What Is Fuel Quality and Why Does It Matter?

Storing diesel fuel is vital for a wide range of industries — your stored fuel allows you to use generators for energy and industrial vehicles during emergencies. Fuel quality directly relates to performance — higher quality fuel works better, igniting quicker. It also protects equipment, which can sustain damage when exposed to poor-quality fuel. Understanding diesel fuel quality is vital for businesses that store and use large quantities of diesel fuel. 

Proper storage methods allow for preserving fuel quality between uses. Regular quality monitoring will help you protect your equipment and ensure you have access to usable fuel when needed. Learn more about fuel quality — how to measure it, why it’s important and what best practices you can follow to ensure the highest quality fuel when you need it. 

What Is Fuel Quality?

The definition of fuel quality depends on the type of fuel and its intended uses. Diesel fuel quality relates to its performance, which depends on its cleanliness and its contents. The contaminants that mingle with stored fuel degrade its performance and quality. High-quality fuel is essential for operating generators, vehicles and other equipment. Additives and stabilizers can help to preserve fuel quality, as can appropriate storage procedures. 

Fuel Quality Standards in the US

Several different fuel quality standards apply throughout the United States. The Environmental Protection Agency (EPA), federal government and state governments set regulations with which fuel producers must comply. Diesel fuel is subject to the American Society for Testing and Materials (ASTM) D975 standard specifications for diesel fuel. 

The specifications describe 13 tests and their acceptable limits. For instance, the EPA restricts how much sulfur diesel fuel can contain. Fuel quality comparison depends on the results of these standardized tests. These standards help to reduce pollution and make diesel fuel safer for the environment and public health. Since fuel quality impacts its performance, it’s crucial to understand how to measure it and how to preserve it. 

How Is Diesel Fuel Quality Determined?

The main way to measure diesel fuel quality is through the cetane number, which describes the fuel’s ignition quality. A higher cetane number means the fuel ignites quicker and burns more thoroughly. In that way, the cetane number rates the quality of combustion. Higher cetane numbers boost engine performance. Determining fuel’s cetane number requires a standardized fuel quality testing method using a single cylinder in a variable compression ratio diesel engine. Pure cetane has the highest possible cetane number of 100

Contaminants, additives and other properties determine cetane numbers and fuel quality levels. Several contaminants and other factors can affect fuel performance. Following best practices and keeping up with routine maintenance can help decrease contaminants buildup and retain fuel quality. 

Storage Contaminants That Can Affect Diesel Fuel Quality

Storage Contaminants That Can Affect Diesel Fuel Quality

Several contaminants impact the quality and performance of diesel fuel. Improper storage or prolonged lack of use can cause these particles to appear. Listed below are the biggest threats to fuel quality.

1. Water

The presence of water degrades fuel quality by causing a reaction called hydrolysis. If condensation accumulates within the diesel storage tank, water droplets fall into the diesel fuel. The resulting chemical reaction breaks down the diesel fuel. Hydrolysis is a general term for any chemical reaction in which water breaks chemical bonds. Water in the storage tank also makes the fuel susceptible to microbial growth, since organisms like bacteria and fungus require water to thrive. 

Water condensation is a common issue when it comes to diesel tanks. The fuel has no vapor pressure to displace air. If the tank is warm, expanding air gets forced out. Then, as the tank cools, humid air gets sucked back in, causing water to condense inside. Avoiding condensation buildup is one of the reasons proper diesel fuel storage is a necessity. 

2. Microorganisms

Since microbial life requires water to live and reproduce, these tiny organisms accumulate as water interferes with diesel fuel. Bacteria, fungi and other microbes degrade fuel quality and performance. Microbial contaminants:

  • Produce acids that eat away at diesel fuel.
  • Clog tank filters, limiting filtration. 
  • Restrict the flow of fluid in and out of the tank. 
  • Corrode the storage tank itself. 
  • Damage the engines in which the fuel is used. 

