Originally Posted By: Jetronic
not quite, as the oil can be 140°C in the big end bearing while the bulk oil is only 90°C. Hence why HTHS is measured at 150°C. And what would be the temperature around the compression rings in the top half of the stroke?
Jetronic,
Apologies for the late reply but I wanted to check on a few things before opening my mouth...
Okay, what you say is very much the same 'received wisdom' which I've heard many times before, from many people who have been in the business longer than me. However, like a lot of oil formulation received wisdom, I'm not sure it stands up to scrutiny.
The basic premise is that the actual oil temperature inside a bearing journal can be significantly higher than that of the bulk oil as frictional energy in the bearing is converted to heat. Because the oil gets hotter, it gets thinner, oil film thickness drops to zip, metal meets metal and bang goes your engine!
However the operative words for me are 'can do' which isn't necessarily the same as 'will do'. Yes, if you red line your engine (for the sake of argument say 7000 rpm), while pulling a boat up a hill (so max load) on the hottest day of the year, then, and only then, will you see risk a real problem and if the OEMs are to believed, then even under such extreme conditions, you will STILL be okay to run a 0W20 because 2.6 cP HTHS guarantees you protection.
Now imagine a different scenario where you drive your little Suzuki at 55 mph max to do the weekly 90 mile round trip to visit your grandson. The engine barely gets above 2500 rpm and you probably never exceed 30% of rated load. Finally, because you believe in global warming, mpg is of paramount importance and rapid acceleration is anathema to you. Now in this scenario, the differential temperature increase across the big-end bearings isn't such a big deal and you CAN use an ultra thin engine oil without destroying your engine.
I do have bits of data to back this up...
First off, I analysed the oil from my old Daihatsu after it's first service (at 9000 miles). The factory fill (I think) was a typical, high Moly, high VI, Japanese 0W20. The KV100 of the used oil was a mere 5.0 cst as a result of 8% fuel dilution. I didn't get the HTHS checked but at a guess, it was about 1.4 cP; way lower than what OEMs would consider acceptable. The thing was though that when I put the used oil through ICP, the wear metals were almost nonexistent. I gave that car away to my daughter after seven years of trouble free service. So where was all that bearing wear and why didn't the big-ends seize???
Here's another bit of info. I have no idea of the number of Sequence IIIF, Sequence IIIG and Peugeot TU5 engine tests I ran when I was in the formulating game but it was a lot. Now all of these tests deliberately keep the bulk oil temperature at 150°C and thrash the engine for upto, in the case of the IIIG, 100 hours continuous. So if you're going to see high differential bearing temperature rises and oil thinning knackering bearings, this is where it will show up. And it just doesn't. I never ever saw a engine fail on bearing failure (although they would fail on many other things, often stuff that was related to high Noack). Why not??
And finally there's my experience of the Sequence VIII test. This is the industry standard test for specifically looking at copper/lead journal bearing wear & corrosion. It's a fairly simple test on a single cylinder research engine which runs for 40 hours and over 3000 rpm. The oil is purposely kept at close to 150°C by an external oil heater. The thing is that I always found this test was always a complete doddle to pass!
Which is why my gut feel is that ultra thin oil is do-able for most ordinary driving. I agree it's potentially unsafe if you include ALL of the engine's design envelope (specifically the highest ambient/highest rpm/highest load condition) but if you're restricting oil viscocity for a condition that most vehicles will never see in their life time, is this really sensible? And if you do want to protect the engine in that most extreme of conditions, why would you not implement a simple oil temperature driven limp mode thing? Remember that ultra thin oils could deliver fuel economy & emissions benefits on a daily basis so why block them because of a condition that they might meet once in a life time and even when that condition hits, the evidence from industry standard test suggests won't actually be catastrophic?
