Fuel properties of biodiesel blended with glycerol-derived acetyl glycerol esters as antifreeze additives

Highlights

• A sulfated superacid catalyst catalyzed the esterification of glycerol for used as antifreeze additives.
• Effects of the blending conditions of ethanol or isopropanol on the fuel properties were systematically evaluated.
• Ethanol-to-antifreeze ratio of 5:95 provided the best cold-flow performance, lowering the CFPP from 2 °C to −4 °C.
• Isopropanol-to-antifreeze ratio of 10:90 reduced the CFPP from 2 °C to −3 °C.
• Acetyl glycerol esters, with a suitable cosolvent, can effectively improve the low-temperature flow properties of biodiesel.

1 Introduction

Biodiesel is a renewable alternative to petroleum diesel, produced by chemically converting oils derived from animal fats, vegetable oils, and recycled waste cooking oil. In addition to biodiesel, the process generates glycerol as a byproduct at approximately 10 wt.%. The carbon-chain structure and fuel properties of biodiesel are similar to those of petroleum diesel (Lin and Wu, 2022); accordingly, biodiesel can be blended with petroleum diesel over a wide range of proportions without engine modification. Biodiesel also generally produces lower emissions of black smoke, particulate matter, sulfur oxides, hydrocarbons, and carbon monoxide (Wasilewski et al., 2024). In addition, its favorable lubricity can reduce engine-component wear, while its high flash point improves storage safety.

Biodiesel is generally produced by transesterification. The resulting crude glycerol byproduct has a purity of only approximately 50% and therefore cannot be used directly in high-value applications. Depending on treatment economics, crude glycerol may be discarded as waste, purified, or converted into higher-value chemical products (Lima et al., 2022). High-purity glycerol is widely used in the pharmaceutical, cosmetic, and chemical industries; however, these applications typically require approximately 99.5% purity, and the associated purification costs are substantial. An alternative is to convert crude glycerol into products such as epichlorohydrin, 1,3-propanediol, polylactic acid, hydrogen, glycerol carbonate, glycerol ethers, or glycerides.

Because glycerol is a versatile feedstock for industrial chemicals, its conversion to acetylated derivatives has attracted increasing attention. In this study, we prepared an SO₄ ^2-/TiO₂ solid acid by impregnating nanoscale TiO₂ with sulfuric acid to catalyze the esterification of glycerol with acetic acid. Recent studies continue to identify monoacetin, diacetin, and triacetin as value-added oxygenated products of glycerol acetylation and emphasize the importance of heterogeneous acid catalysts for improving conversion and selectivity (Keogh et al., 2024; Moklis et al., 2023; Yadav and Katole, 2025). At the same time, biodiesel application in cold climates remains constrained by crystallization of higher-melting fatty acid esters, which can increase viscosity and obstruct fuel filters.

Cold-flow improvement can be pursued through additive blending, winterization/fractionation, fatty-ester modification, or combinations of these approaches. A recent review showed that many cold-flow-improvement methods can improve CFPP, cloud point, or pour point but may trade off oxidation stability, cetane quality, viscosity, or process complexity (Pradana et al., 2024). Controlled winterization combined with branched ester additives has also been shown to reduce CFPP by altering the saturated-ester fraction (Bouaid et al., 2024). More recently, catalytic skeletal isomerization has been investigated as a route to introduce branching into fatty esters and improve low-temperature behavior (Pradana et al., 2025; Indarto et al., 2026). These approaches provide useful benchmarks for evaluating the simpler downstream strategy of adding a glycerol-derived cold-flow formulation to finished biodiesel.

Our group previously characterized a SO₄ ^2-/TiO₂ catalyst for glycerol acetylation and reported how reaction conditions affect glycerol conversion and acetyl-glycerol-ester formation (Lin and Chen, 2024a). A related study subsequently evaluated glycerol-derived acetate/cosolvent formulations in biodiesel at a 5 vol.% additive level using methanol and butanol as cosolvents (Lin and Chen, 2024b). Those studies established the feasibility of converting glycerol to a cold-flow additive, but they did not define the performance of a substantially lower additive loading formulated with ethanol or isopropanol, nor did they systematically frame the cold-flow benefit against practical fuel-property penalties.

