George, Walter, et al,
It has been suggested that a 1.5
psi differential between the air supply to an injector and the deck pressure
(manifold) would optimize atomization of the fuel.
Verrrrry Interrrrresting!
Factoids:
Let’s remember that 1.5 psi is
approximately equal to 3” Hg. At
sea level standard conditions, 29.92” Hg is approximately 14.7 psi (one
atmosphere).
Theoretically, at 200 Kts a
natural ram air system can raise the pressure about 2” Hg (1 psi, Lancair
320/360) and at 245 Kts the rise is about 3” Hg (1.5 psi, Lancair Legacy).
An effective air cooled engine
cooling system should see a pressure drop between the plenum and the exit
air. Some reasonable estimates are
5 to 7 inches of H2O (.37 to .51 inches of Hg or .18 to .25 psi). While the intake plenum is not a high
pressure chamber (sealed plenums notwithstanding), the plenum pressure (in
Lancairs) is higher than ambient – I couldn’t find my old measurements – For
now, let’s use .5” Hg at cruise.
How could a naturally aspirated
engine in a commercially built airplane achieve the optimal pressure
differential? This assumes no ram
air, filtered induction air and perhaps (as in the case of some Continentals)
the air is drawn from the engine cooling plenum thus making no pressure
differential at WOT.
Answer: Reduced throttle
operation. For example, a
Continental powered Skymaster operating under std. conditions at 3000 feet MSL
(approx 26” Hg) and 2500 RPM with the throttle reduced to 23” MAP would provide
the desired pressure differential (1.5 psi) and be running at about 75%
power.
Those of us flying Lancairs with
good cooling plenums and taking advantage of ram air boosts to the induction
system whilst also always using WOT are not optimizing fuel atomization at the
injector. As a matter of fact, we
are pushing back against the air the injector is trying to utilize. Some of us have neutralized this problem
by using shrouded injectors to which we have supplied the same type of ram
pressure boosted air. This is not
air from the post throttle-body induction system since this would steal air from
the induction system much like a leak and always operate with no
differential.
Using my Lancair and the above
example, 3000’ MSL, 2500 RPM, 23” MAP and about 170 Kt cruise, the pressure
differential would probably be closer to 2 psi because the injectors are
receiving ram pressure boosted air.
At 8000’ (21” Hg), 2500 RPM, WOT,
180 KIAS, 23” MAP, the pressure differential is zip because of the WOT – but it
isn’t negative. Note that the
engine thinks it’s operating at 6000’ whilst the wings are at 8K and I’m still
about 73% power.
Solutions to get constant 1.5 psi
differential:
- Electric
air pump with regulator.
- Always
fly at less than WOT.
- ?
- Don’t
worry about it.
- Get
Honda Motorcycle 12 port electronic injectors that don’t need no stink’n
air.
- Get
a Turbo-Prop.
- Get
a glider.
Scott
Krueger AKA Grayhawk
Sky2high@aol.com
II-P N92EX IO320 Aurora, IL
(KARR)
LML, where ideas collide and you
decide!