Quote:
Originally Posted by digitaldissent
-A vehicle requires 22-miles (35.5 km) of continuous driving before it reaches equilibrium temperature.
-Tires will come up to temp in about 5-miles (8 km ),however coolant, engine oil, transmission and differential require a full 22-miles before their viscosity reaches it's final status.
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I'm not entirely sure I believe all of this. In fact, I'm not sure that it even makes sense to talk about the "final status" of the viscosity of the fluids unless the car is being driven at a constant speed and load for a long period of time.
My one way drive to work in a 2003 Protege5 is about 7.5 miles and takes 20 minutes. The radiator is up to equilibrium temperature in about 4-5 minutes and the car switches from open loop to closed loop at that point. This transition can often be felt because when the car is idling the transition causes a very brief engine stumble, with the RPM dropping a bit and then immediately recovering. Anyway, once the car has gone closed loop and the temperature gauge is fixed at midpoint I am pretty sure the coolant really is at a stable temperature. That temperature is pretty darn hot so I imagine whatever tiny separation in water vs. other coolant components which has taken place overnight will have been thoroughly remixed by that point. So what physical mechanism could possibly account for a delay of 15-16 minutes from this time before the coolant's "viscosity reaches its final status"? On the other hand, the viscosity of the coolant isn't going to make all that much difference in anybody's test results, unless they are running the test in Anchorage.
Oil, on the other hand, could well take 20 minutes to reach a stable temperature. The outside of the motor is definitely not yet at its equilibrium temperature in 5 minutes, and the oil is flowing through various parts of the motor and pump which are at different temperatures, so it is reasonable that it would be warming up more slowly. I googled for a while and could not find a plot of engine oil temperature versus time. Perhaps someone here has one? Anyway, 20 minutes sounds about right for the oil/motor to come to equilibrium temperature for
the given load. And there's a problem - I'm certain the viscosity is lower the higher the temperature of the motor, and the temperature of the motor can go up and down depending on the load.
The transmission, like the motor, will heat up and cool off depending on how much work it is doing. Especially if it is an automatic running at speeds below torque converter lock. The differential I'm not so sure about. It would tend to heat up (lowering viscosity of the lubricant) with speed, but that would be offset by the air blowing past many differentials, which would tend to cool it.
What I'm trying to get at here is that I don't think a long warm up period is going to guarantee the reproducibility of test results. I do think that the test conditions should as closely as possible match the warm up conditions. Specifically, driving around town for half an hour warming up the car, and then going out on the highway for an A-B-A-B with 5 minutes each leg, could result in a fair amount of variation from the car moving from the in town temperature equilibrium to the highway speed temperature equilibrium. I suppose this is obvious, but for the first A to be comparable to the last B, the warm up period before A should be as closely as possible the same conditions as for the first test.
This also may be relevant to block heaters. If the point of heating the block is to get the oil up to temperature and so reduce its viscosity, it would seem like it might be more effective to attach some sort of (regulated!) heater to the bottom of the oil pan. That is, heat the oil as directly as possible in order to get it to a lower viscosity as quickly as possible. It should take a lot less power and time to heat up the oil in the oil pan to a nice toasty temperature than it would to warm up the big chunk of metal that is the motor. Block heaters work the slow way on purpose because they are used in places that are very cold - shooting very hot oil into a very cold motor is not a good idea!