Elastic air and inertia AD V7 #877

["C.M. Smith" <cmsmith@domain.elided>]

Being a man of few words, I can't resist prolonging the discussion about
ram effects of intakes and extraction effects of exhausts. BTW, I hope
readers aren't confusing the resonance ram effect with the more
straightforward ram effects due to forward motion of the vehicle. This
latter ram effect is just that, compression of the air due to forcing the
air intake into the airstream by the vehicles forward motion. Apparently
this is so significant in F1 cars that the design of the driver's helmet
affects the efficiency of this ramming of air into the air box. Supposedly,
M Schumacher had to tilt his head to one side in the 98 season on the long
straights to increase the efficiency of this effect!

 A particularly amusing example of straightforward ramming at work is
illustrated by  the NA Pontiac Trans Am which had a backwards hood scoop
with a flap that opened with the accelerator pedal. Now a backwards hood
scoop seems ridiculous since the airflow is coming from the front of the
vehicle, but in fact the highest pressure area at the front of the vehicle
is at the base of the windshield (an ideal place to put the intake for the
ventilation system for the interior!) so Pontiac knew what they were doing.
When the flap opened the engine started breathing from the high pressure
area at the base of the windshield, a mild supercharging effect at speed.
The amusing part is that the benefit is present all the time the car is
moving forward so why the flap? So the driver could see how much power he
was getting! Weird science.

I don't think air is elastic exactly.  For airflow at subsonic speeds it is
treated as incompressible, but it clearly is compressible even below the
speed of sound, if not highly compressible. While logic dictates that the
intake is only open on any given cylinder for just over 25 % of the time, I
don't see this as meaning the airflow actually "stops". At high rpm's the
time intervals between successive valve opening events are pretty short.

 Waves in a fluid are not the same thing as movement of the molecules of
the fluid i.e. a wave travels but the individual molecules just oscillate,
generally continuing their forward motion if already in motion, or not if
not, (ocean waves are a good example because everyone is familiar with the
waves moving in, even carrying surfers etc, but the net movement of the
water is zero). I still think resonance supercharging is not really
inertial so much as a wave effect, but I concede the distinction is
somewhat a matter of point of view. Inertia i.e. the tendency of the column
of air in motion to remain in the same motion, or to remain at rest if at
rest, velocity creates the wave in the first place, but not by reversing
the direction of the airflow, or even by stopping it. 

When the compression wave reflects back up the intake tract it will indeed
"dissipate" as it reaches the lip of the intake, but the result is a
reflected rarefaction wave, which you definitely do not want to reach the
intake valve as it opens, this being the source of the negative effects of
resonance tuning at unsuitable frequencies, causing "flat" spots as noted
by "Godfrey",  whom I suspect knows more than either of us on this subject.

The "dropping" of the intake valve generates a rarefaction wave which
travels back up the intake tract and "dissipates" upon reaching the lip of
the tract, resulting in a reflected compression wave which, at resonant
frequency, returns to the intake valve just as it opens causing a mild but
noticeable supercharging effect (using this term quite loosely, though in a
good engine I believe it is a slight positive pressure effect). The inertia
of the air crowding against the closed valve actually causes the problem of
the reverse effect if the resonance is such that the reflected rarefaction
wave reaches the valve as it opens.

I agree with your remarks about the extraction effect of properly designed
header systems and the challenges facing the designers of those systems.
The resonance works the same way as for intakes, but it is possible to
harness the cylinders together. I know of no really high performance engine
that attempts to harness the intakes together in an analogous way, but even
F1 engines connect their exhausts (and F1 is the peak of specific power
output for real engines that can be driven, as opposed to drag engines
which are about thrust only.)

Valve timing is only incidentally related to resonance effects, clearly the
timing is important for the resonance to operate. Valve timing is more
concerned about inertia effects and that is the explanation for how some
fairly extreme valve overlaps work. The airflow into and out of the engine
at the rpms involved is pretty well continuous, though fluctuating in
pressure continuously, and valve overlap exploits this. Higher rpms allow
more overlap because of inertia effects. Supercharging pretty well
eliminates the advantages of valve overlap because the inertia of the
airflow is overwhelmed by the supercharger pressure (speaking figuratively
of course.)

BTW are you sure the "standoff" you refer to  was caused by resonance
rather than valve overlap? it seems surprising that a wave effect could
actually reverse the flow of air and fuel droplets back through the
carburetor. Interesting dialogue. Maybe we should both just re-read the
books!!!
Michael Smith
Calgary, Alberta
Canada
91 Alfa 164L, White, original owner 

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