Went looking on the internet for a curve of heat transfer vs glycol/water
ratio. Didn't find that, but did find that the state of the art in coolant
has changed a bit in the last 5 years. A short summary which may be useful
to Rotax users follows.
There are two glycols commonly used for coolants: ethylene glycol and
propylene glycol. Ethylene glycol has better heat transfer characteristics
but is poisonous and needs care in disposal. (Lots of info on symptoms
caused by ingestion.) Rotax specifies ethylene glycol as the coolant of
choice.
There are two ethylene glycol based coolant varieties commonly available,
differing in the additives used for corrosion resistance. (Corrosion is a
particular problem with aluminum engines and radiators.) The older version
uses a silicate based corrosion control and is distinguished by the greenish
dye added for identification and leak tracing. The silicates settle out as a
clear gel after 18 months, the shelf life of this product. While ethylene
glycol itself continues to provide boil/freeze protection, the anti-corrosion
and water pump lubrication properties are reduced with time. Addition of an
anti-corrosion/water pump lubricant concentrate can extend the useful life of
the coolant although there is some thought that in this case particulates in
suspension may cause erosion. The ph of the silicate is alkaline and its
anti-corrosive properties decrease as the alkalinity falls over time.
Manufacturers recommend replacement of coolant yearly.
The newer ethylene glycol based coolant has a red (pink) dye added for
identification. This uses a different anti-corrosion formula which is not
silica based. This anti-corrosion additive does not settle out and has a
shelf life of 6 years. One of the major differences is that the ph of the new
additive is much closer to neutral and tends to remain stable for longer than
the earlier silicate type. Manufacturers recommend replacement every 3 years.
In the US the new coolant is called Dex-Cool -- this is what Lockwood
Aviation recommended when I called them to inquire about use of the new (to
me) type coolant. Apparently, General Motors changed to this in 1996,
calling it an extended life coolant.
The recommended glycol/water ratio is 1:1, with the maximum ratio being 2:1.
The boil point rises about 10 degrees with the higher ratio but the heat
transfer rate apparently drops slightly. Tap water should not be used with
glycol since it can change the ph and/or result in erosive particulates.
Distilled water or rainwater (from un-polluted environments at least 100
miles from the ocean) are recommended. In some areas a 1:1 mix is sold
instead of a concentrate, thus avoiding any problem with water quality.
The identification dye is fluorescent to aid in tracing leaks. I tried using
a UV lamp as an aid but didn't find it particularly helpful; the fluorescent
properties were fairly muted.
John A044
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