Today's Message Index:
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1. 03:50 PM - Two power signals, one wire (Paul Millner)
2. 04:19 PM - Electronic Ignition Competitive Comparison (Justin Jones)
3. 04:57 PM - =?utf-8?Q?Re:__Two_power_signals,_one_wire? ()
Message 1
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Subject: | Two power signals, one wire |
I remember the fun stump-the-freshman boxes we'd build in school to
control, one, two or three lights on one box with one two or three
switches on another box, with only two wires running between them. (It
was done with isolated power sources AKA batteries and diodes.)
I've got a similar challenge. I've got one #18 wire that runs out to the
tip of the tail for the beacon and aft position light. I'd like to
separate them. I guess I could do a similar trick with a diode and an
isolated power supply (instead of a second battery, I guess I could use
an IC that generates isolated power via a charge pump...). Is there a
more traditional way to separately control two loads at the end of a
single wire?
Paul
Message 2
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Subject: | Electronic Ignition Competitive Comparison |
With much talk about the EFII system by Robert Paisley here lately, I
thought I would share the results of a test between the EFII system, a
Slick Magneto, the Pmag 114, and the Lightspeed Plasma II+ systems.
http://www.flyefii.com/ignition/ignition_comparison.htm
Bob, I am interested in your thoughts on this test.
Thanks
Justin
At the EFII facility at Cable Airport in Upland, California we gathered
up the most popular choices for ignition systems on Lycoming engines to
do some comparison testing. The results were very interesting and
brought up some important differences between the systems tested.
The systems tested were:
Lightspeed Plasma II+
P-mag 114
Slick Magneto
EFII
There are other ignitions available for Lycoming engines. Those listed
above appear to be the most popular choices in today's market for
experimental aircraft.
IGNITION 101
Energy Storage
In general, ignition systems are categorized first by how they store
energy to do their job. Their job of course is to produce sufficient
voltage and current to generate a spark across the gap of the spark
plug, and to create this spark at some nominal point during the rotation
of the engine.
Most vehicle ignition systems fall into one of two categories depending
upon how they produce and store energy.
Capacitive Discharge - The first category of ignition is "capacitive
discharge" or "CD". CD ignitions store energy in a capacitor and then
discharge the stored energy through the primary winding of an ignition
coil which in turn has a secondary winding connected to the spark plug.
In CD ignitions, the storage capacitor is typically charged through a
DC-DC converter circuit which takes the available charging bus voltage
(commonly around 13.8V) and converts it up to around 400V. Charging the
capacitor at 400V allows for much greater energy storage than if the
capacitor was charged at bus voltage. The ignition coil in a CD ignition
is used as a step up transformer. When the 400V charge in the capacitor
is dumped through the ignition coil, the voltage is stepped up to
several thousand volts by the coil. This provides the required spark
voltage to jump the gap of the spark plug. These days, CD ignitions are
found primarily on small vehicles such as scooters, dirt bikes, and
other small engines which typically have a minimal electrical system.
Common characteristics of CD ignitions are a relatively low spark energy
and relatively short spark duration.
Inductive Discharge - The second general category of ignition systems is
"inductive discharge" or simply "inductive" ignitions. Again, this
refers to how the ignition stores energy to do its job. In an inductive
ignition, the energy is stored directly within the ignition coil in the
form of a magnetic field. When current is passed through the primary
winding of the coil, energy is stored in the magnetic field. When the
charging current is shut off, the magnetic field collapses very quickly
and the energy is discharged through the secondary winding of the coil
which is connected to the spark plug. There are a few sub categories of
inductive ignitions. Magnetos were one of the earliest forms of
inductive ignitions. Magnetos store energy in a magnetic field by
passing a current through the primary winding of the ignition coil like
all inductive ignitions. However, they generate their own electrical
power with an internal generator and do not rely upon the vehicle
electrical system.
Cars made before the 1980s typically used a points triggered, slow
charging type of inductive ignition in conjunction with a distributor.
These ignitions were powered by the vehicle electrical system and had
relatively low energy.
Modern cars all use high energy inductive ignitions. High energy
inductive ignitions use an ignition coil that has a very low resistance
primary winding, typically in the 0.5 to 0.7 ohm range. The low
resistance coil can charge very rapidly to a high energy level. This
type system works very well with larger engines that have a capable
electrical system that includes a battery and alternator. With the high
energy inductive ignition, the coil can draw a fairly high current
during the time it is charging, but this charge time is very short and
the average current draw is low. Common traits of a high energy
inductive ignition are high spark energy and long spark duration.
