Understanding Diesel Fuel Stability and Testing
In 2007, the EPA set a strict limit of 15 ppm on sulfur content in diesel fuel. While this regulation has significantly reduced emissions, it has also increased the risk of fuel instability and microbial growth over time. If you store diesel fuel for extended periods, proactive testing and maintenance are essential to ensure usability. Below are the key tests used to evaluate diesel fuel quality and stability.
Essential Diesel Fuel Tests
1. Ash Content
This test measures the amount of residue left after burning a fuel sample. Excess ash can damage fuel injection systems and engine components like piston rings. Lower ash content indicates cleaner-burning fuel.
2. Aromaticity
Aromaticity is calculated using density and viscosity data. Higher aromatic levels help prevent microbial growth, but excessive aromatics can increase emissions and ash content.
3. ATP Testing for Microbes
Adenosine Triphosphate (ATP) testing detects microbial contamination. If microbes are present, a biocide should be added, followed by manual fuel circulation and polishing to eliminate contamination.
4. Carbon Residue
Fuel is distilled, and the remaining residue is heated to 1022°F. A high carbon residue level suggests that the fuel may cause combustion chamber deposits, affecting engine performance.
5. Cetane Index
This test evaluates the combustion quality of diesel fuel, crucial for cold-weather performance. Low cetane ratings cause rough engine operation and smoke at startup, while excessively high levels can make engines hard to start. A cetane improver can correct imbalances.
6. Cloud Point
This test determines the temperature at which wax crystals begin forming in the fuel, affecting filter performance. Poor cloud point results can lead to clogged fuel filters and engine failure.
7. Color Analysis
Fuel is examined under standardized lighting and compared to a scale ranging from 0.5 (lightest) to 8.0 (darkest). Darker fuel may indicate instability and contamination.
8. Copper Strip Corrosion
A copper strip is submerged in diesel fuel at 122°F for three hours. Corrosion on the strip suggests fuel may damage fuel storage and distribution systems.
9. Density Measurement
Density is calculated from other test results, such as distillation, aromaticity, and cetane index. If those parameters are within normal limits, density will also be appropriate.
10. Distillation Test
This measures how efficiently the fuel combusts. Poor distillation characteristics can lead to rough engine performance, black smoke, or even failure to start.
11. Flash Point
This determines the lowest temperature at which fuel vapors ignite. A low flash point can lead to premature ignition and engine damage.
12. Lubricity Test
A steel ball is immersed in fuel while rubbing against a steel plate for 75 minutes under load. The wear pattern on the ball indicates how well the fuel prevents premature wear on injectors and pumps.
13. Oxidation Stability
A fuel sample is heated to 203°F for 16 hours while oxygen is bubbled through it. The resulting sediment formation determines how well fuel resists oxidative degradation.
14. Sulfur Content
This test ensures compliance with EPA regulations. Fuel exceeding the 15 ppm sulfur limit must be disposed of properly to avoid environmental penalties.
15. Thermal Stability
Samples are aged at 302°F for up to 180 minutes. The presence of insoluble material after aging indicates fuel degradation under high-temperature conditions.
16. Viscosity Measurement
Viscosity affects how efficiently fuel is injected into the engine. No. 2 diesel should have a viscosity between 1.9 and 4.1 mm²/s to maintain optimal performance and emissions control.
17. Water & Sediment Test
This test detects free water and sediment, which can indicate fuel instability and microbial contamination. Contaminated fuel may cause black smoke, deposits in injectors, and poor engine performance. Fuel polishing and tank cleaning may help restore usability, but in severe cases, replacement may be necessary.
Conclusion
Regular diesel fuel testing is critical for maintaining fuel stability, optimizing engine performance, and avoiding costly repairs. By conducting these key tests, you can ensure your stored diesel fuel remains reliable and compliant with industry standards.