Microbial contaminants are a greater concern today than they were in the past, following the Environmental Protection Agency’s (EPA’s) regulations of sulfur content in diesel fuel. Since 2017, the cap for sulfur in gasoline has been 10 parts per million (ppm). Restricted sulfur content reduces air pollutants and allows for stricter emissions standards. For that reason, it’s essential for reducing environmental impacts and preserving public health. At the same time, restricted sulfur content makes fuel more susceptible to microbial growth. 

3. Dirt

Another major threat to fuel quality is dirt. Sediment buildup in the form of sand, dust, rust and other debris can lead to serious performance issues upon use. Using dirty diesel fuel can cause poor engine performance, trouble starting up, stalling, misfiring and part failures. In that way, dirt within diesel fuel can reduce equipment life spans. 

Other Factors That Can Affect Fuel Quality 

Certain factors tend to impact fuel quality by increasing the opportunities for contamination. Proper storage and regular fuel replacement can decrease these factors and protect fuel quality. Here are some of the concerns regarding stored diesel fuel.

1. Time 

The longer stored fuel sits, the more likely it is to undergo a decline in quality. Diesel fuel will remain usable in storage for a matter of months, but adverse storage conditions shorten its shelf life. Over time, various environmental factors will affect the fuel, causing chain reactions. Long-stored diesel may appear sludge-like in consistency rather than oily. The change in consistency is due to microbial buildup and chemical reactions. Thick, sludge-like fuel will not perform well and may damage equipment. In extreme cases, it can prevent an engine from starting. 

2. Temperature 

Another factor impacting fuel quality is temperature. Diesel fuel composition varies depending on region and temperatures. Fuel intended for warm weather will not store well in cold weather and vice versa. Warm ambient temperatures can speed the deterioration of diesel fuel. On the other hand, cold ambient air can contribute to condensation within the tank. As a result, temperature control surrounding a diesel storage tank is essential. 

3. Humidity 

Because water is a major diesel contaminant, humidity levels also affect fuel quality. The amount of water in the air correlates to the amount of water condensing in a diesel storage tank. Diesel fuel in storage will maintain its quality longer in low-humidity settings. You can also cut back on condensation buildup by limiting the amount of empty space in a storage tank — empty space allows water droplets to form. 

4. Oxidation

Oxidation is the chemical reaction that occurs due to oxygen exposure. As diesel fuel mingles with oxygen, the resulting chemical reaction leads to higher acidity levels and sludge buildup. The acidity corrodes the tank while the sludge clogs filters. Both effects decrease diesel performance and damage equipment like storage tanks, fuel injectors, fuel lines and engines. Of course, exposure to oxygen is inevitable. However, certain fuel additives can lengthen diesel fuel’s shelf life by slowing down oxidation. 

How to Store Diesel Fuel to Protect Fuel Quality

It’s essential to practice proper storage methods to maintain the best quality fuel possible. How you store your diesel fuel can guard against contaminant buildup, ensuring you always have access to good quality fuel and protect your equipment from wear or damage. Here are some best practices for storing diesel fuel.

1. Keep the Tank Cool

You should ensure your storage tank is cool. High temperatures speed up the oxidation process, resulting in sludge accumulation and higher acidity levels. You might consider investing in an underground storage tank to maintain consistent cool temperatures. Another method is to build an enclosure for your storage tank. In addition to reducing ambient temperatures, an enclosure will limit water and humidity exposure. 

2. Treat the Fuel 

Treated fuel will last longer in storage than untreated fuel. You’ll want to look into fuel additives and stability treatments to maintain diesel fuel quality. Additives and treatments can increase your fuel’s shelf life by:

  • Dispersing water molecules or particulates in the storage tank. 
  • Counteracting microbial growth or killing microbial life. 
  • Stabilizing the fuel. 

Which additives will help you maintain your fuel depends on how and when you plan to use it — experts can determine a customized additive content for your fuel. 

