biodiesel and their characteristics.
Table 1. Different generations of biodiesel and their characteristics.
GenerationFeedstockProduction MethodAdvantagesChallenges
1st [56,57]Soybean, Rapeseed, Palm oilAlkali transesterification (FAME)Mature technologyFood competition, Deforestation
2nd [58]Waste cooking oil, Animal fatAcid/enzymatic transesterification, HVOWaste utilization, Drop-in fuelCollection logistics, HVO cost
3rd [59]Microalgae, Yeast oilsExtraction and Transesterification/hydrogenationHigh yield, CO2 absorptionHigh production costs
4th [60]CO2, Water, SunlightArtificial photosynthesis/electrosynthesisCarbon-negative potentialLab-scale, Low efficiency

2.2. Fuel Properties

Analyzing the atomization characteristics of biodiesel fuels, the spray and atomization mechanisms are substantially different from those of conventional diesel fuel due to its intrinsic physical properties such as liquid density, viscosity, and surface tension. In general, the mass flow rate through the injector nozzle varies with the square root of the fuel density, influencing spray penetration and the interaction with ambient gas, thereby altering secondary atomization dynamics [61,62]. Higher viscosity resists liquid breakup due to reduced jet instability and suppressed surface wave growth, leading to larger primary droplets and slower ligament disintegration. This elevated viscosity also causes greater energy dissipation, converting more kinetic energy into heat rather than the surface energy needed for effective droplet fragmentation, which may reduce atomization efficiency [63,64]. Higher surface tension increases the energy barrier for liquid sheet breakup during injection, resulting in larger initial droplet formation due to stronger molecular cohesion forces [65,66]. This effect modifies the spray morphology by reducing the spray cone angle and limiting air entrainment, ultimately decreasing fuel–air mixing efficiency [67,68]. The elevated surface tension also stabilizes droplets against secondary breakup, extending their lifetime in the combustion chamber.
Table 2 lists the physical properties of different types of biodiesel fuel, in comparison with diesel fuel. In practice, the significance of the fuel density for atomization is diminished because most liquid fuels experience only minor differences in fuel density. From this viewpoint, the influence of biodiesel density on spray droplet size during the atomization process is quite small.
Table 2. Physical properties of different types of biodiesel fuel.
Table 2. Physical properties of different types of biodiesel fuel.
FeedstockDensity
kg/m3		Kinematic Viscosity at 40 °C
mm2/s		Surface Tension
mN/m		Lower Heating Value
MJ/kg		Cetane NumberReference
Diesel820~8502~3.525~3042.7~43.540~55
Waste cooking oil8784.43238.8551.34[69]
Soybean biodiesel8874.034.437.5351[70]
Palm oil874.45.5326.240.0364.2[71]
Fatty acid methyl ester858.44.7131.2--[72]
Oxidized jatropha biodiesel873.74.3729.720.49-[73]
Hydrogenated catalytic biodiesel785.96.0827.244-[74]
Cottonseed biodiesel8644.1432.436.852[75]

3. Spray and Atomization Characteristics of Biodiesel

3.1. Macroscopic Spray Characteristics

Macroscopic spray characteristics, including spray morphology, spray tip penetration and spray core angle, are key factors for engine combustion chamber design, fuel injector selection, and control system development. They are influenced by biodiesel’s physical properties, which differ from those of conventional diesel. Figure 2 presents a comparison of the spray evolution characteristics between diesel fuel and biodiesel fuels derived from soybean oil (BD100S) and canola oil (BD100C) [76]. In this figure, SOI means ‘start of injection’. As shown in the spray images of the three fuels, under the same injection conditions, the differences in the shape and magnitude of the spray evolution between the biodiesels and diesel fuel are quite small. However, quantitative analysis shows that higher momentum from biodiesel injection contributes to deeper penetration, which can improve air–fuel mixing but may cause wall impingement in small combustion chambers. Biodiesel has a narrower spray cone angle which may reduce air entrainment, affecting combustion efficiency and emissions. Similar results have also been found in Refs. [71,77].
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Figure 2. Comparison of spray evolution between diesel and biodiesel fuels [76].
Figure 2. Comparison of spray evolution between diesel and biodiesel fuels [76].
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3.2. Droplet Size and Velocity

