Barry Gardner wrote:
Here are my two cents on the display.
I think Tracy's post of Tuesday night has the correct perspective
when he uses the *occasion* and *frequency of use* as criterion
for figuring out what engine measurements go where. However, I'd
tinker with the list of measurements displayed just a bit.
Let's look at the question abstractly.
What variables are there that an EFI system is using to control
an engine? The EFI computer is solving for:
--Air mass density. (both pressure and temperature, if using an
air flow meter, or observed directly if using a hot wire mass
airflow sensor)
--Load (using some combination of throttle position sensing and
manifold vacuum/pressure)
--RPM
and the output of this calculation is:
--Fuel Flow (controlled through pulse width of the injector,
premised upon some fuel rail pressure)
--Spark advance
There are other measurements that can be helpful for *tuning* and
engine:
--Fuel/Air ratio, and/or
--EGT.
[As a side note, I read Tracy's earlier comment that fuel/air
mixture ratio measured by an O2 sensor was faster to react than
an EGT. This might be true but hot rodders working on chassis
dynos will tell you that the ordinary O2 sensor, while fast,
isn't accurate enough for tuning and that they prefer EGTs. There
is a special wide-band O2 sensor that is both fast and accurate
but it also costs $$$$. Before going too far down the road built
upon the ordinary O2 sensor, one might investigate this question.
If Brian Cain and John Vitamas are still on this list, they may
be able to add something to this discussion.]
I think that's the list of helpful stuff to know about the
engine.
However, there are also some things that are helpful to know
about an airplane:
--Propellor speed
--Propellor pitch
--Density altitude
Prop speed is directly related to engine rpm, so I think its
inclusion is redundant. Engine rpm is more important for
monitoring the engine's limits whereas, after one makes sure the
tips don't go supersonic during propellor choice time, it matters
less. And you're stuck with your choice in the air anyhow.
I think that is all the stuff that relates to the powerplant
(i.e., engine/prop combination). It occurs to me that these
should be the variables that are candidates to be monitored on
the instrument panel.
Which of these has to be in the EFI display is just a business
judgment about what satisfies the maximum number of potential
customers. As my post on the weekend stated, some of these can be
monitored using easily available off-the-shelf stand-alone
instruments and need not clutter up the display.
It also occurs to me that the airflow measurement for the engine
is the same thing as the density altitude for the airplane. It
might be an interesting idea for a density altitude measurement
to be directly observable on the panel which, on the ramp, helps
the pilot figure out the take-off characteristics of the plane
and which, in the air, helps him understand the potential power
that can be developed by the engine. Perhaps rather than display
intake temp, unadjusted for pressure, use density altitude which
takes both variables into account and even has value on the
ground before engine start.
One other variable that hasn't been mentioned for inclusion on
the panel is spark advance. I don't know the extent to which that
is controllable on Tracy's engine computer but that could be a
subject of great interest for some, particularly turbo users.
Another reason to incorporate that on the display is that I don't
know how else you get it. I can order a lot of these instrument
sensors from aircraft or automotive suppliers if they are not on
the EFI display but a real-time reading of spark advance isn't
one of them.
Lastly, there are some things that are helpful to know about the
EFI unit itself, and that's where you have the variables:
--Table data in use (Can I assume that this is like a software
version number?)
--Table address in use (Interpret this for me; does this change
in flight?)
--Staging flag (I'm not sure what this means but it seems to have
been classed with the EFI data. Perhaps you can clarify, Paul.)
A number of readers have wondered whether these last three
variables need to be so prominent. I recognize that being able to
identify the dataset is important but let me offer two other
common methods to keeping its intrusion to a minimum. On the
Loran on the plane I fly, as well as my GPS, the dataset version
cycles up briefly upon start-up, then disappears. That's one
solution, though I don't know how hard that is to program.
Another conserving mechanism on display space would be to code
the data. Are those labels "TA," "TD," and "SF" required or could
it be that one line of alphanumeric code, perhaps with the
variable components separated by dashes or asterisks, be just as
helpful? Like a VIN number, it could be something that is
interpreted with a short table of some kind but need not be
immediately recognizable in English. In the days of the 80 column
punched card, a lot of data was coded in just such a manner.
[Please don't write if you don't know what an 80 column punched
card is. I don't think it matters any more unless one wants to
understand the historical roots of the Y2K issue.]
Now, the last question--which indicator goes where?
I started by saying that *occasion* and *frequency of use* were
the two criterion I liked. Those overlap, of course. Here's my
take on the implications of that.
I can think of two occasions when I think I want to be able to
glance at the EFI and get data. The first is the most crucial:
the engine is having some kind of problem. I'd want to be able to
see all the engine data at once, whether on the EFI or on some
combination of standalone instruments plus the EFI:
rpm, spark advance, fuel pressure, pulse width, manifold
pressure, perhaps some assurance of measured airflow.
The second occasion is cruising, an occasion for monitoring or
tweaking with controls:
miles per gallon, gph, total fuel used, propellor pitch, EGT,
fuel/air
Well, that's my contribution. It also grew to be more than just
two cents.
Barry Gardner
Wheaton, IL
One thing Barry. Right at the moment we are limited to the data the
EFI computer sends over the RS232 interface to the display computer.
Mass airflow is not one of them. Tracy uses a lookup table which
has as its dimensions intake air temp, RPM and MAP.
The other thing is we want only one display page most of the time.
The other display page (or pages) will be used only for setting up
the map initially.
This is the list of stuff coming from the computer.
1. O2 sensor output. fuel/air mixture (10 bits) 255 = stoic. mixture.
higher = richer
2. MAP manifold pressure (10 bits) 841 = 30" Hg. , 200 = 10" Hg.
3. Air temp (engine inlet ait temp) (10 bits) 511 = 25 degrees C. (curve
not defined yet)
4. PGM current setting of programming knob. (10 bit) 0 - 1023)
5. Mixture current setting of mixture knob 0 - 1023
6. Fuel Flow (10 bit) 0 - 511 511 = approx 20 GPH on 13B, 30 GPH on
20B
7. Table data in use. (10 bit) Current mixture correction table entry.
8. engine speed (10 bit) (1 mhz clocks during 30 degrees of crankshaft
rotation)
9. Table address (8 bit) 0 - 63 Address of current table entry in use.
10. Staging flag (8 bit) 0 or 1, If 1 then fuel flow must be divided by
2 (one injector per rotor in use)
11. EOX end of message byte (8 bit) F7 hex
Paul Lamar
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