George, not exactly certain what you mean
by “calculating Air/Fuel ratio” .
I used the ratio of the mass of the air to
the fuel – which basically relies on the approximation of 0.0765 lbs per
cubic foot of air. So you calculate your air flow in CFM then times
0.0765 to give you the air mass. The you can take your desired A/F ratio –
say 15:1 and divide 15 into your air mass and that would give you the
fuel mass required to achieve that ratio. Of if you have any two of the three factors
you can find the third.
IF you mean from a run time
perspective - how do you know you Air/Fuel ratio - then there are expensive
testing instruments and I believe some fairly accurate air/fuel ratio meters
(based on the newer broad band O2 sensor) But still several hundred dollars
when I last checked.
The narrow band O2 sensor is much cheaper
and works just fine with 100 LL for OUR use. I generally get closer to
200 hours using 100LL before the sensor appears to lose too much sensitivity to
continue to be useful. The common notion that a few seconds running on leaded
fuel will “Kill” an O2
sensor (at least the narrow band ) is simply not true – at least not for
our use. Doing that WILL apparently degrade it for its intended use in an
automobile fuel system (where it needs to help the engine computer maintain a
14:1 A/F ratio), but if you just want a general indication of whether you are
lean, rich or in the middle, the cheap O2 sensor works well.
You can even find some narrow band units
which will read out A/F in numeric values but if using a narrow band O2 sensor,
I question the accuracy of such units myself. I suppose you could use a microprocessor
and accurate analog/digital converter and if you had the “Z” curve
of your O2 sensor – you might get close.
So basically if you know the mass of air
and mass of fuel, you have your Air/Fuel ratio
So how to arrive at those two factors, if
you know the air pressure and temperature (at normal atmospheric values)
then you essentially know the air density from which you can calculate air mass
and then using your engine flow rates and with your chosen Air/Fuel ratio - calculate
your fuel flow, etc. But, frequently it’s easier to use our fuel
flow (which can be measured fairly accurately)
So one r way to approach the problem is as
follows: You know the displacement of your engine and assuming some Ve (volumetric
efficiency) (85% - 110%) you can calculate your air mass flow through the
engine for any rpm. So how to get an approximation of our volumetric
efficiency (at least at WOT), its fairly simple to get close.
Note the ambient atmospheric pressure
(manifold gauge pressure without engine running), fire up your engine and when
warmed up advance it to WOT and note the atmospheric pressure inside your
intake (i.e. your manifold pressure). Lets say ambient pressure is 29.92
inches HG and lets say you are so lucky to read 29.92 “ Hg in manifold
pressure - then theoretically your Ve is 100%. But lets say your design
is not perfect (few are) and your read 28.75” then your Ve is 28.75/29.92
= 0.9608 or 96.08 % Ve –not bad, but not perfect.
OK
calculate your volumetric flow using our old displacement formulas and as best
I recall at 6000 rpm with a 13B at 100% VE = 277 CFM. Since our intake is
not perfect we take our Ve of 0.96.08Ve*277 = 266 CFM actually going
through your engine. Recalling that a cubic foot of air approx = 0.0765
lbm/Cubic Foot, we have 266 * 0.0765 = 20.36 lbsm of air per minute.
Now we don’t know our Air/Fuel
ratio – but we do know our fuel flow at that rpm. Lets say its 16
gallon/hour, turning that in to lbm/min we have 16 / 60 = 0.2666 gallon/min and
we know mass of gasoline is approx 6 – 6.25 lbs/gallon. So taking
6.0 lb/gallon we have
0.2666 * 6 = 1.6 lbm/min of fuel based on
our fuel flow indication.
Now taking both the air mass 20.36 lbm/min
and the fuel 1.6 lbm/min and we get 20.36/1.6 = 12.725 air/fuel ratio.
Which is very close to the common “Best
Power” ratio used by many.
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of George Lendich
Sent: Sunday, June 14, 2009 6:21
PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re: Wire
separation?
Can you tell me what sensors you use to calculate fuel
ratios.
I notice the mention of a MAP sensor (vacuum type), but that
would give pressure, not density.
I notice the absence of an oxygen sensor in the case of
100LL is used - this is understood. I suppose an oxygen sensor could be used
independently.
I assume RPM would be in the calculations for fuel flow, but
I wonder how you actually arrive at air density? perhaps OAT is used.
Basically I wondering what the simplest air density sensor
arrangement would be. I thought Oxygen sensor (not for 100LL), with a RPM and
throttle position sensor.
Old time distributors and carburetors didn’t care how badly you
wired the electrical system. Digital systems are not so forgiving of poor
wiring practices. Noisy power or ground wiring is one of the most common
causes of initial builder problems with computerized equipment (including
ECUs).
In addition to the wire bundling answers you got, the basic power wiring is as
or even more important. Here is a basic aircraft wiring scheme that I
recommend in latest version of the installation guide. Not detailed but
somewhere to start. Note the independent power feeds from the battery for
the various systems. This is a physical as well as electrical
representation.
Tracy
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