Glycerol can react with other compounds to form glycerol ethers or esters; examples include reactions with isobutylene and acetic acid derivatives. In the presence of acidic catalysts such as sulfuric acid or strongly acidic ion-exchange resins (e.g., Amberlyst-15), glycerol and isobutylene can be etherified to form tert-butyl glycerol ethers. Gupta. (1995) conducted this etherification at 50 °C–100 °C and 30–100 psi, using catalyst loadings of 0.5–2.5 wt.% and reaction times of 1–2.5 h. Higher yields were obtained at 60 °C, 100 psi, and 2.5 h. The products comprised mono-, di-, and tri-tert-butyl glycerol ethers at proportions of 96%, 3%, and 1%, respectively. Noureddini et al. (1998) synthesized di- and tri-tert-butyl glycerol ethers as fuel additives using acid catalysts or sulfonic-acid-modified solid catalysts. Improved results were obtained at an isobutylene-to-glycerol molar ratio of 4:1, a temperature of 75 °C, a catalyst loading of 5 wt.%, and a reaction time of 4 h. Melero et al. (2007) used Amberlyst-type acid catalysts to produce di- and tri-tert-butyl glycerol ethers from isobutylene and glycerol. As diesel additives, these ethers can improve emissions, viscosity, and cold-flow properties. They can also serve as octane enhancers in biodiesel, diesel, and their blends. More generally, glycerol can be converted into commercially valuable oxygenated components for diesel-fuel blends. Zahid et al. (2020) reported that acetalization of glycerol with acetone in the presence of various heterogeneous catalysts produces solketal.

Researchers can prepare heterogeneous catalysts by impregnation, sol-gel synthesis, and hydrothermal synthesis. In the impregnation method, porous metal oxides can act as supports for strongly acidic species. For the sulfated titania system used here, we impregnated TiO₂ nanoparticles with sulfuric acid to form a recoverable solid superacid for glycerol acetylation. The preparation and physicochemical characterization of the related SO₄ ^2-/TiO₂ catalyst have been reported previously (Lin and Chen, 2024a). The present manuscript therefore focuses on formulation and fuel-property effects rather than repeating catalyst-structure characterization.

Accordingly, this study addresses the performance and practical formulation window of a low-dose glycerol-derived acetyl-glycerol-ester additive. The mixed ester product was added to biodiesel at only 1 vol% and ethanol or isopropanol was used as a cosolvent over ratios of 0:100–20:80 within that additive package. In addition to CFPP, kinematic viscosity, flash point, calorific value, water content, acid value, distillation behavior, a non-standard carbon-residue index, and a calculated cetane index were evaluated. This design allows identification of not only the strongest cold-flow response but also the accompanying trade-offs relevant to practical formulation selection.

2 Experimental methods

2.1 Preparation of acetyl glycerol esters

We prepared the SO₄ ^2-/TiO₂ solid superacid by the impregnation protocol previously used for this catalyst system (Lin and Chen, 2024a). Nanoscale TiO₂ (1 g) was immersed in 10 mL of dilute sulfuric acid (0.1 M) for 24 h, vacuum-filtered, dried at 100 °C for 4 h, calcined at 450 °C for 3 h, cooled to room temperature, and ground to a powder. Glycerol (J.T. Baker; purchased from Yuho Trading Co., Ltd.) was used as the feedstock. In the reaction screening, temperature was varied from 90 °C to 130 °C; catalyst loading from 4 to 6 wt%; and reaction time from 0 to 16 h. The crude acetyl glycerol ester product was centrifuged at 4,000 rpm for 30 min, filtered, and vacuum-distilled at 105 °C under 650 mmHg until no additional distillate or condensate was collected.

The residual acetic acid content of the reaction mixture was then determined as an acid value (mg KOH/g) using a Metrohm 785 DMP Titrino (Switzerland), after which the mixture was neutralized with an equimolar amount of sodium hydroxide. An ion-selective electrode was used with automatic addition of standard KOH solution. The instrument converted the titration response into the acid value of the sample, expressed as mg KOH/g.

2.2 Composition analysis of acetyl glycerol esters

The esterification product was analyzed by GC-14A (Shimadzu, Japan) equipped with a Phenomenex ZB-5HT capillary column (30 m × 0.32 mm × 0.25 μm). The oven temperature was held at 100 °C for 5 min, increased to 135 °C at 5 °C/min and held for 2 min, and then increased to 240 °C at 5 °C/min and held for 5 min. In the present data set, authentic external standards were available for glycerol and triacetin. Consequently, quantitative calibration was limited to residual glycerol and triacetin; monoacetin and diacetin were not independently quantified with authentic standards. To avoid overinterpretation, the product is therefore referred to throughout this manuscript as a mixed acetyl glycerol ester product rather than as a fully composition-resolved ester mixture.

