In a message dated 9/30/2010 4:22:51 P.M. Eastern Daylight Time,
SBoese@uwyo.edu writes:
Lynn,
I believe we all appreciate your input and benefit from your
experience and insights. I would like to respond (in
italics) to points you made below to make sure I understand what you
are saying. I for one am looking forward to "more
later".
Steve Boese
Let us review...............
We take an engine from a car that usually operates below 2,200 RPM, and
at partial throttle for 99.5% of its life, and put it in airplane and run
it at 6,000 RPM and wide open throttle for hours on end, and even a cold
street plug is boiling cement and rounding electrodes? Now who could have
guessed that would happen.
Many times the statement
has been made that we are not pushing the rotary engine nearly as hard in
the aircraft application as it is in the racing situation and racing
modifications are generally not necessary for the aircraft. Is the
spark plug an exception to this?
Yes it is. The racing stresses involve bearing wear at say 9,600 RPM and
the rotors weigh about 9.7 pounds.
The rotors have the side shaved down for weight reduction and to allow the
rotors to tip over a bit as the crank shaft bends into an "S" shape at high
revs. Along these lines you oil the crank from both ends to keep the bearing in
the front rotor. The mechanical stuff is obvious and relatively easy to deal
with.
The spark plug has been more difficult. There is no off cycle as in a 4
stroke piston engine. every rotor face gets two spark plugs firing. Two power
strokes per revolution. So the heat load is higher. The long reach of the body
provides the maximum amount of contact with the water jacket. The racing plugs
gifted to Mazda competitors is the NGK R6725 11.5. So it is the Mazda
competition department that thinks this plug is the one to use. In this
numbering system street ported racers us a 10 heat range. Bridge ported and
turbos use the 11.5 heat range. This is a style 6 tip D on the chart. It is ice
cold and has a retracted tip The plug ends down inside the shell.
As for duty cycle, race cars are limited to full throttle in doses of about
40 seconds on longest straights, and that includes zero throttle at each shift.
Now racers have 200 channels of data coming in from the car, and time at each
percentage of full throttle is now known. So let us say that we end up with 60%
of the time above 3/4 throttle. At the top of each of those runs it will be
sitting just above 9,600 RPM. Our races are 45 minutes long.
In the home built leaving Georgia going to Kentucky, at 6,000 RPM and
6,000 feet (about 75% power) and wide open throttle for 3 hours. No throttle
reductions until the field is at hand. So the aircraft duty cycle far exceeds
the racer duty cycle and approaches 100%.
If the spark
plug heat range was inappropriate when new, it is puzzling that the
problem with it takes 30 or more hours to show up when using 100LL fuel and
hundreds of hours for the problem to develop when using auto
fuel.
The spark plug is not the wrong heat range for the car. The tip design is
probably designed to run a bit hotter than is required, and the heat range is
broadened by the shell and grounds shielding the tip.
The overheated plug boils the cement near the tip of
the plug, thermally disconnecting the electrode from the ceramic, and allowing
the electrode temperature to run away. This is typical of an overheated tip
(wrong heat range for the application) You may not get the electrode back with
the plug. Check in the muffler or in a competitors tire.
We do not observe that
type of damage to the spark plugs. The information I have seen indicates
that it is the insulator where it pinches out at the tip that becomes too
hot. The electrode itself is much more closely thermally coupled
to the cooling system.
This could lead to preignition (an
ignition event that occurs before the planned ignition). Unless the mixture is
well rich of ideal or well lean of ideal, the pre-ignition would lead quickly
to detonation followed by the apex seals getting stuck in the
muffler.
Which is precisely why it would
be nice to know what is actually happening. How close to the edge of
disaster are we really operating?
The high performance Mazda street
plugs (turbo plugs) have shielding on the shell ends similar in appearance to
some aircraft plugs. They are not aircraft plugs. They may not survive
hours at full load wide open throttle. The heat range is cooler than most
street plugs it is true, but nothing like a racing plug. If the cement has not
boiled, the plug is not too high a heat range.
Racing Beat recommends the same plug for racing as Mazda Competition. NGK
R6725 11.5
Cement boil is the first indication of too high a
heat range for the application. That is all the warning you may get in a rotary.
In a piston engine you might also see aluminum balls stuck to the
porcelain.
I think the SAG is a result of lead salts fouling
of the porcelain insulator, in conjunction with the use of the Kettering style
ignition systems. With no leaded fuel there seems to be no limit to plug life.
And with 100LL about 30 hours. So it is lead salts related. The mechanism of the
failure is open to conjecture.
I contend that the dismally slow rise time of the
Kettering system lets voltage leak across the lead deposits as the field begins
its collapse. Whatever leaks is subtracted from the peak voltage, and voltage
across the gap begins to decay. Poor ignition slowly develops into
accessional misfires and that SAGGING feeling.
Since the cylinder pressure is a factor in gap
voltage requirements, removing a bit of throttle seems to cure SAG. Less
throttle, poorer cylinder filling, less pressure, the lower voltage now fires
the plug. So all is well until full throttle is tried again.
In racing there are only 2 ignition systems I know
of. The Mallory High fire CD system or the MSD6AL CD system. I gap plugs to
.010". Never an ignition related problem. The racing plug gaps can be adjusted,
unlike the stock Mazda plug. You can revive a SAG engine by gapping the plugs
shorter. Less voltage required to jump the gap, and you are back in
business.
If you want to buy NGK racing plugs, its $28.00
each. Probably the B10EV is cold enough for aircraft use.
Take a wrench and a box of new plugs. Be sure you
can change the plugs with no additional tools. You can use the heat range
equivalent in an Autolight plug from Autozone for about $6.00 for a
box of 4. Same price for Autolight racing plugs, but they have to order for
delivery the next day. an AR2592 works fine, and they have one number colder
than that. I think. I got those free and
used them in the racer a few weekends. They run fine in my Kia as well and free
was the right price. The NGKs were free as well so those went back in the racer.
You get more smiles when you run the plugs they give
you....................
The melting center electrode works just fine for me
as a cause, but I am not feeling a connection to lead salts causing
that.
The Racing Beat manual/catalogue is free to
download from their web site. Worth a read.