Peter van Schoonhoven wrote:
>
> Fred,
> Don't forget that at 16000 feet the OAT is maybe minus 30 degrees, so the
> inlet air is that temp, then raised by compression in the turbo, and cooled
> by the intercooler to about 30 degrees warmer than ambient, so it goes into
> the cylinder at around 0 degrees, not 110 degrees F. Maybe a bit more HP
> than you figured.
>
> Add to that the effect on true airspeed at altitude and you might convert
> 145 knots normal cruise to 220 mph cruise at altitude, especially if the
> wind was in your favor to boot!
>
> Peter
I an't really get there from here, Pete. Absolute OAT is irrelevant
in intercooler theory, since induction air to the turbo is the same as
through the cooling fins of the I/C. The relevant factor is the
amount of boost in psi (heat created), and at 16,000 feet (highest for
100% power, but we don't cruise there), there will be a 8 psi boost.
That's not an intolerable temp rise, and yes in much cooler air,
airbox temp should be fine. Some "hot rod" web sites say the
threshold of intercooler benefit is about 7 psi.
There's detailed technical articles on avweb.com about turbos. They
say the principal use of an I/C is to reduce the chance of detonation
down low, where in climb both boost pressure and ambient temp are
high. And to quicker get into the 20's without heat damage, as the
ambient pressure gets lower and lower, meaning induction air gets
hotter through more turbo compression.
On the 914, an I/C will push the 100% power altitude from 16,000 up to
wherever, depending on your I/C efficiency. But once up up at say
FL200, the principal difference will be cruise at 75% means throttle
back. W/o an I/C, you leave it at 100% for cruise power, just like on
a non-turbo engine at around 9,000. As Mike has noted, the lower
relative induction air temp will give some handful of percent
efficiency, but it's a few knots at best by the usual cube-root
formula.
This is all consistent with Karim's original post re Rotax advice.
When climbing in 115% full-boost, an I/C is very good. Especially for
those operating in hot climates, or from high altitude strips. Useful
in the 20's too, but both FAA and your doctor will point out that
above FL180, you need a pressure oxygen mask, or the consequences
could be fatal! In cruise at realistic altitudes, like 5,000 or so
tops, the boost is so small that the I/C accomplishes little.
Regards,
Fred F., A063
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