Very confused...need an engineers help maybe??

[shelbyman]

Ok...I will admit it... I am confused as to JUST EXACTLY how the standard spin on oil filter works. I know this is a long post but very important...I think. I mean I don't understand this and can imagine a lot of others are in the dark as well.

I have thought all my life that the oil flowed out of the engine and entered the filter through the round outer holes into the filter. Then the oil FLOWED completely through the paper element into the inner portion of the filter and then was routed through the hollow center section of the filter through the threaded hole back into the engine. I was under the assumption when cold and thick the oil could be routed to the bypass valve at the far end of the filter and not go through the paper element until it was sufficiently warmed up and viscous.

Well an acquaintance and supposed chemical engineer has told me the following. That the oil enters the filter in the manner described above BUT rather than flowing COMPLETELY through the paper element it flows ALONG or parallel to the paper element with the debris in the oil just randomly attaching as it moves along the paper. Then I was a bit unclear on how it made it's way back to the center section to return to the engine. Whether or not it went the length of the filter and went past the bypass to return wasn't clear.

Well in light of this information I cut a filter apart and took the folded paper element inside and stretched it out a bit and poured some 30w oil on it. I was surprised to see the oil NOT go through the paper element...eventually a few drops did go through. But I can't imagine with the engine running and the oil under pressure that very much actually passed completely THROUGH the paper element. Now I know the oil I poured on the element was not hot like in an engine and that may affect the result but I still can't imagine much going completely through the element. The paper was so restrictive to the flow.

So now what I would like to know from an expert is just what happens in one of these filters. I have seen cleanable filters that use a fine stainless steel mesh screen to let the oil pass through and thought this is a good idea as its not as restrictive as the paper and would filter all the oil all the time. Any comments on these filters would be appreciated as well. I would like to get comments from a source that has no vested interest as that can distort the facts.

Thanks much for this forum. I'll look forward to reading the responses.

 

[Brian Barnhart]

In the engine, pressure resulting from the flow of oil from the oil pump creates enough differential pressure across the paper filter element to force the oil through the paper. A few drops of oil on the filter paper create too little (almost zero) differential pressure. The weight of the oil (gravity) wasn't enough to overcome the paper's resistance to flow.

 

[Gary Allan]

I'm not an engineer ..nor do I play one on TV ..and Holiday Inn Express is still too expensive for me

..but..

I'll expand just a little on Brian's explanation here.

Sure, your filter, by itself, appears restrictive. But what would you say if you saw this flow piping below (upper image)



The first orifice represents the filter. The second represents the rest of the engine. Now can you see how the filter won't (normally) represent a resistance of consequence??

There's more to it ..but if you know anything about series circuits ..everything must add up to applied pressure over a total resistance. In our case, however and against our normal perceptions of flow, the FLOW dictates the pressure over the resistance ..and not the pressure through a given resistance determining flow. Flow is king in the equation ..the rest, normally, just fills in the blanks. The engine, typically, represents FAR MORE resistance to the flow then the filter does. You should look at it in terms of "back pressure" more than "applied pressure". It's applied flow that results in a pressure.

The exception to this is when the oil pump relief limit is reached. Then flow can be shunted ..and it's no longer a series circuit. Then the filter's bypass valve will limit the PSID that can be reached across the element. Since the pressure attenuates at a given level ..the amount of flow that can be diverted from the engine is limited as well.

 

[4esorselrahc]

I'm no engineer, but this site gives a really good explanation of how an oil filter works. The dirty oil flows in to the filter through the little holes along the perimeter of the base plate and is forced trough the media under pressure, and then it enters back into the engine via the large central hole in the filter.

http://www.synlube.com/oilfilters.htm

 

[4esorselrahc]

Emphasis: the oil must go through the media in the filter order to get back into the engine. Seems pretty straight forward to me anyways.

 

[shelbyman]

Thanks fellows. This has helped a lot...so now I know the oil DOES flow through the paper element under pressure.

