Diesel Fuel Doesn’t Last Forever
Modern fuels like ultra-low sulphur diesel (ULSD) and biodiesel aren’t as stable in storage as older, high-sulphur mineral diesel. While removing sulphur is a critical step toward emissions reductions, the process of hydrodesulphurisation also strips away natural antioxidants that once helped preserve fuel quality. As a result, today’s diesel often needs stabilizers to reduce the rate of degradation in storage.
Depending on conditions and biodiesel blend ratio, fuels can start to oxidise in as little as 2 months, and many blends have a shelf life of under a year.
What Happens When Fuel Degrades?
When diesel fuel becomes unstable, it breaks down into compounds that can impact your engine and fuel injection system. Common byproducts of fuel degradation include:
- Gums – Sticky varnishes that contribute to corrosion and injector deposits
- Sediments – Solid filter blocking particles that cause abrasive wear to the fuel pumps and injectors.
- Acids – Corrosive to fuel tanks and systems, leading to hard particulate formation and premature component failure
- Thickeners – Increase fuel viscosity,y reducing combustion efficiency
You may notice symptoms like black smoke, hard starts, poor engine performance, or increased fuel consumption. Over time, degraded fuel leads to higher maintenance costs and reduced equipment reliability.
Why Storage Conditions Matter
Diesel quality declines faster under poor storage conditions. Extreme or fluctuating temperatures, water contamination, and extended storage times contribute to degradation.
Key contributors include:
- Heat – Oxidation rates double roughly every 18°F (10°C) temperature increase
- Water – Contributes to oxidation, corrosion, stability issues and provides the environment for microbial growth
- Contaminants may increase the rate of oxidisation
Here’s how storage temperature affects biodiesel blends:
- 68°F (20°C): Fairly stable for up to 6 months
- 77°F (25°C): Likely degraded after 6 months
- 86°F (30°C): Often degraded after just 4 months
Once fuel degrades, there’s no way to restore it, so prevention is key.
What About Fuel in the Vehicle?
Degradation doesn’t just happen in storage—it can also occur on the vehicle. High-pressure common rail (HPCR) systems expose diesel to extreme pressure and with pressure comes heat. Fuel returned to the vehicle's tank may carry oxidisation products or carbon particles, sometimes referred to as graphite.
As injector pressures rise to meet stricter emissions standards, the thermal stability of diesel fuels will be more important than ever.
Preventing Fuel Degradation
To maintain fuel in “fit for purpose” condition and protect your equipment:
- Consult your fuel supplier for specialised advice
- Treat new fuel with oxidisation stability enhancers for long-term storage
- Store diesel in clean, cool, and dry environments
- Regularly inspect tanks for water and contamination
- Use only high-quality filters and deliquescent breathers
- Turn and burn fuels to ensure engine performance
