Sender: <marv@lancaironline.net> (Marvin Kaye)
To: lml
Date: Fri, 20 Sep 2002 11:19:16 -0400
Message-ID: <redirect-1740249@logan.com>
From: "Fred Moreno" <fredmoreno@bigpond.com>
Subject: Ram Air and carburetors
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Jeff Peterson wrote in part:

"But the problem I have is that the hole in the carb throat
is about the size of the scoop entrance. Doesn't the air have to
speed back up again to get through the carb?  So the pressure falls
and we are back where we started."

Correct.  The flow accelerates through the carburetor venturi creating a
low pressure region that "sucks" fuel into the air stream.  The venturi
is a convenient way of measuring air flow and inducting fuel in the
correct proportions.  (But you have to adjust the mixture to compensate
for variations in air density since the venturi measures volumetric flow
rate, not mass flow rate.)

So why not let the air flow at this high velocity from inlet to venturi?
Two reasons.  First, if the velocity leading up to the carburetor was
very high, the pressure losses along the way would be very high,
especially going around bends.  And you could forget using an air
cleaner which requires very low air velocity to minimize pressure
losses.  The idea of gentle bends, smooth surfaces and low velocity in
the ducting is to minimize friction losses and allow the highest
possible pressure in the intake manifold when the throttle butterfly is
fully opened.  Second, the carburetor needs two pressures to work right:
a relatively high pressure (approximately ambient) at the carburetor
inlet, and a low pressure in the venturi.  The high pressure is
communicated to the float bowl and sets the air pressure in the float
bowl above the fuel.  This higher pressure drives the fuel toward the
low pressure in the venturi with the fuel passing through a fuel jet
which controls the fuel flow.  Upon arriving at the venturi, the carb
uses the high velocity in the venturi to shear the fuel into tiny
droplets (atomization) to promote combustion in the cylinders.

In theory one could maximize performance (maximize manifold pressure) by
sticking the carburetor forward with the inlet jutting out of the cowl
directly into the air stream, but doing so would cause monkey business
with the intake manifold and the engine would ingest dirt, rocks, birds,
rain, and anything else you might find out there.  Jet engines do just
this, attempting to gain the maximum ram pressure at the face of the
first stage of the compressor or at the face of the fan by sticking the
engine directly into the face of the oncoming flow.  Look at a fighter
aircraft with a long curving intake duct to see where lots of taxpayer
money got spent tweaking the duct for maximum performance.  The same
basic principles are at work with hugely greater air flows, so the price
of friction loss is much greater than with our dinky little piston
engines.

Hope this helps.

Fred Moreno