The more I think about it, the more I wonder whether I should have an
intercooler at all.
(Apart from the fact that I think that the commonly used Bell
intercooler behind the naca inlet isn't doing much at all). Let's
suppose we have a proper setup, and the intercooler functions as intended.
There are three reasons for having an intercooler:
1) It reduces thermal strain on the engine by cooling the inlet air.
2) It allows the engine to produce a bit more power, as cooled air is
denser and thus contains more oxygen per volume. Cooling the air lowers
the pressure as well, so the TCU will boost up the pressure a bit more
until it reaches the target pressure in the airbox again. Thus, you will
end up with more dense air containing more oxygen per volume (and at the
same time of a lower temperature). The carbs will automatically squirt
more fuel in this denser air. You will end up with more power, but of
course to make this power you need more fuel as well. These HP's are not
for free.
3) If the OAT is extremely high, it could be possible that the heat
generated by boosing the pressure causes the inlet temperature to reach
the limit programmed into the TCU. The TCU will then reduce boost
pressure to stay below this temperature limit. This is 72C in older
TCU's, 88C in newer TCU's. The intercooler will prevent this
TCU-intervention from happening.
The downside of the intercooler:
1) Weight. This one is obvious. But of course in HP per weight these are
still "light-weight" HP's.
2) Drag. Intercoolers need loads of air to work well. The flow of air
the engine needs is high, and the temperature difference is small. So
you need to cool a lot of stuff that is just above ambient temperature.
This needs a lot of cooling air. Dragging air through a radiator is a
costly process. It might well be that the intercooler will cost you a
few knots cruise speed.
Do we actually need an intercooler?
No doubt, a properly setup intercooler will gain you a few HP's. The
commonly used Bell intercooler is worth 5 to 7 HP's (provided the setup
allows sufficient cooling air through it).
But, we use these HP's only during the take off run, when we run the
engine at 115%. It doesn't make sense to make more HP's when the
throttle is partially open: the throttle is partially open with a
reason, and if the intercooler increases the power of the engine, you
have to throttle back a bit more to stay at your intended fuel
consumption, MAP, speed and altitude.
Apart from this, the intercooler only works if the turbo is actually
boosting the pressure. Without boost, the air won't heat up so there is
nothing to cool at all.
During the cruise at moderate altitudes, as long as MAP is lower than
the ambient pressure, the turbo is not doing much, so the intercooler is
useless here. Even if MAP is slightly higher, the heat produced by
boosting up the pressure this tiny amount doesn't produce enough heat to
justify even thinking about it.
At high altitudes, let's say 10.000 feet or higher, the turbo is
probably working quite a bit during the cruise, for any higher MAP than
the ambient pressure can provide. Still, if you don't fly with wide open
throttle, the HP gain itself is useless. We then of course still have
the advantage of cooling the air, reducing thermal stress of the engine.
But how warm is the air actually at 10.000+ feet to begin with? Does it
actually make sense to cool it? Maybe the increase of temperature is
even beneficial, as this reduces the chances of icing and promotes
better fuel atomization.
The 5 to 7 HP gain allow you to take off in a bit shorter distance. This
would be great, but the fact that an Europa with a 914 can take off
already at a shorter distance than it needs to land, so this makes this
a theoretical advantage only. To practically benefit from the ability to
take off from an extremely short runway, you first have to land there,
which is impossible.
Then the final advantage is that it could be theoretically possible to
reach the warning limit of the TCU, 88 Celcius of the inlet temperature,
and if that happens the TCU will reduce boost and hence power. It must
be awfully warm if this happens, and I wonder if anyone has ever
experienced this loss of power in extremely hot OAT's.
So, to summarize:
1) During the cruise at low altitude the turbo isn't heating up the air,
so the intercooler is worthless.
2) During the cruise at high altitude the turbo is heating up the air,
but at that altitude the air is not very warm to begin with so it is not
a stress for the engine anyway.
3) During all this cruising we are carrying the weight of the
intercooler, and dragging loads of air through it. The intercooler is
costly in terms of cooling drag.
4) During take off we have 5 to 7 more HP's available, and cooler air
for the engine. But it doesn't allow us to take off from shorter runways
as the landing distance is already larger than the take off distance.
So.... I'm in serious doubt whether I should take out the intercooler.
Another option is to have a flap on it, to close off the useless air
flow through it during the cruise. An interesting way to achieve this is
to operate this flap by a piston, fed by the manifold pressure. As soon
as the manifold pressure is higher than the ambient pressure, it pushes
the flap open. So the intercooler is only producing drag when it
actually makes sense do to so. We could also achieve this with a servo
operated flap, controlled by a differential pressure sensor (combined
with a lower temperature limit). All this of course is quite a fuss for
a small gain in HP's, only used (not needed) during a fraction of the trip.
Opinions?
Frans
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