|
Ok, Al
That as you said, explains it. My
recollection of reported coolant temps were clearly in error and what you cited
about coolant temps using NPG does correlate. My mind is
happy!
I have fortunately never experienced cascade
boil-over and hope never to do so, clearly a terrifying and hopeless feeling to
encounter why flying. Shutting off the engine in such a situation might
bring things back under control - if you have plenty of altitude. But, I
would assume most boil overs happen after a high powered takeoff on
a hot day, so altitude may be lacking.
I can certainly understand why that would
induce someone to go to the NPG coolant.
Long, Long ago and in a place far, far away - Japan
to be exact. Mazda provided that 180F coolant temps (out of the block) and
210F Oil temps (into the block) were the limits. These can also be found
in Racing Beat's technical catalog. I have on occasion - and for
very limited amounts of time - had oil temps as high as 230F and coolant as high
as 250F. These were cases where coolant was still flowing and
pressurized. After the first incident with an 86 N/A block, I found I had
coolant "O" ring compromised (coolant leaking into combustion chamber), I then
switched to the Teflon Enscapulate Silicon "O" rings and on the one or two
occasions when the temps reached 220F oil and 230F coolant - there appeared to
be no damage.
I believe that those limits were established for
the mid 1980 era 13Bs and that block coolant galley improvements and casting
changes (as well as use of higher temps rubber seals) have probably raised the
permissible operating temperature. But, I do not know of anyone who has
tested an engine to destruction to find out {:>)
Ed
----- Original Message -----
Sent: Saturday, February 04, 2006 9:32
PM
Subject: [FlyRotary] Re: NPG Coolant
Temperature vs 50/50
The Egg guys no longer operate at the normal 200F when they install NPG.
I don't recall the exact number, but they operate around 215F (cruise). Which
explains most of your theoretical differences.
They gain a significant safety bonus in the boil over temp. That safety
margin is what it's all about. At least from the risk perspective.
So by operating at a higher Delta T, they compensate for the reduced
efficiency of the fluid.
I've heard you guys describe the importance of operating coolant around
185F or so? Are you sure of that limit? Is that just a recommended thing,
or hard and fast limit. I'm always skeptical of stuff like that.
-al wick
Hi Al,
Appreciate your (anybody else welcome also)
views on one other thing that is still bothering me about the use of
NPG.
Given that NPG+ has a specific heat of 0.66 at
212F or 20% less than the 0.82 for the 50/50 mixture and
given that NPG+ is approx 7% more dense than the 50/50
- then that for the same flowrate for both it would seem that NPG
still has a 13% lesser overall capacity for heat transfer (at the same
temps 212F and flow rates).
Also assume that the engine is produce the same
heat load (Q),lets take alook at what temperature we might see with NPG+
compared to the 50/50 solution. We have from the oldie but goody Q =
c*M*DeltaT the ability to solve for the temperature increase,
DeltaT.
Delta T = Q/cM, now if the combined effects of
c and m provide 13% less heat transfer capability than the 50/50
mixture that would indicated that to carry away the same Q at the same flow
rate, the delta T of NPG+ would need to increase by 13%. So if I were
getting 180F with the 50/50 for the same Q load (and flow rate) then with
NPG+, I would expect 180 *1.13 = 203F. Yet, if I understood
correct we have reports that lesser temperatures results noted by
users of NPG - this leaves me a bit puzzled.
If my assumption is correct thus far, then I am
at a loss to understand the reports of lesser cooling temps when using NPG+,
it would seem just the opposite would happen, that is - the
coolant temperature would increase.
Now, If the coolant temp with NPG+
actually does remain the same (or decreased) for the same Q and
flow rate, then it could indicate the engine heat load is not being
carried away as well as with the 50/50. IF the reports of lesser
cooling temps occurs when switching to NPG+ are correct, then it
would appear to me that the engine must then be operating under a higher
heat load (i.e heat not being transfer to the radiators as effectively)
. This doesn't even take into consideration the possibly lesser flow
rate of NPG+ for the same pump speed as the 50/50 due to its higher 3 times
higher viscosity (at 212F).
Yes, I have no problem understanding that
boil-over has been eliminated - but, is NPG actually cooling as well as
coolant temps may lead one to believe? I mean with NPG, I could have a
excessive block temperature and still not have boil-over - but my rotary
engine would likely have suffered damage.
What am I missing? Inquiring minds (even
old ones) want to know {:>)
Ed A
-al wick
Artificial intelligence in
cockpit, Cozy IV powered by stock Subaru 2.5
N9032U 200+ hours on
engine/airframe from Portland, Oregon
Prop construct, Subaru install, Risk
assessment, Glass panel design
info:
http://www.maddyhome.com/canardpages/pages/alwick/index.html
|