Hi Rick,

Most filters operate in what is called the laminar flow regime (Re << 1). In this regime, the inertial capture probability/efficiency for particles ALWAYS increases with increasing filter face velocity. The caveat on this is that as inertia increases, the probability that particles will bounce off the fibre rather than adhere also increases as velocity increases.

It is hard to give an actual number - as it depends on a lot of factors, such as the elastic deformation ability of the fibre and the particle (to absorb the impact), etc. However, most air filters operate at v<1 m/s, and in this range, rebound is unually negligible.

So in short - yes, thereis some wisdom in what you were told. Also - as velocity increases, the effectiveness of the interception and DR (interception-diffusion combined) filtration mechanisms may become less effective as well.

This is one instance where the oil coating on the K&N may help - in reducing the likelyhood of particles bouncing off the fibre when they impact. HOWEVER - I saw some data from K&N filters which were used in F1 race applications (with catastrophic results). When tested under identical conditions (v = 10 m/s !!!), the filtration efficiency peaked at about 85%, then started to DECREASE at about 20 microns. This was all due to particle bounce - so under such extreme condionions, all sorts of strange effects can occur!

It would be interesting to see a comparison of traditional oil bath air cleaners (e.g. Series) included.

John

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JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol

A Texaco rep in Sth America (Bolivia) on the BITOG forum mentioned above had quite a few clients change their air filters from the OE paper in Toyota's to oil baths (being pedalled as superior by salesmen) and when the oil was tested the silicon levels were astronomical. They were also needing rebuilds in a very short space of time.
It could be the case they were undersized and the velocity was miles too high through them.

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thanks Ben.
IIRC our basic rule of thumb used to be to keep the face velocity <2.5m/s on most commercial a/c applications.

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Or just plain lousy design! Don't forget that all it takes is a design that allows intake air to bypass the medium and any filter comparison is meaningless.

John

__________________
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol

The other problem with changing filters too early/inspecting too often is the clean side contamination which can occur during the actual process of changing/removing the filter.

On the topic of a slightly different type of filter - oiled foam, do these increase in efficiency when dirty? Am thinking of the foam filter in my dirt bike - I have read many times that its best to clean and re-oil after every dusty ride for best engine life.......

I haven't done a test, however I would be very surprised if oil bath filters do much more than remove the rocks. The principle they operate on is much the same as a cyclonic pre-cleaner (inertial separation only (i.e. no interception or diffusion)). A good cyclonic pre-cleaner removes 50-80% of the dust mass entering it.


ALL air filters increase in efficiency as they load with dust. The only exception is when there are excesses of moisture or oil in the air, which creates regions which are clogged by large water/oil droplets, rather than a porous dust cake. So if you are riding in muddy/wet conditions, maybe cleaning the filter after is a good idea, but if it is just dusty...

I assume you are talking about HVAC intake filters? They normally operate at fairly high velocities (for a filter) and can often be quite porous, so yes, perticle bounce could be a problem if the velocity is increased further.

Damn excellent test. I have tried most filters over the years on several of my 4x4s . Usually never keeping them long term to find out if they contribute to longevity of internals. More concerned with performance at the time.




2000 Disco II stock
2000 Hilux 3l,ome,sus,extractors,k&n filter
98 2.8 ome lift,long range tank,k&n
96 coil zook 40mm lift,k&n
84 mk patrol 50mm ome lift,50mm body lift,finer then k&n
84 hj47 lifted,diff locked,finer
IIa stock
II 265 hemi rangerover auto,dualrims
74 fj45 lifted,low ratio diffs,worked rebuilt 257eng,finer

It would still be interesting to see the results, though - the principle is much the same, but not exactly. Some cars with oil bath air cleaners seem to have very durable engines - one example is the post 1967 Citroen D series, where 200,000km + is usual. A friend of mine, who ran a Citroen dealership in Melbourne stocked a set of pistons and sleeves when the engine was introduced in 1967 - he still had them when he retired in the early nineties.

Interestingly, if you look at motoring history, air cleaners were the exception in cars and trucks until the late 1920s, although they were standard with tractors almost from the start of internal combustion engines in tractors, which often work in very dusty conditions (but so did early cars!). They were almost invariably water bath filters, occasionally oil bath.

What seems to have made air cleaners essential was the almost universal adoption of alloy pistons in cast iron bores - these are much more susceptible to grit than cast iron pistons in cast iron bores, as the grit embeds in the softer material, turning it into a grinder. Oil bath cleaners remained almost universal until the end of the fifties, and common into the seventies (e.g. Landrover), and since this period produced some quite durable engines, presumably they were reasonably effective.

John

__________________
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol

I have only limited knowledge of this subject so experts please excuse my fumbling thinking but doesn't this thought flow path apply:

1. Particulate matter in the combustion chamber is a recognised cause of extraordinary wear;

2. Fuel and air supply are the sole sources of that particulate matter;

3. Engine power and efficiency objectives are dependent on there being no air supply quantity and temperature "constraints" and this is before considering "opportunities" like cold-ram-air-fuel mixtures associated with modified air intakes;

4. The only air supply "constraint" is that associated with air supply, cleaning and monitoring devices; and

5. Assuming particulate matter in fuel will cause a problem with fuel supply system before it causes a wear problem in the combustion chamber, at what level does particulate matter in air supply cause a unacceptable wear problem having regard for avoiding the air constraint objectives identified in #3?

I am a fan of oiled air filters sitting in large appropriately designed air boxes (not found often in modern engine bay design) largely because while I have seen plenty of critisism of their being not as efficient in collecting particulate matter compared to other filter designs I have never seen any evidence that this lack of efficiency (what ever it maybe in particulate quantum and size) is engine threatening.

Having said that, this critisism of my K&N has had a impact as I keep a paper element handy if ever caught in a extreme dusty situations...LOL LOL