3. Only Use Diesel-Specific Treatments

Be sure to stick to additives and treatments meant specifically for diesel fuel rather than generic options. Not all treatments and additives will help to preserve diesel fuel quality — some might even speed up degradation. It’s essential to use the right solutions for diesel, specific to your unique needs. 

Empty the Tank Every 10 Years

4. Empty the Tank Every 10 Years

Long-term stored fuel management calls for deep tank cleaning at least once every decade. Schedule a time to empty and clean your storage tanks once every ten years. Doing so will:

  • Ensure high-quality fuel. 
  • Extend the life span of the tank itself. 
  • Protect equipment from accumulated tank contaminants. 

Regular preventive maintenance can help you avoid unexpected downtime and equipment damage. While the tank is empty, a thorough inspection should occur and any necessary tank or component replacements should take place. Replacing storage tanks and accessories as needed will help make stored fuel last longer. 

5. Consider Underground Storage

Where and how you place your storage tanks can also affect diesel fuel quality. For instance, underground storage provides better surroundings for a storage tank by minimizing exposure to heat, humidity and other environmental influences. In addition to keeping the tank cooler and dryer, it will also decrease opportunities for damages or leaks. 

The initial expenses related to underground storage might be greater. However, the superior fuel protection it provides is worth the investment in the long run. If you plan on implementing long-term diesel fuel storage, consider investing in underground tanks. If you opt for an aboveground storage tank, be sure to follow placement and maintenance guidelines for fuel preservation and safety. 

6. Monitor the Tank for Water Buildup

Regular monitoring is also a fundamental part of fuel storage. You should check for water buildup on a regular basis, especially during humid or rainy parts of the year. Humid conditions lead to iincreased condensation within the tank, and pooled water on the top of the tank will lead to rusting in metal containers. Monitor for water in, on and around your fuel storage tank. 

7. Keep the Fuel Away From Ignition Sources 

For safety and fuel preservation, it’s vital to keep your storage tank away from any ignition sources. Diesel fuel is flammable, with a flashpoint of 100 to 204 degrees Fahrenheit. Make sure any nearby electrical outlets are rated for explosions. You should also put up signage prohibiting smoking or open flames surrounding the tank.  Any fuel storage tanks should be located in isolated areas away from residences. 

8. Keep the Tanks Full 

Minimizing empty space within the tank limits water buildup and microbial growth. It’s a good idea to enroll in a regular, automatic fill program to keep your tanks full. Doing so will ensure you always have access to high-quality fuel for routine or emergency use. The last thing you’d want during a power outage is to find your stored fuel has degraded and is no longer usable to power your generators or equipment. Unexpected power outages can result in lost revenue, unturned inventory buildup, decreased productivity and other undesirable effects. Being prepared with usable diesel fuel can prevent these issues. 

Keeping your tanks full will also preserve your tank and related equipment by reducing contaminant accumulation. In that way, maintaining full tanks can help you extend your equipment’s life span. Be sure to keep your diesel storage tanks full with regular refilling services. 


Contact Foster Fuels’ Mission Critical Division

Storing large quantities of diesel fuel is a necessity across many industries. Those who do not use diesel fuel regularly may still require stored fuel to run generators and power equipment during emergencies. No matter why you need to store diesel fuel, following best practices will help to preserve its quality, maintain high performance and protect your equipment. Contaminants to avoid include water, microbial life and dirt. Fuel additives, proper storage and regular maintenance will help to minimize contaminant buildup. 

It’s essential to store your diesel fuel tank in cool, dry and covered conditions far from any possible ignition sources. Underground storage may be the best option, but enclosures can make aboveground storage viable, as well. Regular tasks should include deep cleaning the tank once per decade, monitoring for water buildup and scheduling refillings. 

If you’re looking for diesel fuel services, consider Foster Fuels. Here at Foster Fuels, we help preserve high fuel quality through testing, analysis and customized additive solutions. We also provide emergency fuel services for immediate fuel needs and fuel pump-outs due to tank leaks or contamination. To learn more about Foster Fuels diesel fuel services, reach out to us today