SC2camaro

hey guys

If gravity and vacuum plays a part in fuel distribution. since fuel and air is being drawed in the TBI downwards. why is there a film of fuel on the under siide of the air cleaner lid?. IF gravity plays a part why is it present there and not falling down to the lowest point of the manifold then running along runners.


along as far as velocity of air is moving within the manifold the greatest amount is along the sides of the runners(top,bottom, left and right sides) rather than in the center.

Kevin Johnson

There is a good explanation here of the phenomenon known as fuel "stand-off":

http://members.aol.com/dvandrews/cams.htm

SC2camaro

thanks kevin for the info. it answered my question



Cams with lots of overlap generally give good engine power at higher RPM, but give that irksome low RPM intractability known as 'off-cam' where the engine growls, spits and jumps, together with a distinct 'coming-on-cam' (very messy) at a particular RPM. This coming on cam feeling results from the harmonics of the exhaust flow reaching a critical point where the exhaust gases stop trying to exit via the inlet port (reversion), and do their proper job of exiting via the exhaust, and promoting scavenging. At low RPM when the engine is 'off-cam', the exhaust gases cause pulses in the inlet tract which lead to a phenomenon called 'stand-off' where inlet mixture is bounced out of the back of the carburettors and hangs in a mist around the inlet trumpets /filters. This contributes to the 'off-cam' feeling as the mixture then fluctuates between too rich and correct and is mixed with spent exhaust gases.

Kevin Johnson

Professor Bardon, co-author with Professor Gardiner back in 1986 of "MIXTURE MALDISTRIBUTION DUE TO MANIFOLD FILMS IN A METHANOL FUELLED S. I. ENGINE" responded to an email I wrote.

In part he replied:

Question: Do you feel that the sustained WOT acceleration from a standing start of a TBI-equipped, gasoline-fuelled automobile would result in distribution errors?

Answer: yes - it seems likely in any single point fuel supply system.

kdrolt

Q.E.D.

Kingtal0n

Heres a few questions:
If a <b>drop</b> in manifold vacuum supposedly <b>reduces</b> fuel vaporazation, Then under a <b>forced</b> induction state of higher-than-atmospheric-pressure what happens to the fuel's ability to vaporize?
Restated:
Is the <b>mid-point</b> of the fuel's ability to vaporize right at <b>atmospheric</b> pressure? or does an <b> increase</b> in manifold pressure only serve to <b>reduce</b> the fuel's ability to vaporize further...

----
And:
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With <b> forced</b> induction does the cylinders <b> pull</b> of air simply <b>stop</b> affecting the distribution?
Restated:
When <b>higher</b> than atmospheric pressure is observed in the intake manifold does the <b>incoming air</b> determine /change the normally observed route of the fuel under <b>lower</b> than atmospheric pressures?

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You would think that with a pressureized Manifold the distribution of air/fuel would be very close to equal, if not totally. Not to mention I bet there is little or no fuel puddling. but I could be wrong.. thats why Im asking!

Kevin Johnson

Kingtal0n:
Heres a few questions:
If a drop in manifold vacuum supposedly reduces fuel vaporazation, Then under a forced induction state of higher-than-atmospheric-pressure what happens to the fuel's ability to vaporize?
Restated:
Is the mid-point of the fuel's ability to vaporize right at atmospheric pressure? or does an increase in manifold pressure only serve to reduce the fuel's ability to vaporize further...

Kevin:
I was gone for a few days…

I am guessing that the troublesome semantics of double-negations are afoot here (drop in vacuum).

Assuming a well-behaved fuel and neglecting the effects of heat from the forced induction as well as sundry mechanical/physical effects of the blower (depending on type—draw through, blow through, etc.) a forced induction state on a fuel would tend to continue the earlier trend, i.e. the ability of the fuel to vaporize would lessen with increasing atmospheric pressure.

Kingtal0n:
With forced induction does the cylinders pull of air simply stop affecting the distribution?

Kevin:
No. This is a similar question as to whether gravity/acceleration will affect the distribution. A different equilibrium would be established depending on rpm, manifold pressure, fuel, manifold temperature, etc.

Kingtal0n:
Restated:
When higher than atmospheric pressure is observed in the intake manifold does the incoming air determine /change the normally observed route of the fuel under lower than atmospheric pressures?

Kevin:
Since the observed route/pattern of the fuel films and rivulets, etc. change with rpm/manifold-vacuum on the test engines in the articles previously cited, I think it can be safely inferred that higher manifold pressures would change them as well. You might find some primary research on this starting back around WWII with respect to aircraft engines.

Someone pointed out earlier that even within a stock manifold, the pressures vary simultaneously at different locations during static engine operation.

With forced induction you might be able to narrow the range of the pressure variations for a given rpm, etc.