Historically the amount of air going in the engine
is measured with a venturi. The vacuum generated was then
used to draw air out of a float bowl. Very simple. The drawback here is the venturi
causes a reduction of air going into the engine due to aerodynamic drag.
There is a trade off between the amount of vacuum you get verses the aerodynamic
drag of the venturi. If you reduce the vacuum signal the carburetors tend to get
more complicated and expensive This one of the reasons that carburetors have
disappeared on cars. For a good discussion on this and the effect of right angle
intake manifolds read the attached pdf.
Aircraft engines assume the amount of air going is is directly
proportional to the intake manifold pressure. This assumes there is some sort of
valve that controls the intake manifold pressure. Be it slide throttle or butterfly.
This is OK until you get near WOT and then changes in air flow result in very
small changes in intake manifold vacuum.
Another way is to measure the throttle valve opening and assume the amount of fuel
is proportional to the area of the valve opening. This too is fraught
with draw backs and suffers from some of the same problems with measuring
the intake manifold vacuum.
Modern cars use a hot wire in the air stream. The more the
air the more the cooling effect on the hot wire. Measure the cooling effect
and you know the mass air. Cold air is more dense than warmer air so this
works pretty good. They are called mass air flow sensors and are an
improvement over measuring intake manifold alone. They too have some aerodynamic
drag and are rather expensive and bulky. The stock versions are cheap as
they can be obtained in junkyards. The drawback is most come with a butterfly
valve that add a lot aerodynamic drag. Even at WOT. A third disadvantage is you
need to know what voltages they put out for X amount of air flow at Y temperature
so they can be interfaced with the required computer. This information is not
readily available. It is not a show stopper however as some experimental
investigation can solve the problem.
Another way of measuring the air flow is use a pito tube. These
can be very small made from hypodermic needle tubing and consequently can
have very low aerodynamic drag. They can be mounted anywhere in the intake system.
They are cheap and easy to make. Almost free. IMHO The best place to mount them
is near the beginning of the intake runners just after the trumpet mouth. This is
where the air is least likely to be turbulent. What pito tubes do is measure
the force of number of high speed air molecules impacting a flat surface.
The force generated is proportional to speed squared so they are very
sensitive to changes in air velocity (volume in a close tube) at all
throttle openings.
A crude form of this was a mass airflow sensor widely used on early
EFI systems that consisted of a spring loaded trap door in the intake
manifold. The air pressure would tend to open the door. How far it
opened was measure and sent to the computer. Bosch built the early
systems and they were widely used on many different cars including
the second gen RX7. The draw back again was the spring resisted
the door opening so they also caused aerodynamic drag. Also
the signal was not very sensitive due to the geometry near WOT
much like measuring the intake manifold vacuum.
Solid state pressure sensors are cheap so a pito tube feeding data to
a cheap PICAXE microcomputer controlling the amount of fuel going with
pulse width modulated electric fuel pump sounds like a cheap and simple
way to go. Measuring the temperature of the air will also
be necessary but that is cheap and easy.
The use of an air filter near the ground would be mandatory.
Paul Lamar ...No rotor no motor.
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