Spark Timing - The next topic of interest when it comes to ignition
systems is spark timing. A good hot spark is only half of the story when
it comes to making an engine run well. The second part of the equation
is making the spark at the right time. Ideal spark timing is not a
simple thing. It varies with engine rpm, engine load, fuel type and
octane, engine compression ratio, and other factors. Spark timing is
handled differently by different ignitions.
The most basic spark timing scheme is fixed timing. This means that the
spark timing is always the same. The engine designers choose a worst
case timing situation that won't cause engine damage regardless of how
all the operational variables stack up and they fix the spark timing at
that point. The fixed timing we find with aircraft magnetos is a prime
example of this method. The problem with this scheme is that the spark
timing is never correct for any given condition. You end up sacrificing
horsepower, efficiency, and starting characteristics when you're stuck
with fixed timing.
Before engine computers, cars used a mechanical means of producing a
timing curve. Commonly, this included a vacuum advance mechanism to
adjust the curve for engine load. With today's computer controlled
ignition systems, a complex spark timing curve can be generated by the
engine computer to optimize horsepower and efficiency, as well as
starting behavior. Aftermarket electronic ignitions typically have a
base timing curve that advances with rpm up to some maximum value and
then retards the ignition timing to some degree as engine load
increases.
BACK TO THE IGNITIONS THAT WERE TESTED
Here is a list of the basic traits of each of the ignition systems in
our test:
Lightspeed Plasma II+ - Capacitive discharge, electronically controlled
timing curve with load compensation.
P-Mag 114 - Magneto inductive, electronically controlled timing curve
with load compensation.
Slick Magneto - Magneto inductive, fixed timing, points triggered.
EFII - High energy inductive, electronically controlled timing curve
with load compensation.
We measured the spark energy, spark duration, and system current draw of
each of these systems. Below are graphs of the data that resulted. The
ignitions were run under load. This means that the instrumented spark
plug was mounted in a pressurized chamber of inert gas to simulate the
electrical load that the spark gap sees when it is inside the combustion
chamber of a running engine at high rpm and high horsepower.
System current draw at 2750 rpm: P-Mag =BB none; Slick Mag =BB none;
Plasma II+ =BB 1.5 amps; EFII =BB 1.2 amps
There are some interesting items in the data:
Notice the increasing energy of the magneto as rpm increases. One
drawback of the magneto ignition is very low spark energy at cranking
rpms. Impulse couplings are commonly used on starting mags to
momentarily speed them up in an effort to get a little more energy
during cranking. This helps, but the energy is still very low during
cranking.
You might expect the P-Mag to have increasing energy with rpm also, but
they have chosen to limit the charge time of the coil such that the
energy does not increase as rpm goes up.
Another interesting item is the very short spark duration of the Plasma
II+ ignition. This is a characteristic inherent to CD type ignitions. If
the air fuel ratio is optimal, this may not be much of an issue.
However, if you are looking for maximum power, you will be seeking an
air fuel ratio on the rich side. Short spark duration ignitions will
tend to misfire before long spark duration ignitions as you continue to
add fuel. If you are a lean-of-peak guy, you have a similar situation
where a short spark duration ignition will start to misfire before a
long duration ignition as you take away fuel. These characteristics tend
to favor long spark duration ignitions for best economy as well as for
best power. This is a primary reason cars all have inductive ignitions.
The long spark duration of the EFII ignition means that the spark is lit
for more than 36 degrees of engine rotation at 2750 rpm. This gives lots
of opportunity for a non optimal mixture to light.
If you click on the thumbnail below, you can view a nice photo of Mannan
Thomason's EFIS display in his RV-8. His Dual EFII ignition is helping
to deliver 155 knots true at 6.0 gallons per hour - that's 178mph at
29.7 miles per gallon! - not too shabby!
EFII Flies Above the Rest
Why does our system seem to come out on top? It's not because we're
clever with our data. There are really two reasons. First, we are a
technology driven company. We value function over form. Clever packaging
or glossy marketing are not what we focus on. Our priorities are
performance and reliability. Second, we have been designing and
manufacturing performance ignitions since the 1980s. We went through our
ignition design learning curves a long time ago. This allows us to
design the correct product for a given application without the teething
pains that others seem to go through.
