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Passing gas- an exhaustive study

To: mgs@Autox.Team.Net
Subject: Passing gas- an exhaustive study
From: Bill Eastman <william.eastman@medtronic.com>
Date: Wed, 21 Jan 1998 13:04:20 -0600
I have been watching this thread for a while and have now decided to pitch
in with my two cents.

I have simplified quite a bit in this diatribe so it isn't perfect but is
does show trends and orders of magnitude.

The typical fuel molecule is made up of a long chain of hydrocarbon
segments.  Each of these segments contain one carbon and two hydrogen. 
When burned, the result is carbon dioxide and water.  I know that there are
other things involved like end hydrogens, double bonds, etc and that carbon
monoxide as well as nitrogen oxides can be made but this is close enough
for now.

So, the chemical reaction is 2(CH2) + 3(O2) => 2(CO2) + 4(H2O) + heat.  Now
assuming that the hydrocarbon in liquid coming in and the water is gaseous
going out due to the high temp, you ingest 3 oxygen gas molecules and put
out 6 gas molecules including 2 carbon dioxide and 4 water.  So combustion
puts out 50% more molecules that it brings in.

However, Oxygen is only 20% of air so you really only see 10% more gas (not
fuel) going out of an engine than going in from a molecular standpoint-
nitrogen et al just go along for the ride.  The size of the molecule make
no difference.

>From a temperature difference, figure 27 C going in and 527 C going out for
an estimate.  All things equal, gas volume is proportional to temperature
but remember that this is in degrees Kelvin, not Celsius so we add 473 to
both numbers and we see that temperature effects increase the volume by 100
percent (1000/500).  So your engine has about 2.2 times the volume of
gasses coming out than going in.  My wife would say that I outperform this
by a good measure ;-)

So why are exhaust valves smaller than intake valves?  Because your engine
relies on atmospheric pressure to force air into it and that tops out at
about 15 psi while the exhaust is mechanically driven out at much higher
pressures by the piston.  If you have and 500cc piston with a 10:1
compression ration, there is 50cc of exhaust left after each cycle (overlap
can effect this but in general, you don't see a lot of back flow.  Assume
that this 50cc is at atmospheric pressure of 20 psi (5 psi back pressure)
and again at 1000 K.  The charge would be cooled to 500 K when it is mixed
with the next intake charge so it's volume would shrink by half due to
temperature but expand 33 % due to pressure so you would have about 33 cc. 
So, for the 500 cc intake charge, about 6.7% would be taken up by old
exhaust gasses.  If you doubled the back pressure to 10 psi you would be up
to 42  cc of leftover exhaust or 8.4%% of the charge for a 1.7% loss of
power.  There would also be some power loss from increased pumping losses. 
Cutting the back pressure in half to 2.5 psi would leave 29 cc of exhaust
or 5.8 % of the charge.  This would give less than a 1% power gain again
ignoring pumping loss changes.  Again this is completely ignoring the
dynamic forces such as scavenging and pressure waves.

So, an increase of 5 psi back pressure costs about 2 % hp in this
simplified example.  On the other hand, adding a 2 psi pressure drop on the
intake reduces power by 2/15 or 13 %!  This is why your engine has two 1.5
inch carburetors and one 1.5 inch exhaust.  

An engine is a balanced system but the balance is much more complex than
air flow.  The big issues is that changing the dynamics changes the fuel
needs of the engine at a given control signal.  A more efficient exhaust
will increase air flow for at a given throttle position or manifold
pressure so the carburetor will not deliver enough fuel.  It is this
balance that is important when considering engine, intake, and exhaust
modifications more so than the balance between the intake and exhaust port
capacities.  Tuning is the key.

Regards,
Bill Eastman
61 MGA tuned as Syd intended- for now

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