On 03/04/2012 12:40 AM, Gilles Thesee wrote:
>> Furthermore the engine can safely be operated at a higher
>> temperature, which is good for performance.
> Please pardon me for jumping in. I would like to point out that running
> an engine hotter implies *greater* heat rejection, all other things
> being equal, since heat rejection is a direct funtion of the the
> temperature difference between metal and coolant.
This temperature difference would remain constant. If I close the cowl
flap to raise the temperature, I'm raising the temperature of the
coolant. Since the temp difference between the coolant and cylinder head
is constant, CHT will follow, and this will cause a higher efficiency.
Furthermore, it is best to keep the engine at the same temperature all
the time. All metal parts expand with a different rate, so if you keep
the engine at one temperature, all parts will "run in" for that
temperature wich will give the best fitting, i.e. least friction and
abrasion, while at the same time keeping the tightest fit so compression
will not suffer.
Keeping the engine always at the same temperature will in reality imply
that you keep the engine temperature close to the upper limit. In cars
we use water thermostats for this, but in the Europa we have to do this
manually, i.e. by using a cowl flap. (Or use a water thermostat which
keeps you dragging air through a radiator without any good reason,
spoiling our good attempts to keep the drag of the airplane as low as
possible).
> Also in a liquid cooled engine, some degree of local boiling (nucleate
> boiling) *does* occur. It is this local boiling that insures protection
> from hot spot, due to boiling removing enormously more heat than just
> heating liquid. Naturally we are speaking of an adequate flow of coolant
> which removes the bubbles and let them collapse in the general flow.
I largely agree with you here. If localized boiling would be a problem,
the engine would destroy itself pretty fast. This doesn't generaly
happen, so the problem doesn't exist. That said, your 'adequate flow of
coolant' is often less at places where nucleate boiling occurs, this
sub-optimal flow is often the cause of mucleate boiling. In a cylinder
head, it is difficult to keep the coolant flowing at the same rate at
all possible locations.
(BTW I wonder whether the "5 minute restriction" of the Rotax 914 has
anything to do with nucleate boiling)
The main reason I like the Evans coolant is that it operates at zero
pressure. In a Rotax we are pressurizing our coolant systems at 1.2 bar,
which is quite a lot. The tiniest leak will cause you to loose your
coolant pretty quick, and there is always a risk that a loose hose will
blow off. Of course, proper maintenance will take away most of the risk,
but not all of it.
Furthermore if something bad happens which causes your engine to
overheat, let's say to 140 degrees C, the 50/50 coolant will boil and
finds its way out, leaving you with no cooling at all. With Evans you
have the option to keep the engine running and get yourself out of a bad
situation before the engine gives up.
So, if everything is equal, I would prefer Evans. But alas it won't flow
properly through my heat exchanger so I'll stick to 50/50.
Frans
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