MH/T 6137.2-2025 in English

Background and Significance of Standard Development

MH/T 6137.2-2025, "Test Methods for Aviation Turbine Engine Lubricating Oil Performance - Part 2: Vapor-Phase Coking Tendency," is a special test standard developed by the Civil Aviation Administration of China for key performance indicators of aviation turbine engine lubrication systems. This standard fills a technical gap in domestic evaluation methods for the vapor-phase coking tendency of aviation lubricating oils and provides a scientific basis for carbon deposit risk assessment in aircraft engine ventilation lines.

Under high-temperature and high-pressure operating conditions in aircraft turbine engines, lubricating oil forms an oil mist/air two-phase mixture, which undergoes thermal decomposition on the inner wall of the ventilation line, forming carbon deposits, seriously affecting the engine's heat dissipation efficiency and operational safety. The development of this standard is based on extensive experimental data and engineering practice experience, establishing a standardized testing process and evaluation system.

Overview of Core Technology Principles and Methods

The gas-phase coking tendency test utilizes a gas-phase coking tester as its core instrument. This instrument simulates actual aircraft engine operating conditions and quantitatively evaluates the coking characteristics of lubricating oils under high-temperature conditions. The test principle involves placing a specified mass of turbine oil into a three-necked flask, heating it at 204°C ± 1°C while introducing dry air. The oil vapor then passes through a test tube heated at 371°C ± 1°C, forming carbon deposits on the tube's inner wall.

The test requires precise control of three key parameters: a turbine oil temperature of 204°C ± 1°C, a heating furnace temperature of 371°C ± 1°C, and an air flow rate of 765 mL/min ± 5 mL/min. The test duration is strictly controlled within 18 hours ± 0.1 hours. These standardized test conditions ensure comparability and reproducibility of test results between different laboratories.

Instrument and Equipment Technical Requirements

Core Test Equipment Specifications

Gas Phase Coking Analyzer, the core of the test system, consists of a three-necked flask, a heating furnace, a heating jacket, a gas supply unit, and a temperature control unit. The three-necked flask must have a round bottom design, a capacity of 2000mL, and utilize a standard 24/40 joint to ensure sealing and compatibility. The air inlet pipe must be made of frosted glass and connected to the air supply unit with a flexible hose. Thermocouples meet stringent technical requirements and must use a 304 stainless steel sheath with a diameter of approximately 3.2mm. The measuring temperature range is 1°C to 400°C with an accuracy of 1°C. The heating furnace must have a heating power of at least 1200W, an outer diameter of approximately 21.5cm, and a height of approximately 17.5cm to ensure uniform temperature distribution. Test Tube Specifications: The test tube is a key component that comes into direct contact with oil vapor and produces coking. Its dimensions and material are strictly specified: length (165±1) mm, outer diameter (12.5±0.1) mm, and wall thickness (1.25±0.05) mm. The material must comply with the 06Cr19Ni10 stainless steel specified in GB/T24511 to ensure chemical stability and mechanical strength under high-temperature conditions.

Test Preparation and Operating Procedures

Cleaning and Sample Preparation

Before testing, thoroughly clean the three-necked flask, thermocouple, air inlet pipe, and other components. Use analytical grade petroleum ether (boiling range 60-90°C) or water meeting GB/T6682 Grade III requirements to ensure no residual contaminants. The heating furnace tube walls should be repeatedly scrubbed with a stiff brush dipped in cleaning agent until the bristles show no color change.

Sample Preparation: Prepare at least 2L of test turbine lubricating oil. Use an electronic balance to accurately weigh (900±0.1)g of the test sample and place it in a three-necked flask. The initial acid value (TAN₁) of the sample must be determined according to GB/T7304, and the initial kinematic viscosity (ν₁) at 40°C must be determined according to GB/T265.

Test Tube Pretreatment

Test tube pretreatment is critical to ensuring test accuracy. Immerse the test tube in an ultrasonic cleaner filled with cleaning agent for at least 15 minutes until no visible impurities are present. Then, place it in an oven at (100±2)°C for at least 30 minutes. Remove it and cool it to room temperature in a desiccator.

Use an analytical balance to weigh the test tube to an accuracy of 0.1mg. Repeat the drying and cooling steps until the difference in mass between two consecutive measurements does not exceed 0.5mg. Calculate the average value and record it as the initial mass m₁. Mark the installation positions of the six thermocouples on the outer wall of the test tube in accordance with standard requirements, with spacing of 3.0cm, 5.5cm, 8.0cm, 10.5cm, 13.0cm, and 15.5cm, respectively, with an accuracy of ±0.1cm.

Data Processing and Result Report

Key Performance Indicator Calculation

After the test is completed, five core performance indicators must be calculated: coke mass in the test tube (Δm), turbine oil consumption (ΔM), acid number change (ΔTAN), kinematic viscosity change rate (Δν%), and coking temperature range (Tmin-Tmax).

Coke mass Δm = m₂ - m₁, where m₂ is the test tube mass after the test and m₁ is the test tube mass before the test, both in milligrams (mg). Turbine oil consumption ΔM = M₁ - M₂, where M₁ is the total mass of the three-necked flask and sample before the test and M₂ is the total mass after the test, both in grams (g).

Calculation of coking temperature range

Calculation of coking temperature range is a technical difficulty of this standard and must be carried out in accordance with the standard method in Appendix B. The minimum coking temperature Tmin and the maximum coking temperature Tmax are calculated using the temperature measurement values of 6 thermocouples at different positions of the test tube and the distribution of coking materials using the linear interpolation method.

Specific calculation method: When the highest point of the coking material is between the i-th and i+1 thermocouples, Tmin = Ti - (Ti - Ti+1)×Le/Li; when the lowest point of the coking material is between the j-th and j+1 thermocouples, Tmax = Tj+1 + (Tj - Tj+1)×Lm/Lj. The calculation result is accurate to 1°C, and the final report is the coking temperature range (Tmin~Tmax).

Standard implementation recommendations and technical outlook

Laboratory construction requirements

Implementation of MH/T Standard 6137.2-2025 requires the establishment of a dedicated aviation lubricant testing laboratory equipped with a gas-phase coking tester, precision balance, constant-temperature oven, kinematic viscometer, potentiometric titrator, and other equipment. The laboratory environment must maintain a constant temperature and humidity, controlled at (23±2)°C and a relative humidity of 50%±10%. Personnel training is crucial for standard implementation. Testers must possess professional skills in chemical analysis, instrument operation, and data processing, and regularly participate in standard promotion and technical training to ensure standardized testing procedures and comparable results. Technological Development Trends As aircraft engines evolve toward higher temperatures and higher efficiencies, higher requirements are placed on lubricant performance. In the future, gas-phase coking tendency testing technology will develop towards automation and intelligence, integrating online monitoring, real-time data acquisition, and intelligent diagnostic capabilities.

At the same time, test methods also need to be continuously optimized to consider more stringent operating conditions, such as higher temperatures, longer test times, and different air flow rates, in order to establish a more complete aviation lubricant performance evaluation system and provide technical support for the reliable operation of aircraft engines.

As an important part of the MH/T 6137 series, this standard, together with Part 1, High-Temperature Bearing Deposition Performance, and Part 3, Load Capacity, High-Speed Gear Method, constitutes a complete aviation turbine engine lubricant performance test system. It is of great significance to improving the quality of my country's aviation lubricant products and the reliability of aircraft engines.