The data in this article highlights only some of the differences between
our system and others. Our Tefzel wire harness, OEM style connectors,
and billet crank trigger are also significant in a thorough comparison.
The end result of our experience and our sound design philosophy is a
product with unmatched quality and performance.
This is why you too, should fly with EFII.
TECHIE STUFF
Just in case you were wondering how to measure spark energy and
duration, here is a little extra info.
It helps to have a nice oscilloscope that can do some fancy math for
you. Otherwise, data can be exported to a computer for computation.
Fortunately, we have an oscilloscope that is up to the task. Below, you
can see a screen shot of the scope with the spark waveforms shown. This
particular measurement was of the spark output of the EFII ignition.
There are four traces displayed on the scope image. Trace #1 is the
spark current. This measurement is made by returning the spark current
through a 100 ohm resistor that serves as a current shunt. The spark
current generates a signal across the shunt resistor that is a few volts
in amplitude and can easily be measured by the scope. Trace #2 is the
spark voltage. The voltage was measured with a 1000:1 probe. The portion
of this signal that lies between the vertical cursor lines in the image
shows when there is a spark present in the gap of the spark plug. Notice
in trace #1, this is also the period where there is current flowing.
Trace#3 is a math channel that is generated by the oscilloscope. This
trace is defined as (trace#1 x trace#2) or spark current times spark
voltage. Current times voltage is power. Trace #3 is a representation of
the instantaneous power (in Watts) of the spark event. Trace #4 is
another math channel. In this case trace #4 has been defined as the
integral over time of trace #3. This can also be explained as the area
under the curve of trace #3. Mathematically, this gives you the spark
power (in Watts) times the spark time (in seconds) which is spark energy
(in Joules). Watts times Seconds equals Joules. In this case, the energy
is much less than one Joule, so we express it in milli Joules, or
thousandths of a Joule. Trace #4 shows the accumulation of energy during
the spark event. Notice that the #4 curve stops rising when the spark
current stops. This is because there is no additional energy being
delivered across the spark gap. Also notice the two small "x" marks on
the #4 curve. This is an amplitude measurement function of the scope.
The amplitude measured in this case is displayed at the top middle of
the screen. You can see it reads 44.4 mU. The scope doesn't know what
units we are acutally measuring, so it has labeled the value generically
as 44.4 milli Units. In this case, milli Units means milli Joules. You
may notice that in this measurement, the EFII ignition is putting out
44.4 milli Joules of energy - which is a lot! The graphs above show that
our ignition puts out 36 milli Joules. Spark energy readings can vary
quite a bit with variations in temperature and humidity. We tried to be
fair to all the systems measured and take data on the same day when we
collected the info for the graphs. The scope screen shot was made on a
different day with high humidity and the reading showed much higher. We
want to emphasize, that we did in fact make every effort possible to
evaluate all the systems fairly and under the same conditions. The data
in the graphs is a result of this.
There is a little more to the energy measurement process, such as having
a pressure vessel for the spark plug to fire into to simulate the load
on the spark gap that is present in a running engine. There is also the
requirement to trigger the different systems properly with crank signals
in the case of the LSE and EFII systems and to properly spin the input
shafts in the case of the P-Mag and Slick Mag. We used an electronically
generated crank trigger signal for the EFII system. To trigger the LSE
system, we mounted a flywheel in our engine lathe and spun it with the
crank trigger assembly mounted on the tool post of the lathe. The P-Mag
and Slick Mag were also spun using the engine lathe. Below you can see
the magneto mount rig on the lathe.
This setup above was used to test the Slick Mag and P-mag. You can see
the spark pressure chamber in the image. There is an old Bendix mag in
the picture. We would have added the data to the graphs from this mag,
but it didn't perform very well at all. This one is overdue for
overhaul. The Slick Mag we tested was brand new as was the P-Mag.
Below, you can see the EFII system under test. In case you are trying to
read the numbers on the power supply next to the scope, it reads 1.1
amps and 13.8 volts. This was at 2500 rpm.
Message 3
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Subject: | Re:_AeroElectric-List:_Two_power_signals,_one_wire? |
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