In combustion engines, optimal fuel spray atomization plays a pivotal role in achieving homogeneous fuel distribution. This uniform mixing effectively prevents the formation of localized fuel-rich or fuel-lean zones, thereby substantially reducing incomplete combustion phenomena and effectively suppressing soot formation. The comprehensive assessment of atomization quality including the primary breakup of the fuel jet [78,79,80], Sauter mean diameter (SMD), and droplet size and velocity distribution [81,82,83] has been performed by combing experimental measurements (Phase Doppler Particle Analyzer [84], Particle Image Analysis [85], Particle Image Velocimetry [86], X-ray Fluorescence Tracking [87]) with computational simulations [88] under various operating conditions.
The SMD values of biodiesel blends derived from both soybean oil and rapeseed oil consistently exceed those of conventional diesel fuel, as demonstrated in Figure 3 [30]. Consequently, the diesel droplets decelerate more rapidly due to the drag force exerted by the ambient gas. This phenomenon primarily stems from the fundamental relationship between droplet breakup characteristics and the Weber number [89], where biodiesel’s inherently higher surface tension results in lower Weber numbers that directly contribute to a larger SMD [90]. Concurrently, the elevated kinematic viscosity of biodiesel blends increases frictional forces at the nozzle interface, leading to reduced injection velocities that further exacerbate poor atomization performance. These combined effects collectively manifest in the observed size distribution patterns, with biodiesel blends consistently exhibiting poorer atomization characteristics compared to baseline diesel across all tested injection conditions.
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Figure 3. Comparison of droplet SMD between diesel and biodiesel blended fuels (0.3 mm nozzle diameter, 60 MPa injection pressure): (a) rapeseed biodiesel, (b) soybean biodiesel [30].
Figure 3. Comparison of droplet SMD between diesel and biodiesel blended fuels (0.3 mm nozzle diameter, 60 MPa injection pressure): (a) rapeseed biodiesel, (b) soybean biodiesel [30].
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3.3. Factors Influencing Spray Evolution and Atomization

The atomization characteristics of biodiesel fuels are governed by a complex interplay of multiple physicochemical and operational factors that collectively determine the resulting spray morphology and droplet size distribution. At the fundamental level, the higher viscosity and surface tension of biodiesel profoundly influence the primary breakup and atomization process. Therefore, blending with other fuels to reduce the viscosity and surface tension of mixed fuels represents an effective technical approach for improving biodiesel atomization performance, as listed in Table 3.
Table 3. Previous studies that researched how ether fuels affect biodiesel (↑ Increases; ↓ Decreases).
Table 3. Previous studies that researched how ether fuels affect biodiesel (↑ Increases; ↓ Decreases).
Base FuelAdditive TypePerformanceReference
Soybean biodieselDi-n-butyl ether (DBE):
DBE15 (15% DBE + 75% biodiesel, by vol.)
DBE30		Spray tip penetration: ↓
Spray core angle: ↑
SMD: 9.1% ↓, 13.1% ↓		[91]
Waste cooking oilAcetone-butanol-ethanol (ABE):
ABE10 (10% ABE + 90% biodiesel, by vol.)
ABE20
ABE30		Spray tip penetration: ↓
Spray core angle: ↑		[92]
Hydrogenated catalytic biodieselEthanol:
E15 (15% ethanol + 85% biodiesel, by vol.)
E30		Spray tip penetration: ↓
Spray core angle: ↑		[74]
Soybean biodieselEthanol:
BDE20 (20% ethanol + 80% biodiesel, by vol.)		SMD: 9%~17% ↓[93]
Soybean biodieselDimethyl ether (DME):
B75 (25% DME + 75% biodiesel, by wt.)
B50
B25		Spray tip penetration: ↓
Spray core angle: ↑		[94]
20% Castor biodiesel + 80% diesel, by vol.Butanol and 1-Butox
BDT5 (20% biodiesel + 75% Diesel + 5% butanol, by vol.)
BDB5 (20% biodiesel + 75% diesel + 5% 1-Butox, by vol.)		Spray tip penetration: 2%~3% ↓
Spray core angle:
2.8% ↑, 5.3% ↑		[95]
The atomization process is further regulated by a range of operational parameters. Higher injection pressure and appropriate injection duration provide more energy to effectively overcome fuel surface tension, promoting the breakup of fuel droplets and enhancing atomization. Conversely, insufficient energy leads to coarse droplets and inadequate atomization [96,97,98]. Ambient pressure alters the atomization process by influencing interactions between the spray and the surrounding gas (e.g., air resistance, mixing rate). Higher ambient pressure strengthens the shearing force of gas on droplets, facilitating their breakup, while low-pressure environments may cause excessive spray diffusion and reduced atomization stability [99,100]. Nozzle type [101,102] or geometry parameters [103,104,105], including orifice diameter, length-to-diameter ratio, and internal surface finish, introduce additional variability by controlling the development of cavitation and turbulence [106] within the injector, which subsequently affects the emerging spray structure. A smaller orifice diameter increases fuel exit velocity and turbulence, making droplets more susceptible to breakup. The length-to-diameter ratio affects fuel flow within the nozzle: an optimal ratio reduces flow losses, enhances turbulence, and promotes cavitation. Furthermore, fuel temperature [107] emerges as a critical control parameter as it directly modulates viscosity and surface tension, with preheating demonstrating particular efficacy for biodiesel fuels in achieving more favorable atomization characteristics. The complex interactions between these factors ultimately determine the temporal evolution of the spray, governing subsequent evaporation and mixing processes. We also note that these factors play a vital role in the development of spray and atomization models [108,109,110,111], which serves as a bridge connecting basic theories and engineering applications. The models require more reliable experimental results for validation to serve the application of biodiesel fuels.

