These two topics (fuel dilution and TBN/TAN) really come down to one concept:
Inputs vs. Outputs
There a lot of things that are to be considered when speaking about maintenance of equipment. In no particular order:
- operational patterns and use/severity
- availability for maintenance tasks
- type of maintenance practiced (Predictive, Preventative, or Panic)
- monetary thresholds
- longevity expectations (realistic or otherwise)
- environmental contributors
- equipment controls (air or liquid cooled? condition? frequency of use? etc)
- other minutia
What we really care about is how well a piece of equipment wears; that is mostly what we want to know. This is about a means to an end; what do we need to know and do to reach a desired goal? The "output" we care about is low wear. Really does not matter how we get there, as long as we get there. There are typically many roads to the same destination. And so, with more than one option, we can then open up our considerations to what maximizes our investments, and minimizes our costs and risks. That approach garners the best ROI (return on investment). Using cheap products that risk the health of the equipment is not a sound decision unless you intend to only hold onto that item for a very short period of time. Conversely, over-spending and over-maintaining something has never proven to be anything but a waste of time and money. The key is to match up as many pros and reduce as many cons as possible, blending the selections noted above into a solid plan.
Fuel dilution and base/acid are two separate topics, but they are "inputs" and not an "output". Wear data is an output; often seen in UOAs, PCs and tear-down analysis. Some theorize that UOAs are not all that accurate, but I see that as uninformed. UOAs will never be perfect; that's true. But they are by far the quickest and cheapest method to track normal wear given the limitations of other alternatives. VOAs and UOAs can help us understand the direct health of a lube, and the indirect view of equipment wear (engine, trans, diff, gearboxes, etc).
I care about outputs; that's what tells me how well (or poorly) something is wearing. Inputs are predictors for the potential for change; nothing more or less. Outputs actually tell us what's happening. There are times when inputs show correlation with outputs; other times there is no correlation. And without correlation, there can be no causation. This is KEY and CRITICAL to understanding the delineation of inputs and outputs.
Extreme cases of fuel dilution (above 10%) and extreme acid presence (inversion with a ratio greater than 2:1 for acid to base) have shown that damage is either already present, or imminent. But short of those extremes, things are not nearly as dire as folks purport. Typically, fuel dilution up to 5% shows no real world effects. TBN that is low, or has inverted it's ratio (crossover) also does not automatically indicate horrid wear. I have seen countless UOAs where fuel was between 3-5%, and yet wear goes completely unaffected. I've seen so many UOAs where TBN was low, or crossover with TAN happens, and yet nothing bad happens in terms of wear control.
There is nothing wrong with tracking fuel and base/acid. But these should NOT be used as a trigger for an OCI. The way the should be used is a trigger for continued, and often more close, scrutiny.
- If you maintenance plan is to run "normal" OCIs (at OEM limits) then tracking those two things is pretty much worthless. The OCIs are short enough that there's no predominant proof that they are a risk; you can pretty much ignore them.
- If your maintenance plan is to run extended OCIs (past OEM limits) then tracking fuel and base/acid are important. But here's why ...
They are inputs to an output. As they would increase, you are going to want to pay closer attention to wear rates. If your plan is to UOA every 10k miles (or maybe 100 hours) in a large sump system, and you begin to see an uptick in fuel (or decrease in TBN), then you might consider shortening your UOA sample duration. As long as you understand the baseline of your wear rates, what you are trying to discern is the shift in wear rates that would predict one of two things:
1) a wear rate that is unacceptable (a rate, not a magnitude)
2) a total magnitude of some attribute that is considered a max for the equipment
In other words, you want to let the wear data talk to you; don't ignore it. it's OK for the wear rate to show a slight uptick, as long as the rate does not skyrocket. It's OK and totally normal to see "x" ppm of Fe increase, as long as it does not exceed a predetermined condemnation limit. Examples might be thus:
Fe wear rate no greater than 4ppm/1k miles
concentration no higher than 100ppm of Fe
Just because you see fuel increase or base/acid inversion, does not mean wear is automatically going to be directly affected. I cannot tell you how many times I've seen people scream the sky is falling, and yet the wear is totally unaffected, or is only altered slightly and well within an accepted norm.
Fuel and base/acid are inputs to a maintenance decision; they are not, in and of themselves, a reason to OCI. That decision they drive is not whether or not to change the oil. Once they cross a predetermined threshold, they are criteria that may cause you to pay closer attention to your outputs. Inputs are not a reason to OCI; they are a trigger to start paying closer attention for the POTENTIAL for changes in wear, so that you realize you may be approaching a condemnation limit sooner, versus later. This is also true of viscosity, flashpoint, soot/insolubles. Those are inputs, not outputs. Should they escalate, they are reasons to pay closer attention to the outputs. But nothing more.
The topic of fiscal expenditures also plays into this. There are many times when it's just plain cheaper to OCI than UOA. Small sumps don't lend themselves to continuous monitoring. The odds are in your favor with well-made modern equipment and quality lubes to just OCI with the OEM schedule. It's a tad wasteful; lubes are typically discarded well before they are at risk. But there are times it's just cheaper to OCI than UOA when the risks are low. UOAs are a great tool, but they are not 100% the only way to do something. I use UOAs to establish where my normal wear rates are at, and then I select a maintenance program that can reasonably be expected to offer a safe wear rate at a low cost. I can prove beyond any doubt that my equipment, with my selected products, can easily go 3x the OEM OCI, and yet wear is totally unaffected by that extension, and is immune to the fuel and acid issues. That may or may not be true for every condition, but it is very true for most conditions for most people. However, I encourage people to test and decide for themselves.
I started out doing UOAs and OCIs at 5k miles. Everything was totally fine.
I extended to 7.5k miles; a 50% increase. All is OK.
I went out to 10k miles; a 100% increase. All still OK.
I have gone out to 15k miles; a 200% increase. Still no signs that wear rates are affected negatively, and no indication that concentration condemnation limits are anywhere near being hit.
I use the monitoring system to assure my maintenance choices are sound.
I see no evidence whatsoever that short to moderate OCIs are ever a risk in terms of inputs affecting outputs. And that's not just my personal UOAs, but the 15,000+ I have in my database from all manner of engines and transmissions, from just about every manner of use you can think of in a traditional sense. This isn't anecdotal; it's proven beyond any manner of reasonable doubt.
Use your wear rates and max concentration limits as a reason to OCI; use the outputs to judge how well things are wearing.
Use the lube attributes as a reason to alter you assessment duration (your UOA plan); these are inputs that help you understand when to take your next sample.