External-standard calibration was performed as follows:

• Glycerol and triacetin standards were diluted with ethanol to prepare solutions at different concentrations.
• The standards were analyzed by gas chromatography, and calibration curves were constructed from detector response versus concentration.
• The regression equations were y = 0.1821x − 0.0607 (R² = 0.9939) for glycerol and y = 0.4737x − 0.1652 (R² = 0.9903) for triacetin. Because monoacetin and diacetin were not independently calibrated, composition-specific effects of those species are treated as a limitation of the present study.

2.3 Scope of catalyst characterization

Catalyst preparation and detailed structural characterization of the related SO₄ ^2-/TiO₂ system, including diffraction and spectroscopic/thermal analyses, have been reported previously (Lin and Chen, 2024a). EDS and XRD are therefore not presented as new outcome data in this study. This work is limited to the preparation protocol required to reproduce the catalyst and to evaluating the resulting glycerol-derived additive in biodiesel.

2.4 Fuel-property analysis of biodiesel blended with acetyl glycerol esters

Acetyl glycerol esters were mixed with ethanol or isopropanol as cosolvents at cosolvent-to-ester ratios ranging from 0:100 to 20:80. The measured fuel properties included CFPP, kinematic viscosity, calorific value, flash point, specific gravity, distillation characteristics, a non-standard carbon-residue index, acid value, and water content. The measurement methods are described below.

• Calorific value

Calorific value is an important indicator of fuel energy content; a higher calorific value indicates that more heat is released per unit mass of fuel. It is commonly expressed in cal/g or MJ/kg, where 1 cal/g = 4.1868 × 10^-3 MJ/kg. We used a Parr 1,261 calorimeter (United States) to determine the calorific values of the solid and liquid samples. The instrument software measured the heat released by combustion in a constant-temperature water bath and calculated it automatically. The stated reproducibility was 0.10% relative standard deviation, and the temperature resolution was 0.001 °C.

• 2. Flash point

Flash point is an important parameter for the safe storage and transportation of fuel. A low flash point indicates greater volatility and a higher risk of ignition during handling. A Walter Herzog closed-cup flash-point tester (Germany) was used. The sample was heated gradually and exposed periodically to an ignition source; the lowest temperature at which a flash occurred was recorded as the flash point.

• 3. Kinematic viscosity

Kinematic viscosity characterizes a fuel’s flow behavior. A Cannon K698 viscometer (United States) was used in accordance with ASTM D445 and ASTM D446. A viscometer constant of k = 0.001045 mm²/s² was applied, and the kinematic viscosity was calculated from the time required for the sample to flow through the specified capillary length.

• 4. Specific gravity

Specific gravity was measured by the hydrometer method at a reference temperature of 15 °C in accordance with ASTM D1298-24. The hydrometer was placed in a graduated cylinder containing 100 mL of sample, allowed to reach temperature equilibrium, and read at the liquid meniscus; the reported values were referenced to 15 °C.

• 5. Distillation temperature

A Walter Herzog HAD-620 distillation apparatus (Germany) was used to determine the distillation-temperature profile based on differences in the boiling points and vapor pressures of the fuel components; the more volatile fractions distilled first. Because distillation temperature increases with recovered volume, we used the temperature at 50 vol% recovery (T₅₀), together with API gravity, to calculate the cetane index (CCI).

• 6. Calculated cetane index

We obtained the calculated cetane index (CCI) from API gravity and the 50 vol% distillation temperatures using the empirical density–distillation correlation shown in Equation 3.1 (Lin and Wu, 2022). This correlation was originally developed for petroleum-derived distillate fuels. We use it here only as a comparative calculated descriptor, not as a substitute for a directly measured cetane number.where CCI is the calculated cetane index, G is the API gravity, and T₅₀ is the distillation temperature at 50 vol% recovery, expressed in °F. Specific gravity (SG) was converted to API gravity (G) using Equation 3.2. Because biodiesel and oxygenated additive blends can deviate from the fuel compositions used to develop conventional cetane-index correlations, Section 3.8 discusses interpreting these calculated values as a limitation (Lin and Wu, 2022; Bezaire et al., 2010).

