mrluke wrote:321apex wrote:
- WRC rally formula is based upon restricting the air flow RATE into the engine.
- F1 limits the fuel flow RATE as well as the QUANTITY consumed.
These are fundamental differences, which set the optimization logic and consequently control methods apart.
"snip"
Sorry, I missed the bit where you explained that by changing boost levels you cant change peak power position on the rpm scale, but only on an f1 engine (right?)
On an NA engine cam timing is more important as it is the main control of the air into and out of the chambers, on a turbo engine you can vary the density of the air to control how much gets into the chambers, rather than just the volume.
In the link describing "choked flow" there is a fundamental relationship explained of a ratio of stagnant pressures before and after the restriction.
http://en.wikipedia.org/wiki/Choked_flow
When an engine "runs out of breath" it is usually due to this "choked flow" effect thru the valve/seat curtain area. If it is inlet curtain area the stagnant pressure delta is approx. 0.528. At such condition, Mach speed of 1 is reached and flow mass rate can not increase.
This "choked flow" is also tied with piston/engine speed and Mach index on the inlet cycle down stroke, as is explained in this link:
http://books.google.pl/books?id=UtxI5gX ... ne&f=false
It is important to note that this phenomena is based upon a ratio of stagnant pressures before and after the restriction. So that it's occurrence is piston speed based and not manifold pressure.
p(u)/p(d)=0.528
To illustrate, if you have a turbocharged, unrestricted engine and you dyno test it at some level amount of boost across the rev band. As an example the engine may reach peak power at say 7000 RPM, after which it just falls off. If you were to strip away the turbocharger and if possible not altering inlet/exhaust tract geometry run that same engine as naturally aspirated.
The power peak would occur at just about the same RPM within +/-150 revs.
In both cases, the "choked flow" would coincide at similar piston speed, which is subordinate to Mach index on the piston inlet cycle down stroke. Usually cam lift and timing is most effective in altering rev band location of power peak. It effectively alters the Mach index relationship.
In closing, I must add that similar effects may take place on the exhaust side first. The principle remains the same, however the properties of exhaust gas are much different than air and so a different Mach index threshold would result as well as different numeric pressure ratio before the "choked flow" is reached.
