Explain it like I'm 5.
What do bypass filters filter exactly? Is it just another filter in line with the main filter?
OK - here's a synopsis ... for a more simplistic explanation ...
First, most 5 year old kids won't be able to follow this, but hopefully you will.
- there are many types of contamination in lubricants. When we speak of internal combustion engines, there are four major sources
a) hydrocarbon byproducts (soot) and other insolubles
d) soluble liquids (fuels, etc)
- As for soot, it starts out sub-micronic in size. It does NOT start out life at 10um or even 5um; it's sub-micron. Therefore there is no practical filter element that will catch it when it's "young". As it accumulates in a sump, it has the ability to agglomerate (co-join) with other soot particles. To counteract this, the oils have additives (dispersants) to keep this from happening.
- Silica comes primarily from air-intake. Varies in size. If it makes it past the air filter, it can do damage easily.
- Metals are typically shed from the engine; vary in size.
- Fuels completely mix with the lube; no ability to separate out a "fuel" from the lube in a traditional filter element as the filter cannot distinguish the difference.
Now we speak to filter efficiency ...
Read some more thorough SAE articles and studies, and a few BITOGers here can decently explain. Essentially, most all filters have pores of various sizes; the better filters have tighter and more consistent pores. Smaller holes catch ever finer stuff. We all get that. But we also need to acknowledge that "bypass filtration" is only a combination of two words, and unless we see Beta Data for the filter elements, it's kind of only getting 1/2 the story. For example, my 1987 diesel Escort had both a primary and BP filter element for the lube system. But the BP element (nominal at 10um) was the same rating as the FF filter (nominal at 10um). Neither element was more efficient. But, the BP side was "slowed" with a restriction orifice; this promotes slower flow which causes a slightly higher capture ratio. So the "Bypass" filter wasn't "better" because of the filter element itself, but because the flow was manipulated. In theory, a BP filter element can be absolute down to 3um, or 15um, depending upon it's design intent. We will, for this conversation, presume that a BP element is absolute at 3um or so ...
BP filtration works (as primarily seen in the market place we'd all recognize) in a parallel loop with the FF filter. Most any commercially available brand will be reasonably absolute around 2 or 3 um. Most of these systems "sample" the full-flow; typically about 10% of the total. So for any particle that is (example) 5um, whereas the FF filter will never catch the particle with any reasonable efficiency, the BP won't even see that particle 90% of the time. As an average, if the full sump flow only sends 10% of the flow to the BP loop, that means 90% of the time that 5um particle goes right through the FF element and right through your engine.
BP filters are great tools, but ONLY when you understand how they work and when their effect moves from secondary to predominant. Follow along ...
A fresh OCI will hopefully push out most contamination (admittedly not all, but a lot of it). That means when the OCI is young, there isn't much contamination present. If you have a decent air filter going, and a decent fuel system, you won't get much contamination in the way of these routes. So the other two routes left are shed metals and soot; sort of inter-related. If you don't have big soot, you'll not shed a lot of metals. So, when soot is small, in young OCIs, the OIL is what controls wear, not any filter element. Remember that soot starts out at sub-micro sizes, and therefore even a BP element cannot stop it. Essentially, a short-to-moderate duration OCI never has soot grow large enough to do any specific damage in large scale. Don't confuse "size" with "magnitude" (volume). There can be a LOT of soot in a system, but it can be small in size; that is until the anti-agglomerate additives becomes overwhelmed - typically as the OCIs mature in much longer duration. There are a bazillion UOAs that show soot loading, along with jet-black visual appearance, but wear is minimal; the soot is predominant in quantity, but small in size. It has not agglomerated in high quantity yet and therefore is too small to affect much.
FF filter elements are there to catch BIG stuff immediately. BP elements are there to catch small stuff over a LONG duration of exposure. But they cannot catch what is not yet large enough to be caught (it's Homer Simpson "Doh" obvious). The only time they catch soot is when it gets to be 2um or 3um in size, and in sufficient quantity to make an appreciable difference it quantity. (remember - 90% of the time a BP element isn't even "seeing" an opportunity to stop anything, because it only samples 10% of the total flow).
The primary advantage to BP filters is that (being our accepted beta of absolute at 3um), the soot is still smaller than what would typically cause damage. Generally, most sources agree that particulate in 5-15 um size range is most damaging. Because a BP can catch stuff at 3um with great efficiency, even though it only samples stuff 10% of the time, that's still small enough AND soon enough that it matters in effect. But ONLY IF
(and this is the key to understanding this entire topic
) the oil has been run long enough to see agglomeration of soot that large in the first place!
In short-to-moderate OCIs, you'll never see any wear-rate reductions using a BP filter in a typical engine. This is because, of the four primary contamination routes, a BP filter element has the following (lack of) effect:
a) soot too small to catch
b) silica generally stopped at the air filter
c) metals most often a result of other influence; not shed by themselves
d) traditional syn and/or cellulose elements cannot distinguish between lubes and fuels
IOW - a BP filter element generally has no effect in wear control in moderate OCIs. ONLY AFTER AN OCI BECOMES SUFFICIENTLY LONG would a BP element have a pronounced, discernible effect in reducing soot loading, AFTER the oil additives were compromised. This is why UOAs don't show BP systems having any tangible effect in most applications; folks don't run the lubes long enough to actually glean the ROI.
Some applications such as OTR and off-highway equipment, typically running much larger sumps and much longer OCIs, can easily get a benefit from BP filtration.
To the question of why someone would run BP filtration
As typical of the world we live in, the answer is "it depends ...."
Either it's a tool or a toy.
* If you're going to run much longer OCIs, and your sump capacity is large (representing a significant cost to exchange lubes), and it's far cheaper to UOA than OCI, then BP systems are a fantastic method to maximize your OCIs and therefore your ROI. It can assist in the control of wear over long periods of time; as the OCI matures the BP will keep soot at that desired small level by removing it from circulation.
* If you've got a small sump (less than 5 gallons), and are Hades-bent on OCIs in a "normal" sense (15k miles or less), BP filtration is a complete and total waste of your money because of the limitations of the system effect, and other influences it cannot control. It won't alter your wear rates in a meaningful manner at all.
Here is an example of which I speak; these values are estimated, but represent what happens in respect to soot loading. NOTE: these are approximated values, not exact values.
As you see, short-to-moderate OCIs don't see appreciable size growth in soot until the oil becomes compromised.
The effect of BP comes AFTER most folks have already changed oil.
Wear isn't affected much until AFTER the soot loading becomes substantial in size (again, not quantity, but size)
(the escalating line on left represents soot size when ONLY FF is used)
(the muted line on right represents soot size when BP is added)
In short, this is no different that the effect (or lack thereof) concerning syn fluids. Syns don't typically exhibit an advantage until longer OCIs are employed. But most folks never run an OCI long enough for the disparity to show up.
The key to understanding the entire topic is this: you cannot have an advantage until one of the competitors is compromised.
When you change oil often enough, the OIL ADDITIVES are what controls wear, not filters.
PS - there is reasonable correlation between the soot-size loading, and wear rates .... I can show very similar charts with specific UOA wear data. Unfortunately, my data is limited because LONG OCIs are actually quite rare here at BITOG.