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What’s the Difference Between a Single-Stage and a Two-Stage Air Cleaner?

The Filter Guys

Air cleaners are defined into two types based on the stages the air and contaminant flow through the assembly. These types of air cleaners are considered single-stage and two-stage.


Single-Stage Air Cleaners

A single-stage air cleaner filters contaminant from the air through a primary filter without the use of a pre-cleaner. The contaminated air enters the air cleaner through the inlet side and flows directly to the primary filter. At this point, the contaminant is filtered out by the media and the filtration is completed. Single-stage air cleaners are typically used in light dust environments such as on-highway applications.

Two-Stage Air Cleaners

A two-stage air cleaner uses both a pre-cleaner and a primary filter to remove dust from intake air. The contaminated air is first sent through a pre-cleaner, which is considered the first stage. The pre-cleaner will remove 75-98% of the contaminant from the air. Pre-cleaners can be a separate add-on accessory or built into the air cleaner. Once the air is pre-cleaned, it is sent to the primary filter where the contaminant is filtered out by the media, completing the second stage. Two-stage air cleaners are typically used in medium- to heavy-dust environments such as construction and mining.


Fuel Filtration F.A.Q’S

The Filter Guys

Q1: Please explain the differences between the primary and secondary fuel filters in terms of the type of medium used, micron rating, and so forth.

Differences between primary and secondary filters vary from system to system, but in general, primary filters are used to separate water and larger particles (7-25 μm efficiency). Secondary filters are for final filtration (3-5 μm efficiency). Primary filters usually will have treated media to provide water separation performance. This can be either cellulose or a multi-layered synthetic media called melt-blown coupled with cellulose like Donaldson’s SynteqTM media. Secondary filters have untreated, multi-layered cellulose or purely synthetic media. These differences mainly have to do with the water separation requirements placed on primary fuel filters.


Q2: Have micron (μm) ratings become smaller and smaller as injection technology has advanced? When replacing filters, how do you make sure you have the micron rating that’s appropriate for your generation of engine and its injection system?

As injection technology has advanced and injection system pressures have increased the filtration

requirements have become more demanding. These systems have required filtration technology to be more and more efficient. When replacing your filters be sure you use an OEM approved replacement or a direct cross from a reputable filter manufacture to ensure you are using a filter that is appropriate for your engine.


Q3: Some truckers used to use a fine primary filter to avoid changing the secondary, while the original equipment concept was to use a coarse primary (on the suction side) and a fine secondary (on the pressure side). This took extra changes, but they liked the idea of avoiding changing the secondary. Is doing this impractical on modern engines?

Primary and secondary filters are usually balanced to provide the required engine protection and the optimum filter life. Placing a fine filter in a primary (suction) filter location is impractical because they can not tolerate as much pressure drop and will need to be changed very often. Generally, fine filters do not contain the required water separation in a primary filter.


Q4: How have new engine designs affected fuel filtration?

In the past, diesel engines had either mechanical fuel injectors or unit injectors. The drive to develop engine that meet emissions regulations has led to the application of common rail fuel injection systems. The higher pressures of common rail systems enables more precise control of fuel delivery and control of the combustion process. The goal of the new technology is to reduce the particulate matter and NOx coming out of an engine system, thereby reducing the burden on after treatment systems. The very high pressures in the common rail systems require tighter tolerances, elevating the requirements for cleanliness and efficiency on new and future fuel systems. This has created the need for increasingly better fuel filtration technology. Donaldson offers a range of products for those demanding conditions and is developing solutions for tomorrow’s requirements.


Q5: Will common rail systems bring any changes in terms of fuel filter requirements? If so, can you say what will they be?

Most fuel injection systems today are already common rail or close derivatives. The technology itself does not drive specific changes, the injection pressures and desired filter service intervals are more influential.


Q6: How important is filtering fuel stored in bulk tanks?

It’s becoming very important and can reduce future vehicle maintenance downtime. If you’re using a bulk fuel tank, filtering the fuel BEFORE putting in your vehicle is another great practice that can reduce contaminant and water from the fuel before refilling your vehicle tank. Over time, tanks can corrode, water condensation can build up, contaminant could enter the tank opening during fills.


Q7: I’ve been handling my diesel the same way for years. Why should I change the way I store fuel?

With the exception of reducing sulfur content, fuel standards have not changed substantially in over a decade. Engines, however, have changed dramatically. In order for new equipment to run trouble-free, they require much cleaner fuel. This means an increased need for filtration. Manufacturers are insistent that damage caused by fuel contaminants is not a factory defect. Therefore, it is in your best interest to filter your fuel prior to use.


