Air Temp effects on Atomization????

[Bernd Felsche]

Steve.Flanagan at VerizonWireless.com tapped away at the keyboard with:
> One week later we bolted the converted intercooler (water/air) on
> and ran the car.  It seemed to buck a little and lay down as the
> car went down the track.  Air charge now only went from 80 deg to
> 140 deg F from start to finish line. We lost about 1-2 tenths in
> the 1/4 mile over the next few passes, from 7.60 to 7.80  It
> slowed down.  Yes I was scratching my head. 

> What do you think would cause the car not running correctly with
> the colder air?  From all we saw, we came to general conclusion
> that at current fuel pressure, with the colder air, the fuel was
> not atomizing efficiently.  These were all assumptions.

Higher pressure post-intercooler. The turbocharger is still putting
out the same amount of air (I hope!) but the reduced temperature
means that the air molecules are more tightly-packed; which is the
same as saying it's at a higher pressure. 

You're injecting into a higher pressure manifold, so your fuel
pressure needs to be increased to maintain the same type of flow
through the injector.  Can your fuel system do that? Increased
pressure and flow?

Have you checked manifold and fuel pressures?
How about the pre-intercooler pressure to ensure that the turbo is
still delivering as before?

> Does this make sense, is it harder to burn the same (pressure) of
> fuel in colder air?  And would this require a harder spray to help
> the fuel to atomize.  If this is the case, then what is the
> coldest you can get the air charge up to before you run into
> problems.

Atomization depends (in part) on the velocity of injection which is
determined by the pressure differential between fuel and manifold
pressure.

Vapourization depends on temperature; a higher temperature results
in more rapid vapourization and less risk of condensation on inlet
surfaces, but the density of the charge falls off rapidly with
rising temperature. 

I don't know what the optimum charge temperature would be for a
turbo engine; VW tries to maintain an inlet temperature (pre-
manifold) of between 25 and 30 degrees C in non-supercharged
engines. Their supercharged G60 engine had an intercooler drop of 55
C from a nominal maximum inlet of 150 C. i.e. inlet temperatures as
high as 100 C or thereabouts.

> I was running the old PV=nrT formula, and trying to understand the
> difference between air charge of 130 deg down track vs 265.

You're taking energy out of the system at the intercooler so more
complex thermodynamics apply.

> Basically before and after the change.  If you convert to Kelvin
> and look at a constant Pressure, the Volume increases by about
> 20%.  I get 402 deg K vs 333 deg K at the same P, so 402/333 =
> 1.20, now wouldn't this 20% increase in volume, provide for a 20%
> increase in CFM which should add a decent amount of horsepower????

The volume doesn't change (much) when you change intercooler types;
the pressure rises. The charge density increases; the mass of air
within the volume.

-- 
Bernd Felsche - Innovative Reckoning, Perth, Western Australia

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