Key Measures to Prevent Coking in Aircraft Piston Engines

Aircraft piston engines, as the foundational power systems for light aircraft and drones, face challenges from fuel coking during long-term operation. This phenomenon not only reduces engine efficiency but also poses safety risks. Below are the technical strategies and practical measures to mitigate coking risks.

Fuel Quality Optimization: Reducing Coking Precursors

The composition of aviation fuel directly influences coking behavior. Aviation kerosene contains heavy aromatic hydrocarbons, colloids, and asphaltenes, which undergo dehydrogenation and condensation at high temperatures to form solid deposits. To address this, refining processes must strictly control the content of these components.

For example, reducing the proportion of benzene rings and heavy aromatic hydrocarbons in fuel can significantly lower the likelihood of coke nucleation. Studies show that fuels with lower asphaltene content exhibit a 40% reduction in coke deposition under identical combustion conditions. Additionally, using additives that disrupt polymerization reactions—such as metal deactivators or oxygenates—can further inhibit coke formation.

Combustion Chamber Design: Balancing Temperature and Mixing

The combustion chamber’s geometry and air-fuel mixing efficiency are critical factors. In high-temperature environments (e.g., combustion chamber inlet temperatures exceeding 850K), uneven fuel distribution leads to localized overheating, accelerating coke formation.

Modern designs integrate swirl-stabilized combustion techniques. For instance, a旋流组合式空气雾化喷嘴 (swirl-integrated air-atomizing nozzle) increases head-end air intake by 30%, improving fuel vaporization and reducing wall wetting. This design minimizes fuel contact with hot metal surfaces, thereby lowering coke adhesion. Another approach involves multi-stage injection, where fuel is delivered in pulses to maintain optimal combustion temperatures and prevent thermal degradation.

Thermal Management Systems: Controlling Component Temperatures

Coking risks escalate when fuel residues in inactive oil passages or post-shutdown engine components are exposed to sustained high temperatures. For example, residual fuel in a non-operating oil circuit can reach coking temperatures within 30 minutes after engine shutdown.

To mitigate this, advanced cooling systems are employed. One solution involves diverting fan bypass air through dedicated ducts to cool fuel nozzles post-shutdown. This method maintains nozzle temperatures below the coking threshold (typically <150°C) for over an hour, preventing residual fuel from solidifying. Another technique uses thermal insulation coatings on critical components to reduce heat conduction from hot structures to fuel pathways.

Material Selection and Surface Treatment: Enhancing Resistance

The choice of materials for fuel nozzles and combustion chambers significantly impacts coke adhesion. GH605 cobalt-based superalloy, commonly used in high-temperature components, forms an oxide layer at elevated temperatures. However, this layer can catalyze coke formation if not properly managed.

Surface treatments like plasma coatings or diamond-like carbon (DLC) films create non-stick surfaces that reduce coke adherence. For example, a DLC-coated nozzle in a test engine showed a 60% reduction in coke accumulation compared to untreated counterparts. Additionally, catalytic inhibitors embedded in the material matrix can disrupt carbon chain growth, further suppressing coke formation.

Operational Practices: Minimizing Coking Triggers

Pilot and maintenance practices also play a role. Prolonged idling or low-power operation can lead to incomplete combustion, increasing unburned hydrocarbon residues that contribute to coking. Implementing standardized start-up and shutdown procedures—such as avoiding abrupt throttle changes—helps maintain stable combustion temperatures.

Regular fuel system purging is another effective measure. Draining sediment from fuel tanks every 36 flight hours, as tested by CFMI, reduces particulate contamination that accelerates coke nucleation. While the direct impact on coking is moderate, this practice enhances overall fuel cleanliness and system longevity.

By integrating these strategies—fuel quality control, advanced combustion designs, thermal management, material innovation, and operational discipline—aircraft piston engines can achieve reliable performance while minimizing coking-related risks. Each measure addresses a specific vulnerability in the fuel-air-thermal interaction cycle, ensuring sustained efficiency and safety in aviation applications.