4. Spray and Droplet Impingement

It is a common phenomenon in CI engines that spray impinges the combustion chamber walls at high velocity. After the fuel spray hits the wall, it spreads on the wall surface to form an oil film, and at the same time, the shape of the spray body changes. The process of spray impinging on the wall is a macroscopic manifestation of a large number of droplets colliding with the wall, and droplet–wall impingement is the basic element in the complex process of spray impingement. The behaviors of these droplets upon impact, such as deposition and breakup, are critical to the engine’s combustion performance and emission characteristics. Secondary atomization induced by the impact can enhance the fuel–air mixing process, which in turn, helps reduce nitrogen oxide emissions and improve combustion efficiency [112,113]. However, the droplet deposition and thicker fuel film formation [114,115] upon wall impingement may induce knocking. This behavior can also lead to incomplete combustion and increased particulate emissions, particularly at lower surface temperatures [116]. Zhai et al. [117] have created schematic diagrams of spray flame development and pollutant formation for flat wall impingement and 2-D piston impingement, as shown in Figure 4.
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Figure 4. Spray flame development and pollutant formation for flat wall impingement and 2-D piston impingement (OH* means Hydroxyl radical) [117].
Figure 4. Spray flame development and pollutant formation for flat wall impingement and 2-D piston impingement (OH* means Hydroxyl radical) [117].
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The probability of wall impingement is higher with biodiesel than with base diesel because of the former’s longer spray penetration. Additionally, biodiesel spray impingement is more prone to fuel deposition and film formation than diesel due to its higher viscosity and surface tension, which will lead to combustion inefficiencies and emissions [43,118]. Studies also show that the combined strategy of mild retarded injection timing, compression ratio and EGR could reduce both NOx emissions and the probability of wall impingement [43].
Due to the complexity of spray droplets impacting on the wall, single-droplet impacting is usually employed in literature studies. The dynamic behavior and the heat and mass transfer between droplets and the wall during impingement are influenced by multiple factors, including the droplet physical properties and motion state, wall surface characteristics, and environmental conditions [47,119]. These parameters exhibit linear or nonlinear coupling relationships. To achieve high-fidelity simulations of spray–wall interactions under various engine operating conditions, spray impingement models must incorporate scientifically accurate descriptions of fundamental droplet phase change, aerodynamics, and transport processes [120].
In recent years, research on the dynamic characteristics of single-droplet wall impingement in the context of internal combustion engines has mainly focused on experimental studies on the influence mechanisms of droplet properties [121], liquid film thickness [122], wall morphology and roughness [123], and wall temperature [120]. Additionally, studies have been conducted on a series of sub-models within the droplet–wall impingement dynamic model, such as spreading models based on the law of similarity and energy conservation [124], and the critical conditions for droplet splashing and transition [125]. These efforts aim to meet the requirements of different fuel types and combustion chamber walls in internal combustion engines. However, most of the existing single-droplet wall impingement experiments in the literature were conducted under atmospheric conditions and using millimeter-scale droplets. In the actual working process of CI engines, the impingements occur in high-temperature and high-pressure environments, and the size of droplets is mostly distributed at the micrometer scale. The ambient gas and droplet size exert significant influences on the mass and heat transfer processes during impingement, which in turn, lead to changes in dynamic characteristics [126,127].
Sen et al. [128] examined the impact dynamics of a biofuel droplet at room temperature within the Weber numbers range of 20~570. Dong et al. [129] experimentally explored the impact dynamics of binary droplets composed of biodiesel and ethanol (ethanol concentrations spanning 10~40%) on a heated stainless-steel surface (temperatures ranging from 25 to 350 °C), under Weber numbers between 40 and 230. Figure 5 presents the typical dynamic behaviors, and the results indicate that higher viscosity reduces droplet spreading velocity and increases the critical Weber number required for splashing, often leading to thicker liquid films upon wall impact. Additionally, biodiesel’s elevated surface tension enhances droplet stability, hindering secondary atomization induced by the impingement. Future research should focus on the impingement of microscale biodiesel and blending fuel droplets on a heated wall under real engine operating conditions.
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Figure 5. The dynamic behaviors of biodiesel droplet impingement: (a) camelina biofuel [128]; (b) methyl oleate droplets with different ethanol concentrations [129].
Figure 5. The dynamic behaviors of biodiesel droplet impingement: (a) camelina biofuel [128]; (b) methyl oleate droplets with different ethanol concentrations [129].
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5. Advanced Techniques for Improving Biodiesel Atomization

