Matt wrote:
Thinking about this more.. If you used an integrator, you need
to set it up so that the integration window is only during the
port-open time.
Otherwise, you might as well use the thing I was talking about
(essentially a mechanical integrator).
Okay, so maybe you need a peak-detect-hold circuit.. The way it would
work is the output from the MAP sensor would be fed to the input of
the peak-detect-hold circuit, and then to the MAP input on the EI
computer.
An output from the EI computer would reset the peak-detect-hold
circuit - preferably when the port is closed on each cycle of
operation. This way, you get to see the full value of the
supercharging effect, and the timing of the MAP sample becomes
a moot issue.. The hold portion of the peak-detect-hold circuit
retains the sampled peak voltage until it gets reset. How's that?
Here's an IC that looks like it might be the right idea. I haven't
priced it..
http://www.amptek.com/ph300.html
Or maybe something like this, though you'd have to rig a switching
power supply to make it work..
http://www.chipcatalog.com/Analog/PKD01EP.htm
I suspect that we are making this problem harder than it really is. If
we make the assumption that the shape of the pressure graph is linear
between IO and IC (which it is in Paul's graph, and likely is whenever
the manifold is "on song" as Paul puts it), then simply take one
measurement at time (Tio + Tic)/2. That is the height of the area under
the curve between IO and IC, and the width of the area is Tic - Tio.
Easy. The less linear the graph shape, the more error this calculation
has, but I suspect that we really only care about the full-throttle
resonant operation anyway.
Russell Kent
We are concerned about the proper mixture over a wide range
of RPMs and that curve will rarely be so simple. Some times it
could even be U shaped.
Paul Lamar
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Okay... How about make it really simple? Just take the samples and sum
them. Do we really care about doing the sum_of_samples/num_samples? It's
not like we are going to get ACTUAL pressure values. Just some binary
representation of a voltage that is proportional to the MAP pressure. If
we want to display it, we'll have to do a bunch of massaging to make it
cubic feet per second. The only thing to worry about is whether the
accumulator register is big enough to hold num_bits_per_sample *
num_samples.
Using manifold pressure to extrapolate mass-airflow from a sensor that is
located so close to the port means that we are stuck with a bunch of
non-linearity, even if the signal is integrated over port-open time. This
is so because the intake port size isn't constant, even when we say it's
'open.' It starts to open (low initial flow, regardless of pressure), is
completely open for a while, and then ramps closed (reducing flow,
regardless of pressure). Integrating the values from a MAF would be
better. The MAP would be more useful if the sensor could be mounted in a
plenum with a throttled inlet. It's probably going to work, but still
take a bit of tuning to get the mixture vs MAP vs RPM to work out.
Regards,
Matt-
No doubt you are totally correct Matt. In fact the flow drops off
as the chamber pressure builds. That is why it is nice to get peak
pressure just before the port closes.
A feedback system based on EGT's so far has been resisted by Tracy
for two reasons.
One; slow response of EGT thermocouples. That can be mitigated
by going down on the size of the thermal couple. Small thermocouples
respond faster.
Two; can't tell what side of the
curve you are one. I suspect that can also be fixed by incremented
the mixture and see if the EGT temp goes up or down. If it goes up
you are on the lean side. If you lean it and it goes down you are also
on the lean side. If you make it leaner and it goes up you are on the rich
side. Simultaneously comparing that technique to an increase or decrease
in engine RPM should also help.
Of course a good wide band mixture monitor would work as well
and I think Tracy needs to try that. Currently the auto tune mode
uses ordinary O2 sensors.
Paul Lamar
The Rotary Engine NewsLetter. Powered by Linux.
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http://www.rotaryeng.net
Copyright 1998-2005 All world wide rights reserved.