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Study: Combustion and emission characteristics of aviation biofuel derived from coconut oil using the co-solvent method: toward eco-friendly micro jet engines. Image Credit: bombermoon/Shutterstock.com

The researchers used a co-solvent process to achieve more than 97% fuel purity, without energy-intensive processing. The carbon dioxide and nitric oxide emissions of the two biofuels remained broadly comparable with kerosene.

Coconut Oil’s Potential as Fuel for Sustainable Aviation

Aviation faces growing pressure to reduce greenhouse gas emissions. Sustainable aviation fuel (SAF) could play an important role in cutting aviation CO2 emissions, particularly for medium- and long-haul aircraft.

However, many existing SAF pathways rely on hydrogenation, hydrocracking, and other energy-intensive processes, which can increase energy demand and associated emissions during fuel production. The researchers therefore investigated coconut oil as an alternative feedstock and used a co-solvent method to produce aviation biofuels.

The study aimed to determine whether coconut-oil-derived FAME and FAEE could perform effectively in a microjet engine when blended with kerosene. The researchers evaluated fuel consumption, thermal efficiency, and four key exhaust components: hydrocarbons (HC), carbon monoxide (CO), carbon dioxide (CO2), and nitric oxide (NO).

Earlier studies have demonstrated the production of coconut-oil-derived FAME and FAEE, while other research had mainly examined these esters as biodiesel in diesel engines.

However, researchers had limited information on how these fuels behave during jet-engine combustion and how they affect exhaust emissions, and only a few studies had directly compared coconut-derived FAME and FAEE. This study therefore provides experimental evidence on their engine performance and emission characteristics.

Producing and Testing the Biofuels

The researchers produced two aviation biofuels from coconut oil, using methanol to produce FAME and ethanol to produce FAEE. The co-solvent process combined coconut oil with acetone, alcohol, and potassium hydroxide.

The mixture was reacted at room temperature for 30 minutes, after which researchers recovered the acetone and alcohol under reduced pressure for reuse. The process produced FAME and FAEE with purities of 98.31% and 97.66%, respectively.

The researchers blended each biofuel with kerosene at 10%, 30%, and 50% concentrations. They tested the blends in a J-850 microjet engine equipped with a single-stage axial compressor, annular combustor, and single-stage radial turbine. The team operated the engine between 80,000 and 100,000 rpm in 5000 rpm increments. They recorded measurements after the engine reached stable operating conditions.

The experiments measured thrust, fuel flow, engine temperatures, and pressures. The researchers calculated specific fuel consumption (SFC) from fuel flow and thrust and evaluated two forms of thermal efficiency. They collected exhaust gases downstream of the nozzle and analyzed them using a gas analyzer.

Engine Performance and Emission Findings

The engine tests showed that coconut-derived biofuel blends can operate without a major loss of thrust, with static thrust remaining comparable with that of pure kerosene.

However, the blends increased specific fuel consumption. As the FAME or FAEE content increased from 10% to 50%, SFC rose by more than 15%. At 80,000 rpm, the 50% FAME blend recorded an SFC about 16.8% higher than kerosene, while the 50% FAEE blend showed an increase of about 19.6%.

The lower heating values of the biofuels largely explain the higher fuel demand; FAME and FAEE had heating values about 16.8% and 14.5% lower than kerosene, respectively. The researchers also calculated mean spray particle diameters that were about 2.2–2.3% larger than those of kerosene at a 50% blend. Larger droplets can reduce atomization quality and affect combustion.

Despite these differences, the blends maintained thermal efficiency close to kerosene. Some blends even produced slightly higher efficiency. The maximum difference in turbine inlet temperature was about 36 K. These results indicate that the biofuel blends can convert fuel energy into useful engine work at a level broadly comparable with kerosene.

A 50% biofuel blend reduced hydrocarbons (HC) concentration by about 5–40% compared with kerosene. The researchers linked this reduction partly to the absence of aromatic compounds in the biofuels.

However, CO concentration increased by about 3–17%. This is likely a result of higher flash points and lower heating values making ignition more difficult. CO2 concentration remained close to kerosene, with changes of only about 1–3%.

Implications for Sustainable Aviation

The study demonstrates that coconut oil has potential as a feedstock for producing high-purity FAME and FAEE through a co-solvent process, with the researchers successfully testing both fuels in a microjet engine.

The process also offers potential energy-saving advantages over more energy-intensive conventional pathways by enabling the recovery and reuse of acetone and alcohol.

Coconut-derived blends maintained thrust and broadly comparable thermal efficiency, but their lower heating values increased fuel consumption. Lower HC emissions provide an environmental benefit, while the relatively small changes in CO2 emissions suggest limited deterioration in the measured exhaust profile.

The increase in CO emissions, however, indicates that further fuel and engine optimization will be necessary.

Further development is also required regarding oxygen content, as the researchers recognize that the oxygen content of their biofuels exceeds the cited ASTM and SAF certification limits. They also identify concerns related to moisture absorption, oxidative stability, and corrosion.

Future research should use more detailed engine diagnostics and computational fluid dynamics (CFD) to better understand the combustion behavior of these fuels. The researchers also recommend measuring polycyclic aromatic hydrocarbons and conducting a full life-cycle assessment to evaluate their wider environmental impact.

Overall, the findings suggest that coconut-derived biofuels could contribute to a diversified SAF portfolio, particularly in applications where lower-energy fuel production and efficient use of renewable feedstocks are priorities.

Journal Reference

Ogawa, S., Hongo, T., et al. (2026). Combustion and emission characteristics of aviation biofuel derived from coconut oil using the co-solvent method: toward eco-friendly micro jet engines. Fuel. 428. DOI: 10.1016/j.fuel.2026.140208. https://www.sciencedirect.com/science/article/pii/S0016236126019630.

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By Akshatha Chandrashekar

Reviewed by Sarah Kelly