5.1. Air-Assisted Atomization

Air-assisted atomization is a promising technology to enhance the spray characteristics. In this system, high-velocity air interacts with the liquid fuel stream, generating intense shear forces that destabilize the fuel jet and promote rapid breakup into finer, more uniform droplets [130,131,132]. This mechanism is especially advantageous for biodiesel, which typically suffers from higher viscosity and surface tension compared to conventional diesel. Studies [133,134] using Phase Doppler Anemometry (PDA) and CFD simulation showed that air-assisted atomization effectively reduces the SMD of biodiesel sprays. The influences of ambient pressure, fuel injection duration, and ambient temperature on spray characteristics of biodiesel with air-assisted atomization injection are also investigated by numerical simulation [135]. The air-assisted atomization injector assembly, simulation model, results and application are shown in Figure 6. Ma et al. [136] further investigated the droplet velocity and size characteristics of biodiesel in an air-assisted pressure swirl atomizer under different oil pressures and air volumes. These findings offer critical insights into the fundamental atomization behavior of biodiesel, establishing a scientific basis for optimizing spray parameters in CI engines, although challenges like nozzle wear and system complexity persist.
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Figure 6. The air-assisted atomization injector and its application [135].
Figure 6. The air-assisted atomization injector and its application [135].
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5.2. Dual-Fuel Impingement