CCI = 0.016 G 2 − 420.34 + 0.192 G log ⁡ T 50 + 65.01 log ⁡ T 50 2 − 0.0001809 T 50 2 (3.1)

G = 141.5 / SG 60 ℉ − 131.5 (3.2)

• 7. Carbon residue (non-standard gravimetric index)

The carbon residue reported in this study is not an ASTM carbon-residue determination. After combustion in the closed bomb-calorimeter vessel, the remaining solid residue in the crucible was weighed and expressed relative to the initial sample mass. We used the result only as a within-study, non-standard gravimetric carbon-residue index. Standard carbon-residue methods such as ASTM D4530 or ASTM D189 instead use controlled evaporation and pyrolysis conditions; therefore, the present values should not be interpreted as ASTM carbon-residue results.

Non − standard carbon residue wt . % = mass of residual solid / net mass of fuel sample × 100 (3.3)

• 8. Acid value

The acid value is defined as the mass of potassium hydroxide, in milligrams, required to neutralize the acidic constituents in 1 g of sample. It indicates the extent of hydrolysis or degradation of oils and esters; a higher acid value generally indicates lower storage stability. Hydrolysis promoted by time, temperature, enzymes, or moisture releases free fatty acids, thereby increasing the acid value. The acid value can therefore indicate free fatty acid content (Meher et al., 2006).

Acid value was measured using a Metrohm 785 DMP Titrino (Switzerland) equipped with an ion-selective electrode and automatic addition of standard KOH solution. The instrument converted the titration response into the sample acid value, expressed as mg KOH/g.

• 9. Water content

Water content was measured using a Mettler-Toledo DL31 volumetric Karl Fischer titrator (Switzerland), because water in fuel can corrode pipelines and machinery and promote microbial growth. The instrument had a stated measurement range of 0.1–100 vol.% and automatically calculated the water content from the volume of titrant consumed after sample introduction. Results were reported as either a percentage or parts per million. Before sample injection and mass entry, set the background moisture reading to zero.

• 10. Cold filter plugging point (CFPP)

CFPP was measured with a manual CFPP tester (Green Tree Scientific Inc., Taiwan) coupled to a Firstek B10/−40 low-temperature circulating bath (Taiwan) in accordance with CNS 15061 and ASTM D6371-24. A 45 mL test portion was cooled in the controlled bath, and filtration checks were made at 1 °C temperature intervals. The fuel was drawn under approximately 2 kPa vacuum through the standardized stainless-steel filter screen (nominal aperture 45 μm); CFPP was taken as the temperature at which 20 mL could no longer pass through the filter within 60 s. The apparatus operating range was −38 °C to 50 °C, and the stated temperature accuracy was ±1 °C. ASTM D6371 also recognizes that sample thermal history can affect low-temperature results. We tested the samples as received, without preheating before CFPP testing.

We measured each reported condition in at least three independent replicate runs (n = 3) and used the arithmetic mean for comparison. Error bars in Figures 1–8 represent ±1 standard deviation (SD) calculated from the three replicate measurements.

3 Results and discussion

The esterification screening described in Section 2.1 used the freezing point of the resulting mixed acetyl glycerol ester as the selection criterion. The lowest measured freezing point occurred at an acetic acid-to-glycerol molar ratio of 8:1, a catalyst loading of 4 wt.%, a reaction temperature of 120 °C, and a reaction time of 10 h; these conditions were therefore used to prepare the additive for the subsequent biodiesel-blending experiments. The present work does not claim composition-resolved optimization of monoacetin, diacetin, and triacetin because this data set did not independently quantify individual mono- and diacetin fractions.

3.1 Cold filter plugging point

As shown in Table 1 and Figure 1, adding 1 vol.% mixed acetyl glycerol esters without a cosolvent lowered the CFPP of neat biodiesel from 2 °C to 1 °C. The modest decrease is consistent with limited miscibility between the polar acetyl glycerol ester fraction and the fatty acid methyl ester matrix. Adding a cosolvent substantially strengthened the cold-flow effect. The lowest CFPP was −4 °C at an ethanol-to-acetyl-glycerol-ester ratio of 5:95, whereas the minimum for isopropanol was −3 °C at 10:90. Increasing isopropanol beyond 10 vol% of the additive package produced no further CFPP improvement. In Figures 1–8, filled symbols/solid trend lines denote ethanol-containing formulations and open symbols/dashed trend lines denote isopropanol-containing formulations.

TABLE 1. Properties of biodiesel after the addition of 1 vol.% acetyl glycerol esters.

