Quote:
Originally Posted by aerobod
The issue with using water temperature as a guide is that it only indicates how hot the top of the engine is, virtually all the heat is created around the combustion chamber. If you have ever had a water pump fail, you will know that the coolant in the cylinder head can boil in a matter of seconds, without circulation.
With cars I've had in the past with full instrumentation, the water temperature will reach normal temperature about twice as fast as the oil. At this point the top end of the engine will be pretty close to thermal equilibrium, but until the oil is up to normal operating temperature, it will be cooling the big-end bearings and preventing them from reaching equilibrium.
If the big-end of aluminium connecting rods are not at the normal operating temperature of about 100C, but at about 60C (likely the approximate bearing temperature when the coolant has just reached 100C, as no coolant reaches the bearings, they are oil cooled), then they will be running tighter than at thermal equilibrium.
Aluminium has a thermal expansion coefficient of about 22x10^-6/C, steel has a coefficient of thermal expansion of about 12x10^-6/C. This means that a 50mm diameter journal bearing will be 0.02mm tighter at 60C than at 100C. Engines with big end bearings of 50mm diameter require a bearing clearance of about 0.06mm when the engine is built at room temperature (nominally 20C), so at 60C the clearance will be 0.08mm and at 100C 0.10mm. With the oil not circulating as well at 60C compared with 100C (of course this can be seen by the oil pressure, which is directly linked to oil temperature at a given pump speed, ie engine speed for mechanical pumps), the flow rate will be lower and the pressure higher due to both oil viscocity and the tighter space to squeeze through in the bearings.
I will not run the engines of my cars at high RPMs until thermal equilibrium has been reached and the oil is lubricating the big ends at the optimum bearing clearance.
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The oil temp gauge has no indication for anything other than temp.
It doesn't show a yello, green, or red zone.
That isn't by mistake. If waiting until the oil temp reaches 240F before normal driving can be done, then why is that not indicated?
Why is it not part of all automobiles operating directions?
And, in this age of superior engine manage, it would be really easy to limit RPM based on oil temp. But, it's not done.
This issue of oil temp is being overstated, and it's because the 1 has an oil temp gauge and no coolant/engine temp gauge.
Also, notice that the oil temp gauge starts at 160F, very close to the typical 170F of most thermostats.
At 77F ambient temp, it takes my 135i about 5-6 minutes of driving at an avg. of 50mpg to get the oil temp gauge to start moving from the 160F position. If we were to wait until we hit 240F-250F, in the winter with 10F, we'd be near to our 15 mile destination before we could actually drive normally. I hope no one has to actually use half throttle to avoid an accident during that drive, as they would think their damaging their engines doing so with this overstated issue.
Your knowledgeable thermal metallurgical post is good, but doesn't prove that the oil temp has to reach 240F before the oil is providing proper lubrication to the needed parts. Modern engines are built with much tighter tolerances than they were back when. Bearing clearance and heat expansion rates are considered when designing an engine. And, if oil temp were that big of a factor, it would be given much more consideration than a simple gauge that indicates nothing other than temperature.
Even way back in the days of using standard oil, daily driven cars went with the coolant/engine temp gauge. Modern oils, and especially synthetics are quite superior lubricants. Synthetic even flows easily at 32F. I've not found data to show that synthetic at even 100F is not providing proper lubrication and protection, nor that it provides better service once it hits 240F.
The other lack of data is that we don't see the large number of engines with bearing failures due to driving normally before the oil has reached 240F.
I didn't read in your post at what temp you feel it's safe to drive normally.
I'm guessing that you support waiting until the oil temp gauge reaches 240-250F.
I'm not saying that one doesn't need to take into account engine warmup before hitting the loud pedal. We're just disagreeing on when that is from a cold start. The ingrained engine wisdom of the 60's and 70's is still alive today. For example, look at how many people still buy into the 3000 mile oil change interval even when using synthetic.
But, many of us have moved on from that, and so have the companies that make these modern automobiles.