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.LocationThe primary fuel filter is typically located on the truck frame. It is plumbed between the fuel tank and the engine’s gear pump.FunctionThe 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.LocationThe secondary fuel filter is mounted directly onto the engine. It receives fuel after it has passed through the engine’s gear pump.FunctionThe 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 ImportanceWhy 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 [...]