Air filters, how they work

An air filter purifies the air before it enters the engine. That makes sense. Yet there is much more to it and we will explain that to you in this blog.

The basics of an air filter

An air filter usually consists of a primary and a secondary filter. The first is for filtering the fine and coarse contaminants that you don't want in the engine. The secondary filter has a different function than many people think. This filter is not there as a second stage to filter even smaller particles, but serves to prevent dust from entering the air intake of the engine when replacing the primary filter. This is one of the biggest misconceptions about the secondary filter.

A pre-cleaner is worth the investment

If you have a machine that often works in dusty environments, it can quickly pay off to install a pre-cleaner on your machine. This is an extra filter that you place on the air inlet of your machine and that ensures that the first coarse dirt is removed from the air before it reaches the primary filter that really has to filter the air before it goes to the engine.

A pre-cleaner works on the basis of air circulation. In the pre-cleaner, the air is, as it were, thrown around, causing the heavier particles of contamination to be thrown to the outside and against the housing of the pre-filter. This housing contains a discharge channel through which these heavier particles are ejected. For extreme conditions there are also pre-filters that consist of two stages. The so-called dual-stage pre-cleaner. The operation of the “second” pre-filter is basically the same as that of the first.

Because a pre-filter already filters much of the pollution from the air, the primary filter is used much less and can therefore last much longer. If your machine does not work much in dusty environments, you do not need a pre-filter.

The importance of an air filter for your engine

It sounds so logical that your engine will last longer if you replace the air filter in time. This is partly true, but more about this later. If dust enters the engine, it is involved in combustion in the cylinders. If this contamination burns in the supply air in the cylinders, this forms a layer on the inside of your cylinders. This layer is often many times harder and coarser than steel and therefore never comes off. This causes extra wear and tear and makes your engine last much shorter.

As a rule, you can say that an amount of just 50 grams of dust contamination in the supply air can permanently destroy a small engine. In the case of larger engines, such as in trucks and excavators, this is approximately 100-150 grams and even the largest engines, for example in mining machines, can fail as soon as more than 200 grams of dust reaches combustion. This is extremely little when you look at the cylinder capacity and size of these engines. It is therefore important for the life of your engine that you prevent the formation of this silica layer and supply clean air to your engine.

How much air goes through your engine?

Modern engines must operate more and more economically and efficiently. This is partly due to emission requirements, but also because companies are increasingly looking at the consumption of the machines for lower costs or certain tenders. In modern engines, a lot of air is often fed into the engine because they are equipped with turbos or extra cooling of the engine is required. This extra cooling, through after or intercoolers, also requires more air to pass through the filter.

A good practical example of this is the Cummins 4B engine. This is available in three variants:

1. Cummins 4B 3.9 liter unblown
2. Cummins 4B 3.9 liter with turbo
3. Cummins 4B 3.9 liter with turbo and aftercooler

The first version produces 76 hp, the second version with turbo 105 hp and the third, with aftercooler and turbo, delivers 120 hp. In practice, this means that the latest version delivers approximately twice the performance of the first.

If you look at the amount of air that these engines need, the first engine, without a turbo or cooler, needs 11,700 liters of air to burn just 1 liter of diesel. The second engine needs 17,100 liters to burn 1 liter of diesel and the third variant even needs more than 20,000 liters of air to burn just 1 liter of diesel.

If you look at volume, the first engine uses approximately 4 M³ of air per minute, the second 7 M³ and the third even 9 M³ of air per minute.

Strange but true, a used air filter is better than a new one

It seems logical that a filter should be replaced as soon as it becomes dirty and full of dust. At first glance, a dusty filter seems to indicate that you need to replace the filter. This is not necessarily true. Did you know, for example, that an air filter filters even better when dust gets on the filter media?

This is because the first layer of dust on the filter often consists of somewhat coarser contamination. This first layer of retained pollution and dust ensures that this layer will help retain smaller particles in the sucked air. The larger particles of the first dust layer still allow air to pass through, but do help filter out smaller particles. In fact, of all the contaminants that a filter lets through in its entire lifespan, approximately 10% is let through during the time the filter is new. This is the period when the first layer of coarse contamination has not yet formed. The efficiency of an air filter therefore initially improves from the first dust that the filter absorbs. In fact, a filter functions best and is most effective just before it is really clogged, does not allow enough air to pass through to the engine and needs to be replaced.

Inspecting your air filter

A visual inspection sometimes gives an indication that a filter needs to be replaced. But with the above in mind, it is not that simple. If a filter is full of dust it can still have better filtration efficiency than a new filter. So this doesn't help you much. Another disadvantage of visual inspection is that you remove the filter from the housing every time. This increases the risk of incorrect replacement and of the filter seal being deformed or damaged. The disadvantages of visual inspection:

• You cannot see whether a filter is still doing its job. A filter with a thick layer of dust often functions even better than a new filter
• During a visual inspection you can deform or damage the seals, which could cause pollution to enter your engine

What is also often done when installing a new filter is to write the date and operating hours of the machine on the filter. This allows you to better monitor planned maintenance. There are also some disadvantages to this form of inspection:

• You must keep track of the operating hours in a document or Excel
• You cannot estimate how much the filter has suffered. Did it have to work under tough conditions or is it not too bad and can the filter actually last for many hours of operation?
• You often replace your filter far too early
• You regularly carry out (unnecessary) inspections to see what date and operating hours were on the filter

A possible solution is the electronic indicators that many filter brands now offer. Such an indicator measures how much air is still allowed through and whether there is still enough air going to the engine. These indicators are well worth the small investment. Not only because of the convenience, but especially because they quickly pay for themselves. Since you cannot see when a filter needs to be replaced and you therefore only have limited insight into how much life is left in your filter based on operating hours, you can easily replace a filter too early. With an indicator you get the maximum lifespan from your air filter. Because you replace the filter at exactly the right time, your engine will also last longer, but this is difficult to calculate. The calculation for early replacement is not that difficult. Research showed that air filters are often replaced about 30% too early or preventively. If your filter lasts 30% longer, you simply have far fewer filters to replace during the life of your machine. Advantages of an electronic indicator:

• Your filters will last longer because you replace them exactly at the end of their lifespan
• You save costs because you can use filters for longer
• You avoid using a filter for too long and that the filter has already exceeded its service life