Dual-fuel impingement systems typically involve injecting two fuels (biodiesel and a liquid fuel like ethanol or biodiesel and a gaseous fuel like natural gas) through separate nozzles or a single multi-orifice nozzle. The collision of fuel jets or fuel–gas streams creates shear forces that disrupt the fuel into smaller droplets [137,138]. The atomization mechanism of spray impingement consists of two stages: the fragmentation process where the spray interacts with the ambient gas prior to impingement, and the impingement process between the two sprays. Due to the short distance before impingement, the impingement process in the second stage plays a dominant role in spray atomization. Spray impingement leads to frequent droplet collisions, and the outcomes of these collisions such as fragmentation and coalescence, are influenced by environmental conditions, injection parameters, and fuel properties. Zhang et al. [139] experimentally and numerically investigated the macroscopic and microscopic characteristics of n-butanol/biodiesel spray collision, and the impinging spray behaviors are shown in Figure 7. They observed that butanol–biodiesel dual sprays with optimized injection delay times (0.5~2 ms) produced SMD values 15~20% smaller than single biodiesel sprays. This is attributed to the intense mixing and secondary breakup induced by jet collision. As follows, they explored the cross-impingement characteristics at small (10%), middle (30%), and large (50%) biodiesel–butanol blended proportions [140]. They found that the cross-impingement is likely to decrease the spray–wall impingement owing to a change in the diffusion direction, and the biodiesel blended with a higher proportion of n-butanol presents faster vapor-phase diffusion, which promotes fuel–gas mixing. Dual-fuel impingement represents a breakthrough approach for improving biodiesel atomization, where controlled spray collisions effectively overcome viscosity limitations. This technique necessitates systematic investigations into impinging spray dynamics to fully characterize the atomization mechanisms and optimize operational parameters for biodiesel applications.
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Figure 7. The behavior of n-butanol/biodiesel dual-fuel impinging sprays [139].
Figure 7. The behavior of n-butanol/biodiesel dual-fuel impinging sprays [139].
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5.3. Nano-Biodiesel

Nanofluids represent an advanced class of solid–liquid composite materials, comprising nanoscale particles (NPs, typically 1~100 nm in diameter) uniformly dispersed in a base liquid [141]. Figure 8 presents several advantages that are linked with the addition of NPs as fuel additives [142]. When this principle is applied to biodiesel blends, the resulting fuel mixture is called nano-biodiesel. Studies [143,144,145] showed that the integration of metal-based additives including Mn, Ni, Mg, and Co and specific oxygenated additives into biodiesel blends leads to reductions in viscosity, density, and flash point. Conversely, such incorporation enhances the oxygen content of the blends. NPs also enhance atomization quality by introducing disturbances that promote the breakup process, thereby widening the spray cone angle, increasing the penetration length, and improving fuel–air mixing [146,147]. The SMD found for D80B20A100 (80% by vol. of diesel, 20% by vol. of biodiesel and 100 ppm of alumina) was 14.3 μm, which is almost comparable with neat diesel (13.1 μm) and much better than D80B20 (17.5 μm) [148]. However, it is crucial to emphasize that most NPs do not fully combust with the fuel and may raise additional environmental concerns when released.

5.4. Water-Emulsified Biodiesel

Water-emulsion fuels are emulsions of water and flammable liquids, typically oil [149,150,151]. Water-emulsified biodiesel is an emulsified fuel system in which water is uniformly dispersed in biodiesel in the form of tiny droplets, and emulsifiers are usually required to maintain its stability. From the perspective of performance, the introduction of water can improve fuel atomization through the “micro-explosion effect”, as shown in Figure 9 [152]. During combustion, the pressure generated by the vaporization of water can promote further breakup of biodiesel droplets, thereby improving combustion efficiency. The spray and atomization performance of biodiesel derived from castor oil and its emulsion with 15% water under an environment temperature of 673K was investigated by Lin et al. [153]. They found that the SMD of the emulsion decreased significantly, coupled with the expanded spread of the spray plume, indicating a greater volume of smaller droplets dispersed in the chamber air. At the same time, the heat absorption property of water evaporation helped reduce the combustion temperature and decrease the emission of pollutants such as nitrogen oxides (NOx) [154]. Currently, findings on emulsions have generally been derived from short-term steady-state and transient tests, long-term durability tests are imperative to examine the corrosive effects of prolonged use of water–biodiesel emulsions on engine components, particularly the fuel injection system.
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Figure 9. Mechanism of a micro-explosion caused by water in fuel [152].
Figure 9. Mechanism of a micro-explosion caused by water in fuel [152].
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5.5. Comparative Discussion of Each Atomization Enhancement Method

