|
|
Villi Seemann wrote in part
"We know we have a lot of losses in this system otherwise, with the
above
equations, we should get more than 64 times the required mass of air
pumped
into our engine. According to Mr Pitot the raw pressure increase is much
less, but according to Mr. Bernoulli, we can really increase the
pressure a
lot, by slowing the incoming air velocity down."
The physics does not work this way. If it did, putting an inlet even
larger would provide even more pressure. Not so. Energy is conserved,
alas. (First law of Thermodynamics.)
The maximum ram air pressure available is the "stagnation pressure"
which arises when the flow is brought to a complete stop in the absence
of friction (called "isentropic deceleration" meaning no entropy is
generated because there are no losses). For subsonic flows up to about
Mach numbers of 0.3 it is given with good accuracy by the expression:
ram pressure = one half times gas density times velocity squared
(For Lancair IV drivers at Mach= 0.5 or thereabouts, the ram pressure is
a bit more than this, but the temperature rise cancels out some of the
benefits.)
One has to get the units right and include the gravitational constant to
get the answer. I just did some quick calculations for another reader,
and in the units that we typically use, you get the following
for 100 feet per second, ram pressure is about 0.17 inches of mercury
for 200 feet per second, ram pressure is about 0.66 inches of mercury
for 300 feet per second, ram pressure is about 1.48 inches of mercury.
(Sea level, 70F day.)
Three hundred feet per second is 178 knots. Thus if your indicated air
speed is 178 knots, the pitot tube is "feeling"about 1.5 inches of
mercury, or about three quarters of a PSI. If you prefer, 100 feet per
second is about 60 knots or about 70 miles per hour. (Full disclosure:
your airspeed indicator assumes sea level and 59F day which would yield
a slightly higher ram pressure than above.)
In the induction system which is flowing a lot of air (unlike the pitot
tube which flows none), frictional losses occurring once the air enters
the scoop or inlet duct deduct from the maximum available. For an
0-320, the flow rate at full power and 100% volumetric efficiency is
about 230 cubic feet per minute. Put this flow in to small a duct or
SCAT hose or flow it around a sharp bend and you can lose a good deal of
pressure. About the best you can expect without a lot of testing and
development is about 75% of the ram pressure. But for an aspirated
engine, getting another inch of manifold pressure is worth the effort,
in my book.
One estimate the performance gain as follows. At constant RPM and
mixture strength, the engine power is proportional torque which changes
roughly linearly with manifold pressure. At 2500 RPM, the engine would
probably put out zero net power at about 5-10 inches of manifold
pressure and about 75% power at about 25 inches of manifold pressure.
So zero percent to 75 percent power occurs over a manifold pressure
range of 15 inches (from about 10 to 25 inches). So each inch of
manifold pressure near cruise power settings is worth roughly a 5%
increase in horsepower.
This little bit of analysis made a LOT of assumptions, but you get the
point. We are talking about changes of about 5% with good ram air
recovery. To get better numbers, look at the engine performance tables
and see the power difference between 24 and 25 inches for a normally
aspirated engine at about 7,500 feet. My guess is it will be about
5-6%. This will give you maybe a 2% increase in speed. Add another 1%
because you can get the power 1000 feet higher (equal to one inch of
mercury) where the air is a bit thinner. Washing the bugs off the
leading edge will probably do more, but with less fuel burn. But if you
can get an extra 5% more horsepower with good induction system design, I
say go for it.
Do not expect a 10% speed increase (which requires about a 30% increase
in power) unless your carburetor is currently sucking through a soda
straw or a clogged air filter or a truly wretched induction system
(although I have seen a few of these).
Fred Moreno
|
|