... - so the uoa doesn’t tell the whole story of wear metals - there could be a large amount of bigger particles that aren’t being reported?
Correct; UOAs do not report all particles, because they can only see up to around 5um. UOAs show you a representation (a sample) of the total population; we take an inference from the wear metals. UOAs have been shown in various studies to have good correlation to represent wear, when compared to other methods such as electron bombardment, component weight analysis, etc. UOAs can best show trends; that's what we should be most interested in. Knowing the averages, standard deviations and trending is a good step towards understand what's happening in the engine.
UOAs see content, but not size. They have no ability to tell you how big or small particles are.
PCs see size, but not content. They have no ability to tell you the composition of the particles.
Short to moderate OCIs tend to have lower particle counts, and that results in lower wear. This makes sense for two reasons:
- less exposure to ingested contamination such as Si
- less exposure to generated contamination such as soot
That second issue, soot, is often misunderstood. Soot starts out super small; like around 40nm. (That's nanometers, not micrometers). So a soot particle would have to grow about 100x larger just to be 4um. As a generality, hard particles smaller than 5um do little damage to an engine. Don't confuse the quantity of soot particles with the size of soot particles. By weight, they could be the same. But it's much preferred to have a lot of little particles which can harmlessly pass through the engine, rather than a few large particles capable of gouging bearing surfaces, etc.
The additive package in lubricants reduces the rate at which soot can agglomerate (cojoin). And so, with short to moderate OCIs, two beneficial concepts are working in concert.
- soot has not had a significant amount of time to be produced in large quantities
- soot has not been very successful in agglomeration, because the add-pack is not depleted
And so, traditional FF filtration (typically only 50% effective at 10um or so), really has no effect in controlling wear caused by soot in short to moderate OCIs. Normal FF filters are quite good at stopping large particles, but they really have a poor ability to control wear in that critical 5-15um size; a lot can get past them.
The reason UOAs don't show significant differences in wear rates when the filter is the variable is because that filter really isn't doing much below 10um in short to moderate OCIs. Whether you have an average filter efficiency (80% at 20um) or a great one (99% at 20um), you're not going to be able to distinguish the filter as a controlling entity because the "normal" variation of wear is larger than the effect of the filter.
It is true, and I completely agree, that tighter filtration is desirable. It's never going to be a bad thing to use a more efficient filter. If you run longer OCIs, where the accumulation of higher concentrations of contamination are more likely, and soot has had a longer period of time to agglomerate, then using a better filter certainly can tilt the odds in your favor.
Most simply put, if you were to always run 20k miles or longer with your OCIs, then it's very likely that UOA wear metal data could show a disparity in performance between an average filter and a great one. But if you're going to OCI every 10k miles or less, it's unlikely you'd be able to statistically distinguish any difference. In short to moderate OCIs, you are flushing out the contamination, rather than filtering it out.
