On Wed, 21 Jan 1998, Bill Eastman wrote:
> 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
>
Say what?
...Art
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