The various biodiesel atomization enhancement methods present distinct trade-offs. Air-assisted atomization offers good droplet breakup and emission reduction at moderate cost, while fuel blending provides the simplest and most scalable solution despite limited effectiveness. Nano-biodiesel demonstrates superior atomization performance but raises concerns about nanoparticle pollution and high production costs. Water-emulsified biodiesel reduces NOx emissions but faces corrosion and stability challenges. Dual-fuel systems deliver strong performance yet require complex infrastructure. The advantages and disadvantages of each method based on parameters such as cost, scalability, environmental impact, and effectiveness are summarized in Table 4.
Among these methods, nano-biodiesel and water-emulsified biodiesel stand out for their high effectiveness in improving atomization, yet they may raise long-term environmental and health concerns that warrant careful consideration. Nano-biodiesel, which incorporates nanoparticles like TiO2 or Al2O3 to reduce viscosity and enhance dispersion, significantly improves atomization by shrinking droplet sizes. However, the release of nanoparticles during combustion and exhaust emission is a critical issue. These tiny particles, often measuring less than 100 nanometers, can easily penetrate biological barriers, such as cell membranes and even the blood–brain barrier, posing potential risks to human respiratory and cardiovascular systems. Environmentally, they may accumulate in soil and water bodies, disrupting ecosystems by affecting microbial activity and plant growth. Their long-term persistence and bioaccumulation tendencies further amplify these concerns, as the full extent of their ecological impact remains incompletely understood.
Water-emulsified biodiesel, while cost-effective and environmentally beneficial in terms of reducing NOx emissions, faces challenges related to corrosiveness. The presence of water, even in emulsified form, can accelerate the corrosion of metal components in fuel storage tanks, pipelines, and engine systems over time. This corrosion not only reduces the lifespan of equipment but also raises the risk of fuel leakage, which can contaminate soil and groundwater. Furthermore, the emulsifiers employed to stabilize water–biodiesel emulsions may themselves pose environmental concerns. These additives could potentially alter the fuel’s biodegradability or generate harmful combustion byproducts, though comprehensive studies are required to fully evaluate these secondary effects.
We should also note that translating biodiesel atomization enhancements from laboratory to commercial engines presents significant challenges that require careful adaptation. Commercial applications demand solutions that maintain performance across variable operating conditions while meeting strict emissions regulations and cost targets—factors often not fully addressed in laboratory settings [155]. The most promising approaches typically undergo extensive prototype testing and iterative refinement to bridge the gap between ideal lab conditions and harsh engine environments, with fuel blending and air-assisted injection currently showing the most immediate commercial potential due to their balance of effectiveness and practicality.

6. Conclusions and Future Outlook

This review article concludes that biodiesel stands as a promising alternative to diesel fuel, well-suited to meet the operational requirements of CI engines. To assess the application characteristics of biodiesel, a comparative analysis was conducted, encompassing fundamental fuel properties, spray atomization behaviors, spray and droplet impingement and advanced techniques for improving biodiesel atomization. The conclusions derived from the current literature review are presented below, along with areas warranting further research and development regarding biodiesel atomization.
Despite the wide variety and diverse sources of biodiesel fuels, their viscosity and surface tension are generally higher than those of conventional diesel fuel, which can lead to poor atomization performance. The significance of the fuel density for atomization is diminished because most biodiesel fuels experience only minor differences in fuel density.
Biodiesel showed a longer spray tip penetration, smaller spray cone angle, larger SMD and faster droplet velocity compared with diesel fuel. Blending other fuels such as ethanol, butanol, dimethyl ether and di-n-butyl ether, to reduce the viscosity and surface tension of mixed fuels, represents a simple and effective approach for improving biodiesel atomization performance. More studies should be performed in this field in the future.
The probability of fuel–wall impingement is higher with biodiesel than with base diesel because of the former’s longer spray penetration. Additionally, biodiesel spray impingement is more prone to fuel deposition and film formation due to its higher viscosity and surface tension, which lead to combustion inefficiencies and emissions. Further studies should focus on combined strategies such as mild retarded injection timing, compression ratio, and combustion chamber design to enhance impact-induced secondary atomization, as well as reduce fuel deposition.
Droplet–wall impingement is a fundamental element in the complex process of spray–wall impingement. Spray impingement models must incorporate scientifically accurate descriptions of fundamental droplet phase change, aerodynamics, and transport processes. A comprehensive analysis of single-droplet impingement must be conducted, particularly using microscale biodiesel and blending fuel droplets under real engine operating conditions.
Advanced techniques such as air-assisted atomization, dual-fuel impingement, nano-biodiesel and water-emulsified biodiesel, collectively contribute to mitigating the atomization limitations of biodiesel, paving the way for its more efficient application in compression ignition engines. Further research is needed to refine these methods and explore synergistic effects between different approaches.

