Determination of Existent Gum Content in Motor Gasoline (Jet‑Evaporation Method)

Determination of Existent Gum Content in Motor Gasoline (Jet‑Evaporation Method)

Overview
  During storage and service, motor gasoline is prone to oxidation and polymerization reactions, generating non‑volatile gum deposits. Such deposits may block oil circuits and adhere to internal engine components, which directly affect oil product stability and engine operating conditions. As a core indicator for gasoline quality control, existent gum can intuitively evaluate the degree of oxidative deterioration of oil products.

Test Objective
  Determine the existent gum content in motor gasoline and assess the oxidative stability and compliance of oil products in accordance with GB/T 8019‑2008 Standard Test Method for Gum Content in Fuels by Jet Evaporation (equivalent to ASTM D381). The test is carried out with the SH8019A Five‑Hole Existent Gum Tester.

Test Sample
Commercial 92# motor gasoline

Test Instruments and Consumables

1. SH8019A Five‑Hole Existent Gum Tester
2. Oil‑free silent air compressor, air purification unit
3. Standard test beakers, trumpet‑shaped jet nozzles, precision glass thermometer
4. Analytical balance (0.1 mg readability), n‑heptane washing solvent

Operating Procedures
1. Clean and dry test beakers to constant mass, and record the mass of empty beakers. Measure and pour the sample into beakers, then place the beakers into test wells of the evaporation bath.
2. Set the temperature controller to constant‑temperature mode at 165 °C. Wait until the bath temperature stabilizes within 160‑165 °C. Adjust the pressure‑reducing valve of the air compressor to keep supply pressure stable at 0.1‑0.2 MPa.
3. Adjust the rotameter for each well to set the airflow rate of each single well to 600 mL/s, and confirm that the temperature at the bottom of beakers stays between 150‑160 °C.
4. Set test duration and start the jet‑evaporation procedure. Upon completion of evaporation, take out the beakers. Wash motor gasoline test samples with n‑heptane and dry the beakers again to constant mass.
5. Calculate the existent gum content of the sample based on the mass difference of beakers before and after testing.

Data Analysis and Result Evaluation
1. Measured data of 5 parallel samples
Five parallel tests are performed simultaneously on the same 92# motor gasoline sample. The results of solvent‑washed gum are shown below:

| Test Well No. | 1 | 2 | 3 | 4 | 5 |
|---|---|---|---|---|---|
| Test Value (mg/100mL) | 2.1 | 2.3 | 2.0 | 2.2 | 2.2 |

The arithmetic mean of five replicates is 2.16 mg/100 mL; the range within replicates is 0.3 mg/100 mL; the relative standard deviation (RSD) is 2.1%.

2. Compliance Judgment
  Per product specifications for motor gasoline, the limit for solvent‑washed gum is ≤ 5 mg/100 mL. The mean value obtained in this test is far below the national standard limit, so the sample is judged qualified.

3. Verification of Instrument Performance
‑ Temperature‑control precision: Integrated aluminum bath block offers temperature‑control accuracy of ±0.1 °C; temperature fluctuation ≤ 0.2 °C within the working range of 160‑165 °C.
‑ Flow stability: Rated airflow rate per well is 600 mL/s with full‑process flow fluctuation ≤ 3%.
‑ Repeatability: Difference between two test results by the same operator shall not exceed 5 % of the arithmetic mean, better than the 15 % repeatability limit specified in national standards, showing good parallelism.

4. Implication of Excessive Test Results
  Excessive gum content indicates poor oxidative stability of oil products. Long‑term application will lead to fuel injector blockage, valve carbon deposition, increased fuel consumption and other failures. Such oil products shall be prohibited from factory release and circulation.