Q8: Shouldn’t it be my fuel supplier’s responsibility to deliver clean diesel?

More than likely, your supplier is delivering perfectly in-spec diesel. The problem is that diesel cleanliness specifications are woefully out of date when compared to the needs of the modern engine. Some distributors are starting to go the extra yard and filter diesel prior to delivery, but this is not an industry requirement. An additional note of caution: the term “clean diesel” can also be used when referring to ultra-low sulfur diesel. This is not the same as reduced contamination levels or fuel “cleanliness”.


Q9: My fuel filters are plugging up really quickly. Should I change brands?

It is important to use high quality fuel filters to protect your engine. In most cases changing filter brands will NOT solve your fuel problems. Remember, a plugged filter did its job. Rapid filter plugging is an indication that there is a problem with the fuel, not the filter. The key to resolving rapid plugging issues is to determine how filterable solids are getting into or forming inside your fuel tank, and then fixing the root cause. Switching to a lower efficiency filter, regardless of brand, will simply spread the problem throughout your fleet.


Q10: The injectors and fuel pumps on my new equipment keep failing; what can I do?

The first step is to speak with your Original Equipment supplier. If you suspect that dirty fuel is behind the problems, a simple test can verify your fuel cleanliness level. Make sure you put the cleanest fuel possible into your equipment and protect your engine with a high efficiency fuel filter. This should eliminate injector and fuel pump problems due to dirty fuel.


Q11: Diesel is diesel, right? Why not buy from the cheapest source?

As with anything, you typically get what you pay for. Diesel is expensive, so it is tempting to minimize operating expenses by purchasing the cheapest fuel possible. While this fuel may meet minimum industry standards, that may not be adequate. Small differences in handling practices can have a huge impact on overall fuel quality and cleanliness. Saving a few pennies on your fuel bill may end up costing you far more in downtime, lost production and equipment repairs. Partnering with a good supplier is one of your best defenses against unforeseen fuel quality issues.


Full-Flow, By-pass or Two-Stage Filtration

The Filter Guys

The difference between the various lube filter configurations can be confusing. There are three common filtration approaches.

Full Flow Filtration

Full flow filters receive near 100% of the regulated flow in an engine lube system. Full flow filters provide essential engine protection for maximum cold flow performance and filter life. Most lube filters available today are full flow.

By-pass (Secondary) Filtration

By-pass filtration is when a small portion of the system’s oil flow (usually 5-10%) is diverted back to the sump or oil pan before reaching the primary filter. A by-pass filter captures smaller particles than the full-flow filter. Because of the increased efficiency of a bypass filter, they are more restrictive. To optimize restriction, a bypass filter should be located in a separate flow path, as illustrated on the right.

Two-stage Filtration

A two-stage filter design attempts to combine the features of both a full flow and by-pass filter. The two-in-one design significantly increases restriction, causing shorter filter life and decreased cold flow performance. Poor cold flow performance starves the engine of oil during start up, leaving the engine temporarily unprotected. This may lead to increased engine wear that could result in premature repairs or even engine replacement.

Filter Servicing Steps

The Filter Guys

Listed here are recommended practices from Donaldson for servicing and handling engine liquid filters. This servicing information is provided as a best practices guide. Donaldson recommends that where possible, follow the filter service instructions supplied by your original equipment manufacturer. It is not intended to replace or supersede the service instructions supplied by your equipment or vehicle manufacturer.


Where Contamination Comes From

The Filter Guys

New Fluids

Adding new fluid can be a source of contamination. Even though it’s fresh from the drum, new engine oil isn’t clean. (It may look clean, but, remember, the human eye can only see a particle the size of about 40 μm.) Also, diesel fuel cleanliness varies from pump to pump. Typical fuel cleanliness levels coming out of the pump are ISO rated at 22/21/18. (ISO cleanliness code of 22/21/18 translates to a particle count of 20,000 to 40,000 per millilitre for particles of 4 μm and greater; 10,000 to 20,000 per millilitre for particles of 6 μm and greater; and 1300 to 2500 per millilitre for particles of 14 μm and greater), and water content is typically 200 to 300 ppm. Never assume your fluids are clean until it has been filtered.



Built-in contamination, also called primary contamination, is caused during the manufacture, assembly and testing of the engine and its components. Metal filings, small burrs, dirt or sand and other contaminants are routinely found in initial clean up filtration of newly manufactured engines.