The observed response can be interpreted in terms of low-temperature crystallization and additive dispersion. Biodiesel cold-flow failure is governed largely by nucleation, growth, and agglomeration of higher-melting saturated fatty-acid esters (Pradana et al., 2024; Bouaid et al., 2024). The mixed acetyl glycerol ester product can interfere with crystal association, but its effect is limited when it is not uniformly dispersed in the biodiesel phase. A small amount of alcohol cosolvent can improve molecular-level mixing between the glycerol-derived ester fraction and the fatty-acid-methyl-ester matrix. Ethanol, which is less viscous and more polar than isopropanol, may achieve adequate compatibilization at a lower ratio, consistent with the 5:95 optimum observed here. Once dispersion is sufficient, additional cosolvent gives diminishing CFPP benefit because the crystallization tendency of the saturated biodiesel fraction remains the dominant limitation. This mechanism is consistent with the observed trends but should be regarded as an interpretation rather than direct molecular evidence, because the present study did not measure crystal morphology or composition-resolved ester interactions.

3.2 Kinematic viscosity

As shown in Table 1, the addition of acetyl glycerol esters caused a slight increase in the kinematic viscosity of biodiesel. After a cosolvent was incorporated, however, viscosity decreased progressively as the cosolvent proportion increased. Ethanol produced a more pronounced reduction than isopropanol (Figure 2), consistent with its lower viscosity (Table 2). At an ethanol-to-acetyl-glycerol-ester ratio of 10:90, the kinematic viscosity decreased from 4.41 mm²/s for the formulation without ethanol to 4.34 mm²/s. For isopropanol, the minimum kinematic viscosity of 4.37 mm²/s occurred at an isopropanol-to-acetyl-glycerol-ester ratio of 15:85.

TABLE 2. Properties of the acetyl glycerol ester antifreeze and cosolvents.

*Estimated value.

3.3 Water content

Acetyl glycerol esters, ethanol, and isopropanol are hygroscopic. Accordingly, the water content of the biodiesel blends tended to increase with increasing cosolvent proportion. Jimenez et al. (2010) similarly reported increasing water content in ethanol–biodiesel blends because of ethanol’s hygroscopic nature. The neat-biodiesel water content in Table 1 has been corrected from the typographical value 0.437 wt% to 0.0437 wt%. In the cosolvent-containing formulations, ethanol generally produced a higher water content than isopropanol, and water content increased with cosolvent proportion. At a ratio of 20:80, the values for the two cosolvents approached one another (Figure 3).

3.4 Acid value

Hydrolysis is a reaction in which water cleaves a compound into new products. Ester hydrolysis produces an alcohol and a carboxylic acid (Hilal et al., 2004). Thus, hydrolysis of fatty acid methyl esters produces methanol and free fatty acids, whereas hydrolysis of acetyl glycerol esters produces glycerol and acetic acid; both pathways can increase the acid value. Figure 3 shows that the water content increased with the ethanol-to-acetyl-glycerol-ester ratio. Correspondingly, at a cosolvent-to-acetyl-glycerol-ester ratio of 20:80, the acid value increased from 0.378 mg KOH/g for the formulation without cosolvent (0:100) to 0.658 mg KOH/g with ethanol and 0.714 mg KOH/g with isopropanol (Figure 4). The increase may therefore be associated with moisture-promoted hydrolysis of the methyl esters and acetyl glycerol esters.

3.5 Flash point

As shown in Figure 5, the flash point of biodiesel decreased as the cosolvent proportion increased. At an ethanol-to-acetyl-glycerol-ester ratio of 20:80, the flash point decreased to 100 °C. This trend is consistent with the low flash points of ethanol and isopropanol (13 °C and 15 °C, respectively; Table 2). The increasing proportion of these volatile cosolvents therefore lowered the biodiesel blend’s flash point.

3.6 Calorific value

The calorific value of neat biodiesel was 40.83 MJ/kg and decreased to 40.18 MJ/kg after adding 1 vol% acetyl glycerol esters (Table 1). As the cosolvent-to-acetyl-glycerol-ester ratio increased, the blend’s calorific value increased (Figure 6). This increase occurred because ethanol and isopropanol have higher calorific values (Calam et al., 2020) than the acetyl glycerol ester product (Table 2). Isopropanol produced a larger increase than ethanol because its calorific value (30.86 MJ/kg) is higher than ethanol’s (26.91 MJ/kg).

3.7 Carbon residue (non-standard gravimetric index)

The non-standard carbon-residue index decreased as the proportion of ethanol or isopropanol in the acetyl glycerol ester formulation increased. At a cosolvent-to-ester ratio of 20:80, the value decreased from 0.223 wt% for the formulation containing acetyl glycerol esters alone (0:100) to 0.117 wt% with ethanol and 0.127 wt% with isopropanol (Figure 7). Adding alcohol increases the blend’s oxygenated fraction and changes volatility and combustion behavior, which may contribute to the lower residual solid mass. Because this measurement is not ASTM D4530 or D189, these data are interpreted only as a comparative within-study trend, not as a standardized coke-forming tendency.

