Eric sez:
>SAE 20w50 means that the oil acts like a 50 weight at 0 deg C, and a 20
>weight at 100 deg C....
****GONG!!!***
Just backwards, in this case. 20w50 acts like a 20 weight at 0 C, and a
50 weight at 100 C. But it *doesn't* get more viscous at increasing
temperature!
>So the viscosity decreases (easier to flow at high temps) with temp....
Right. True of just about every fluid (Newtonian).
The idea behind 20w50 is to have plenty of protection at high temps without
the sacrifice of too much viscosity at low temps.
I'll try a little graphic. Here's a cheesy plot of viscosity vs.
temperature for the three oils in question. The point is that the *rate*
of change in viscosity per change in temperature is *less* for multi-
viscosity oils than for single-grade oils:
| a
| a
| a
| a
Relative | a
| x a---50wt
Viscosity| ox a
| o x a ___20w50wt
| o x a /
| o x a/
| o x / a
| o x a
| 20wt---o x a
| o x a
| o x a
| o xa
| o x
| o
| o
+---------------------------------x
0 temperature, deg C 100
The idea is that you'd like to use 20wt in the winter (that's what the "W"
is for) and 50wt in the summer without having to switch your oil back and
forth. So multigrade oils give you the better flow characteristics of a
lighter oil at low temperature and the better protection characteristics of
a heavier oil at high temperature.
Eric knows all this, too, he just experienced brain fade while trying to
explain it.
Lee M. Daniels Laboratory for Molecular Structure and Bonding Texas A&M
daniels@tamu.edu (409) 845-3726 Fax (409) 845-9351
|