Author Contributions

Conceptualization, Z.F.; software, J.Z. and J.G.; investigation, Z.F., J.Z. and J.G.; data curation, Z.F., J.Z. and J.G.; writing—review and editing, Z.F., X.L. and K.N.; visualization, Z.F.; supervision, Z.H.; funding acquisition, Z.F. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by the China Postdoctoral Science Foundation (No. 2024M751177). We also acknowledged financial support from the Natural Science Foundation of Jiangsu Province (BK 20240859), and the Jiangsu University Foundation (Grant No. 22JDG040).

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Processes 13 02527 g008
Figure 8. Positive impacts of nanoparticles as fuel additives.
Figure 8. Positive impacts of nanoparticles as fuel additives.
Processes 13 02527 g008
Table 4. Comparative analysis of the biodiesel atomization enhancement methods.
Table 4. Comparative analysis of the biodiesel atomization enhancement methods.
MethodCostScalabilityEnvironmental ImpactEffectiveness
Blending other fuelsMedium–LowHighExcellent (depending on blend): Alcohol/ether blends improve sustainabilityMedium: Atomization improves via adjusted viscosity/surface tension, with results varying by blend ratio
Air-assisted atomizationMedium–HighMedium–LowGood: Improved atomization reduces particulate emissions, but compressed air energy use indirectly increases carbon emissionsMedium–High: Air shear significantly reduces droplet size with stable atomization
Dual-fuel impingementMediumMediumGood: Synergistic combustion lowers emissionsMedium–High: Fuel impingement creates uniform droplet breakup
Nano-biodieselHighLowModerate: Enhanced combustion reduces emissions, but nanoparticle release poses potential toxicity and environmental accumulation risksHigh: Nanoparticles reduce viscosity and improve dispersion, markedly boosting atomization
Water-emulsified biodieselMediumMediumModerate: Water emulsification reduces NOx and particulates; micro-explosion enhances efficiency with no additional pollutants but faces challenges related to corrosivenessHigh: Water evaporation triggers “micro-explosions” that shatter droplets, yielding significant atomization
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Feng, Z.; Zhang, J.; Gu, J.; Leng, X.; He, Z.; Nishida, K. Improving Biodiesel Atomization Performance in CI Engines: A Review of Spray Behavior, Droplet Impingement, and Advanced Techniques. Processes 2025, 13, 2527. https://doi.org/10.3390/pr13082527

AMA Style

Feng Z, Zhang J, Gu J, Leng X, He Z, Nishida K. Improving Biodiesel Atomization Performance in CI Engines: A Review of Spray Behavior, Droplet Impingement, and Advanced Techniques. Processes. 2025; 13(8):2527. https://doi.org/10.3390/pr13082527

Chicago/Turabian Style

Feng, Zehao, Junlong Zhang, Jiechong Gu, Xianyin Leng, Zhixia He, and Keiya Nishida. 2025. "Improving Biodiesel Atomization Performance in CI Engines: A Review of Spray Behavior, Droplet Impingement, and Advanced Techniques" Processes 13, no. 8: 2527. https://doi.org/10.3390/pr13082527

APA Style

Feng, Z., Zhang, J., Gu, J., Leng, X., He, Z., & Nishida, K. (2025). Improving Biodiesel Atomization Performance in CI Engines: A Review of Spray Behavior, Droplet Impingement, and Advanced Techniques. Processes, 13(8), 2527. https://doi.org/10.3390/pr13082527

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