3.8 Distillation temperatures and calculated cetane index

Tables 3, 4 show that, among the measured distillation temperatures, the initial boiling point (T_IBP) changed most markedly after adding the cosolvent/antifreeze formulations. T_IBP decreased as the cosolvent proportion increased because ethanol and isopropanol have relatively low boiling points of 78 °C and 82 °C, respectively.

TABLE 3. Distillation temperatures (°C) of biodiesel containing different ethanol-to-acetyl-glycerol-ester ratios.

TABLE 4. Distillation temperatures (°C) of biodiesel containing different isopropanol-to-acetyl-glycerol-ester ratios.

Figure 8 shows the calculated cetane index (CCI) after adding different cosolvent-to-ester ratios. The CCI increased slightly as the cosolvent proportion increased. This trend results from the density and T₅₀ terms used in the empirical calculation and should not be interpreted as direct evidence that ignition quality increased by the same amount. Conventional cetane-index correlations were developed primarily for petroleum distillates, and previous work has shown that calculated indices can fail to capture changes in derived cetane number caused by biodiesel content or fuel additives (Lin and Wu, 2022; Bezaire et al., 2010). Accordingly, this study retains CCI values only for internal comparison among the tested formulations; direct cetane-number or ignition-delay measurements would be required to establish actual compression-ignition quality.

3.9 Practical formulation trade-offs and comparison with other cold-flow strategies

The present additive route differs from structural or process-intensive cold-flow strategies. Winterization improves low-temperature properties by removing higher-melting saturated fractions but can reduce product yield, whereas catalytic isomerization introduces branching into fatty esters and requires an additional reaction/separation step (Bouaid et al., 2024; Pradana et al., 2025; Indarto et al., 2026). In contrast, the present formulation can be introduced by downstream blending and simultaneously valorizes a glycerol-derived coproduct. These features may simplify implementation in an existing biodiesel supply chain, although storage compatibility and engine-level validation remain necessary.

The data also show why the practical optimum should not be chosen from CFPP alone. Within the 1 vol% additive package, the ethanol/ester ratio of 5:95 achieved the lowest CFPP (−4 °C), while higher cosolvent fractions produced little or no additional cold-flow benefit but continued to lower flash point and tended to increase water content and acid value. Therefore, the preferred formulation is the lowest cosolvent level that achieves the required low-temperature performance while retaining acceptable handling, storage, and fuel-quality characteristics. The present study establishes this trade-off experimentally but does not include long-term storage stability, oxidation stability, or engine durability testing.

4 Conclusion

The major findings for biodiesel blended with a 1 vol% additive package containing mixed acetyl glycerol esters and different cosolvent-to-ester ratios are summarized as follows:

• The lowest CFPP was obtained with an ethanol-to-acetyl-glycerol-ester ratio of 5:95, which reduced CFPP from 2 °C for neat biodiesel to −4 °C. The minimum with isopropanol was −3 °C at a ratio of 10:90. The response plateaued at higher cosolvent fractions, indicating that only a small cosolvent fraction was required for the strongest cold-flow benefit observed in this study.
• Increasing cosolvent proportion generally increased water content and acid value while decreasing flash point. The neat biodiesel water content in Table 1 was corrected to 0.0437 wt.%. These changes indicate that cold-flow improvement must be balanced against volatility, moisture uptake, and hydrolytic/storage-stability concerns.
• Kinematic viscosity and the non-standard carbon-residue index decreased as cosolvent proportion increased, whereas calorific value increased. Isopropanol-containing formulations generally retained higher calorific value and viscosity than corresponding ethanol formulations. The reported carbon-residue values are non-standard comparative measurements, and the calculated cetane index is an empirical descriptor rather than a directly measured cetane number. Overall, the 5:95 ethanol-to-ester formulation provides the strongest CFPP reduction among the tested blends with a relatively low cosolvent fraction; longer-term storage and engine tests are required before practical deployment.

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Footnotes

Edited by: Nanda Kishore, Indian Institute of Technology Guwahati, India

Reviewed by: Nanhang Dong, Northeast Electric Power University, China

Youngho Jeong, Korea Institute of Energy Technology, Republic of Korea