In spite of what Rotax say in their Manuals, I do not think that monitoring
coolant
temperature is necessary. This is why:
1. The coolant is heated by the cylinder heads. So obviously coolant temperature
is always below cylinder head temperature.
2. Conventional water based coolant boils at 120C at the 1.2 bar pressure, at
sea
level.
3. Anytime the hottest cylinder CHT exceeds 120C, there will be at least some
local
boiling.
3.1. Then, if the engine is running at cruise RPM or more, the water pump
insures
a good flow through the cylinder heads, the bubbles are pushed downstream to
the cooler and condense, so there is no boiling over.
3.2. If the engine is at idle on the ground, while you are waiting for your take
off clearance, then it will boil over. Moreover, as soon as CHT reaches 110C,
you must either shut down the engine or take off quickly. If you wait a few
more minutes and let the CHT reach 115C, then the only choice is to take off
immediately.
Then the CHT will decrease instantly. If you shut down at this point,
then the heat transferred from the hot engine to the coolant will make it
to exceed its boiling point and you are facing a big mess.
So the only advantage of monitoring the coolant temperature would be to allow
the
CHT to slightly exceed 120C, for example during a full power climb in very
hot conditions, while the coolant temp would stay slightly below 120C. It seems
to me this is a very marginal and risky advantage.
So in my opinion, monitoring the CHT of the hottest cylinder and considering the
redline is 110C on the ground and 120C in flight is good enough. This is
especially
true on the Europa where cooling is very good in flight but very marginal
on the ground.
Remi Guerner
F-PGKL, XS Monowheel, 912ULS, 1195 hours
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