This topic raises some questions in my
mind that just won’t go away. Pesky things.
The fuel injectors have holes in them so
that air can flow from a high pressure region into the injector to the lower
pressure region in the intake port. Along the way the flow of air helps to atomize
the fuel assisting combustion.
There are the bits of “data”
(I use the term loosely) rumbling around in my mind that suggest the injector
air flow is not important at wide open throttle and thus irrelevant to high
power cruise performance.
The question is: True or False?
Data bit #1: I recall the following when talking
to an auto engine “expert.” Modern electronic fuel injection
systems control fuel flow by varying pulse length, but the fuel pressure
remains constant. A jet of fuel is pointed at the back of the intake
valve. The intake valve which runs moderately hot helps to
vaporize the fuel. Further vaporization occurs as the intake air is drawn
by the intake valve into the cylinder. The space between valve and valve
seat creates an annular venturi of sorts, and as we know, venturis are highly
effective at atomizing liquid fuel. So in auto engines higher fuel pressure
squirting on the intake valve plus a bit of heat plus the high shear stresses that
occur when the flow passes the valve and seat all combine to provide good atomization
over a wide range of power conditions.
Data bit #2: I spent some years working
in gas turbine combustion to reduce NOx emissions. This led me to Arthur
Lefebvre’s book Gas Turbine Combustion which reports on the huge
amounts of work done on fuel atomization in jet engines. The
problem is that between cold day sea level take off power and flight idle at
the start of descent at 36,000 feet, there is a huge difference in fuel
flow. This creates major problems for atomization over the entire range
of operation, and has resulted in a lot of fancy fuel injector designs.
Our simple constant flow fuel injection
systems are more like turbine injectors than modern electronic auto injectors.
Data point #3: Aircraft fuel injectors
tend to squirt at the wall of the intake port (also warm) and not at the back
of the intake valve. So cold, low power (idle) atomization maybe
particularly poor. And low power which calls for low air flow in the
intake port also leads to poor atomization. Help needed.
So methinks the following:
In our constant flow fuel injectors, at
idle and low power settings, the fuel flow is so low that the injector by
itself does not squirt, but dribbles fuel out in a low pressure stream. Poor
atomization = lousy combustion. So to improve atomization, we add air
assist atomization. Fortunately, low power also means low manifold
pressure, and so ambient pressure (or upper deck pressure) will force air
through the fuel injector where it can atomize the fuel dribble and –
magic! – good (or at least improved) combustion. One can expect at
least 5 or 10 inches of mercury delta P between intake port and ambient, good
enough to spray gasoline.
At high power and in cruise (in particular,
wide open throttle at altitude) the manifold pressure virtually equals the
ambient pressure. No delta P means no atomization air through the
injector. But not to worry – the cylinder head is hot so the fuel
is hitting a hot surface inside the intake port, and moreover, the MASS FLOW RATE
of air through the intake port is high so that the fuel is well sheared and
mixed when air and fuel pass through the venturi between intake valve and valve
seat.
Conclusion (Sorry Scott, if I am right) –
The extra effort to gain a few inches of WATER pressure using the pitot ram
tube set up to pressurize the injector air is not worthwhile at higher power
settings. At lower power settings, the delta P IS large enough to provide
atomization.
So for higher power settings or wide open
throttle, there is little to be gained by fancy air distribution systems for air
injection flow. More importantly for Scott, the speed contribution
may be zip.
There are two things that can modify this
argument.
The first is that the pressure in the
intake port is not constant at the “manifold pressure” but surging
up and down as the cylinder goes gulp-gulp-gulp, and so there may well be some
delta P during the intake stroke when the cylinder is sucking on the intake
port. If this pressure fluctuation is not too fast for the flow in the injector,
it may assist in atomization. But the effect is due to periodic low
pressure in the intake port due to induction, not a tiny bit of additional
pressure from the pitot ram air set up.
The second thing that can modify the
argument is DATA! One sound data point is worth 1000 speculations.
Has a good before and after test under carefully controlled conditions shown performance
improvements in terms of power, mixture distribution, or beneficial effect on
the harmonic convergence? If so, Publish or Perish!
What say you, Walter? Teach us some
more about fuel atomization at higher power settings and wide open throttle.
What does the data show? If no data, you are invited to speculate as
well. One Walter speculation = 1000 Fred speculations.
Always Curious Fred