Understanding the Difference Between Primary and Secondary Fuel Filters

Modern diesel engines are true marvels of technology, capable of powerful performance, improved fuel economy, and durable enough to run for decades. Like any engine, diesels require clean fuel to operate properly. However, the more refined the engine—like today’s cutting-edge diesels—the cleaner the fuel needs to be.

Most diesel engines have a primary fuel filter and a secondary fuel filter to ensure that the fuel is clean before it enters the engine. Filtration systems typically use a measurement system based on micrometers or microns. That’s how you know the level of filtration they offer. A grain of salt is around 100 microns in size. Diesel fuel filters are designed to filter out particles anywhere from 50 microns down to 2 microns.

Drivers are often confused about which filter is which and why they even need two filters. Let’s look at what each filter is for, so you know exactly what you are looking for when choosing among diesel engine fuel filtration systems.

What is a Primary Fuel Filter?

As the name indicates, a primary fuel filter is the first line of defense among truck fuel filters.

Location

The primary fuel filter is typically located on the truck frame. It is plumbed between the fuel tank and the engine’s gear pump.

Function

The primary fuel filter is designed to remove larger particulates from diesel fuel. These first-line filters can be designed to catch particulates ranging from 50 microns to 7 microns. Many primary filters can also remove free water from the fuel. This filter helps clean up the fuel before it goes into the fuel pump to be sent to the injectors.

What is a Secondary Fuel Filter?

The fuel quality in diesel engines can vary quite a bit depending on where it comes from. The primary filter cleans out the larger particles while the secondary filter takes care of the smaller particles.

Location

The secondary fuel filter is mounted directly onto the engine. It receives fuel after it has passed through the engine’s gear pump.

Function

The secondary filter is responsible for finer filtration. It uses filter mediums with tighter micron ratings, typically 5 microns to 2 microns. These filters need to get the fuel as clean as possible before it reaches precision components like the fuel injectors. They also operate at much higher pressures than primary fuel filters.

Differences in Function and Importance

Why don’t diesel engines just use one filter? Multiple benefits come with the two-filter approach, which is why it’s the go-to design for most engines. The primary filter and the secondary filter are each located at different stages of the fuel system and need to accomplish specific things at those stages. A one-size-fits-all approach would also sacrifice performance and be highly inefficient.

There are several components that filtration protects, including:

• Rubber tubes and metal fuel lines. These lines operate at intense pressure and are damaged by particulates in the fuel. Effective filtration removes those particles so that they don’t corrode the tubes and lines in the system.
• Fuel tank. Water and other contaminants will always make their way into the fuel and therefore must be removed. Circulating fuel through the filtration system protects components like the fuel tank from excessive corrosion.
• Fuel pump. The fuel pump takes fuel from the tank to the fuel injectors. It can be damaged by particulates, water and other contaminants.
• Fuel injectors. The injectors spray a fine, pressurized mist of fuel into the engine to ensure optimal combustion and performance. They are sensitive to contaminants like water and particulates, and are easily damaged if you run dirty fuel through them.

Why have a primary filter?

The primary filter is responsible for removing larger particles and water from the fuel. It operates at a lower pressure than the secondary filter and therefore needs to be designed to be effective at that pressure.

If engineers tried to design the primary filter to remove all particulates, it would not be as effective at removing water from the fuel, and it would struggle to remove both large and small particulates at the same time.

Having only a single filter would also mean changing that filter more often. It would not protect as well as two filters and the system components would wear out faster.

Why have a secondary filter?

A secondary filter has a vital job, which requires fine-tuning and precision – a job impossible with the primary filter. The secondary filter is located right before the fuel injectors, because it’s incredibly important that the fuel entering the injectors is as clean as possible. The pressure at this point in the system is quite high, and the filter must be capable of operating consistently at that pressure.

If there was only a secondary filter, it would be much harder to remove water from the fuel and larger particulates. The conditions in which the secondary filter operates would make removing all these things at the same time difficult, if not impossible.

Why do some trucks only have one filter?

Some trucks, particularly older models, might only have a single filter. If that’s the case, then that particular engine was designed to run with only a single filter. Having two fuel filters in a diesel engine system was an innovation that helped create conditions where cutting-edge engines could operate at the highest levels. While older engines might run forever and get the job done, they do not meet the fuel efficiency, emissions and other standards demanded of modern engines.

The Role of Modern Diesel Engine Systems

Modern diesel engines are more powerful, more energy efficient and run much cleaner than their older counterparts. The engineering work that went into creating such engines was substantial and impressive. Of course, there are always tradeoffs in these situations. For modern engines to work like they do, they need very clean fuel.

Advanced injection systems make optimal performance possible, but they are sensitive to contaminants in ways older systems did not. The same is true for modern emission systems. They protect the environment and keep the air we breathe much cleaner than possible with older diesel engines. These emission systems are also sensitive to excessive fuel contamination.

The AirDog Advantage

The AirDog fuel filter is a superior primary fuel filter solution. Using our patented technology, the AirDog cleans fuel in ways that other systems cannot, which is why it’s proven to improve engine performance. AirDog provides the cleanest fuel supply to the transfer pump, and extends the lifespan of secondary fuel filters and other system components in the process.

AirDog is an investment in the efficiency, performance, and lifespan of your diesel system. It removes water, entrained air and vapor, particulate matter, and anything else that makes its way into your diesel fuel. That way, your engine is only getting pure, clean diesel, at net positive pressure, so your truck’s fuel system can perform optimally.

Conclusion

Understanding the fundamentals of how your diesel fuel system operates can help protect your engine and get the most out of your vehicle. There are plenty of truck fuel filters out there, but none can provide the level of protection, peace of mind, and complete fuel system optimization that you will find with AirDog. Please check out our other blogs to learn more!