Ingressed or external contamination comes from the environment surrounding the engine or vehicle. Dirt can enter the engine liquid supply through crank case breathers or fuel tank breathers and vents and the air intake system. Ingressed moisture, particularly, can cause longer term problems. As a hot system cools at night, cool moisture-laden air can be drawn into the engine or fuel tank; as the air condenses, water is released into the engine or fuel tank. Water in excess of 0.5% by volume in a hydrocarbon-based fluid accelerates the formation of acids, sludge and oxidation that can attack internal components, cause rust, and adversely affect lubrication properties. The severity of ingression and type of contaminant are dictated by the applications and environment.



Maintenance procedures can introduce contamination into the engine. Opening the engine allows airborne particles to enter. Removing air filters, opening oil caps, fuel tank caps and removal of oil and fuel filters are all possible sources for introducing contamination to an engine. Keep your system closed as much as possible and take care to be sure everything that goes into the engine is as clean as possible. One common example is very often funnels are used fill the engine with oil. The oily funnel will collect dirt between uses. The funnel should be properly cleaned before using it to fill the engine with oil.



The major sources of contamination in the engine are the combustion by-products (soot) and oxidation of the fluids in the engine due to the thermal stressing. Wear-generated contaminants are a hazard during engine operation.




The circuit actually generates additional particles as the fluid comes into contact with the precision machined surfaces of cylinder walls and pistons, injector needles and pistons and crankshaft bearings. Contaminant levels can keep doubling with every new particle generated. The result can be catastrophic if these contaminants are not properly filtered out of the system.


Rubber & Elastomers




Biodiesel can support biological growth and generate organic contamination and microbes.


Donaldson Filtration

Filter elements for compressors.

The Filter Guys

Filter elements for compressors.





Pre-line Fuel Filtration Housing.

The Filter Guys


Features and advantages

  • Up to 600 l/h volume flow rate
  • Use of multigrade high performance media
  • 93% water separation
  • Modular system with excellent flexibility
  • Easy to service
  • Integrated water collector
  • Parallel operation possible with a number of systems



Compatibility of Filter Media with Hydraulic Fluids

The Filter Guys

Compatibility of Filter Media.

Filter media can be divided into two broad categories, natural fibres (usually cellulose) and synthetic or man made fibres (usually fibre glass or woven wire mesh), although many different variations and types of media do exist, many filters fall into one of these two categories.

Fluid to be Filtered Recommended Media
Petroleum-based Cellulose, Glass fibre
Phosphate Ester Glass Fibre
Diester Glass Fibre
Water Glycol Glass Fibre
Water-Oil Emulsion Glass Fibre
Biodegradable Fluid HWCF* Glass Fibre
Course Filtration Wire Mesh

* High water content filtration

Notes on seals:

Filters with seals made of BunaN are appropriate for most applications involving petroleum oil and some high water content fluids. Filters with seals made of Viton® of Fluorel® (both fluoroelastomers) are required when using diesters, phosphate ester fluids.

(Viton and Fluorel are registered trademarks of DuPont Dow Elastomers and 3M)

Don’t Throw Away a Good Filter Just Because it Might Look Dirty.

The Filter Guys


Although this air filter may look “dirty” – It can go plenty more miles. Installation of a restriction indicator can save you money and time.Why Service By Restriction?

Proper air cleaner servicing will result in maximum engine protection against the ravages of dust. Proper servicing can also save you time and money by increasing filter life and dust cleaning efficiency. By using proper filter restriction measurement tools you will use the full life of the filter at maximum efficiency. Don’t be fooled by filter appearance: it should look dirty.

The only way to determine when a filter is plugged or plugging is to measure the restriction on the system with the engine working at max airflow.

Two of the most common air cleaner servicing problems are:

1. Over-servicing: the least efficient time in the life of the filter is when it is new. Filter elements increase in efficiency as dust builds up on the media.

2. Improper servicing: your engine is highly vulnerable to abrasive dust contaminants during the servicing process when the filter is removed from the housing. A leading cause of engine damage is due to careless servicing procedures.

Watch Out for Old Compression Gaskets.

The Filter Guys

Watch Out for Old Compression Gaskets. When changing any filter that has a gasket – use caution as old gaskets may stick.

A compression seal is a means of preventing migration of liquids, gases or solid contaminates across a joint or opening in an assembly or housing. A seal not only prevents the escape of fluid from inside and foreign material from entering the system from outside, but it must provide for easy installation and removal. A new gasket is critical for proper filter function.


  • Remove used gaskets and clean the sealing area thoroughly
  • Always use a new gasket with a replacement filter
  • Over-tightening the filter may damage the head
  • Dispose of used filters properly
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