Understood, Graham. Correct you are, and nice to know that the folks at
Rotax also understand what is going on (no surprise, that). I did not to
get too detailed with the physical chemistry and heat transfer "lesson"
because I feared that it would bore too many others. It is rather important
for builders to understand the implications of changes we might make in the
amount of glycol, especially in the absence of recommendations from either
Europa or Rotax.
Best regards,
Rob Housman
A070
-----Original Message-----
Behalf Of Graham Singleton
Subject: Re: 2 items - service bulletins on Rotax and
> However, adding glycol to water has an adverse effect on heat
>transfer so engine cooling will be adversely affected by having higher
>glycol concentration. In other words, keeping the coolant mixture from
>boiling is not the same as providing more efficient cooling for the engine.
>So, one must ask: is the cooling problem indicated by high coolant
>temperature (or boiling) or by high cylinder head or oil temperature?
>Rob Housman
Rob
In the early Rotax 912 installation notes 100% glycol was specified.
Perhaps to prevent boiling at cylinder head hot spots, (my guess, we
weren't told the reason). Spot boiling would also reduce heat transfer,
vapour conducts heat worse than liquid.
We soon found that diluting the coolant to 75% glycol had a big effect on
cooling. Much better. Eventually Rotax and Europa suggested 50% glycol but
the effect on boiling point was not discussed AFAIK. Max CHT was still
specified at 150 deg C. Way above the boiling point of the coolant.
PFA now require coolant temp to be measured rather than CHT. Very
sensible,IMHO but please note the boiling point, 115deg C or 130deg at 1
bar overpressure, (the relief valve is set at 1 bar)
Graham
|