Would anyone care to dive in on the benefits, if any, of the cleanable filters that are available on the market? There are at least 2 or 3 out there.

 

[Gary Allan]

My opinion ..no benefit. They're too expensive. They may reduce landfill volumes ..but at what cost? There's also the chemicals needed to clean them ..their cost ..environmental impact ..and disposal.

The way this would work effectively is if it were a service. That is, you buy the right to have a cleanable filter forever. Currently, you would be hard pressed to keep the same filter number for long enough to make it pay. They run $150+ ..that's about 40-60 filters ..or, even @ 3k OCI, 120k-180k for a payback. Plus there's no assurance that the filter will be appropriate for your next vehicle (hence the need for a trade in policy).

Add to all that the fact that they don't filter all that well compared to cheap paper filters ..and they just don't make a whole lot of sense economically.

Now if they start putting a $5 disposal fee on your oil filter purchase ..then a whole lot of people may be buying Motor Guard bypass filters and getting the reusable/cleanable elements.

 

[Gary Allan]

Quote:

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so now I know the oil DOES flow through the paper element under pressure.

---

Just to get back to the engineer that described the filter to you ..that WAS sorta how old bypass filters worked in engines that didn't have full flow filtration. Essentially the oil percolated through a cotton ball or shredded newspaper media and drained back to the engine. It was under "pressure" but only saw a small % of the total oil flow. The practice has been abandoned as the primary filtration modality of choice for many decades. It's now used as a supplement to full flow filtration ..reducing the full flow to a safety net for bigger particles.

 

[dwendt44]

To add more confussion: when the oil is cold and thick, SOME of the oil will flow through the filter media, some will go through the bypass valve; until the oil is thin enough, then the bypass valve closed completely, and all oil flows through the media.
How much crud oil flowing to and through the bypass valve picks up along the way is anyones guess; I'd say: not much.
Once a piece of 'stuff' is trapped by the paper(or what ever) media, it's pretty well stuck there. Back flushing MAY dislodge it, but we don't typically do that to our oil filters(at least not on purpose).
My 2¢

 

[TeeDub]

With regards to your supposed chemical engineer - somewhere there is a village missing an idiot...

 

[Gary Allan]

Quote:

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when the oil is cold and thick, SOME of the oil will flow through the filter media

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Even that needs qualification based on my observations. The bypass activity is most likely (other then due to extreme filter loading) when the oil pump is in relief. That IS typically when the oil is cold and thick ..once the relief is closed ...then we revert back to the upper image in the thumbnail I posted above ..where the filter is trumped big time by the engine. The relief, when closed, mandates the resistive characteristics of the circuit.

Look at a 6 lane highway ..bumper to bumper with cars. You encounter a reduction in lanes to 2 lanes ...and the road goes on for 10 miles. Now normally you would look at this as a "bottleneck" ......until you figure out that the REAL restriction that governs flow is the single lane exit at the end of the 10 miles stretch. Your intermediate velocity may change .. just like "rapids" in a river ..but the mean velocity will not be altered ....10 car/min (or X gpm) will still be 10 cars/min.

Some engines run in perpetual relief. That is, they start at XX psi ..and stay there all the time. The filters in these engines are most likely to be in bypass more often. They will have higher PSID when cold ..and lower PSID when the oil is hot. They NEVER have to fall into the mandated proportional relationship of resistance with the engine. They never have to conform to that linear relationship with the engine. They can be reactive and have a higher apparent impedance to flow.

Look at an oil pump as the drive wheels on a steam locomotive. Replace the cold/thick/heavy conditions with a loaded train. As it starts ..it spins the wheels. There is, at this point, a differential between motion applied and motion realized. This would be what creates your filter differential. As the speed of the wheels and the speed of the train reach unity ..this differential would be reduced. A pump that was always in relief would be a train where the wheels are never at the same applied speed as the train in realized speed ..it will vary in the differential. It may be slight ..it may be up to the limit (in our case the bypass valve setting that will limit that differential).

Not a perfect example ..but one that may give you another approach to viewing the event.