Scott,

I have a problem with your problem with Doug's argument :-).  Well, not much of a problem, but it does make for an interesting discussion.  True, for any condition there is a range of timing that produces the most effective pressure profile - I wouldn't call it "efficient cylinder pressures".  That profile has been stated by the GAMI guys as the timing that produces a peak cylinder pressure at 16 ATC.  But it's not like you're falling off a cliff on either side of this timing.  And this peak pressure point does move with mixture changes - it's unavoidable.  So one of the things you do with mixture is to try to put the peak cylinder pressure at the right angle.  Indirectly, because there is no instrumentation to go by.

Yes, there are other characteristics of "electronic" ignition that have an effect.  Usually, a battery-power system that is electronically switched (Is that "electronic"?  I suppose) provides more spark energy than with a magneto.  And all magnetos (and some electronic systems) use shield spark plug wires and they "suck" some of the energy out of the spark.  What's the energy good for?  To get more a combination of time and current in the spark that exceeds what would otherwise be available.  There the approaches of different suppliers diverge.  The Lightspeed system uses a capacitor-discharge (CD) method that charges a capacitor to something over 100 volts and then discharges this into a coil.  This produces a very high current, low duration spark.  The duration is perhaps 1 or 2 microseconds(very, very short).  The other suppliers typically use an inductive system just like in a magneto.  This produces a much lower current spark that lasts a "long" time maybe 2 milliseconds.  Each method works, but they have different limitations.  A short duration spark has one chance to light the fire and if it misses that, there is no second chance.  A long duration spark might have less of a chance to light the fire in the first microsecond (less current means less "intensity"), but it keeps trying.  So Klaus built the "III" that double-fires the spark.  Problem is that the second spark can't come immediately after the first one.  I don't know the delay, but is suspect it might be 1 millisecond, maybe half that.  At 2500 rpm the the crank turns 15 degrees per millisecond, so the second spark would be 15 degrees "late".  Better late than never, I suppose.  The biggest advantage of the double strike comes in starting and idling, when finding a combustible mixture near the plug at any given instant is not likely.  When attempting to run lean mixtures, automotive OEM's tried everything and finally gave up on CD technology.  The inductive system lights a lean mixture better.  And either technology can fire a larger plug gap, but the CD system can take advantage of it better.

Another advantage of higher-energy sparks is that they reduce the "initial combustion time".  They can light more fuel molecules and that speeds up the initial portion of the fire.  Imagine a bonfire where you light more kindling.  The fire reaches its peak faster, but once the end result is the same.  Now imagine the pressure in the cylinder immediately after ignition - there is almost no change initially, but once a substantial portion is burning the pressure goes up rapidly and then there is no difference.  So, what I'm getting at is that a higher powered spark results in combustion that behaves exactly like that lit with a low-power spark, but with a little more advance (maybe 2 degrees or so).  That change shouldn't be construed as an advantage - you can get the same result by advancing the magneto timing.

Oh, oh, we might have drifted into the "theological" discussion Doug was trying to avoid...:-)

Gary

From Scott:

Doug, 

I have a problem with your argument.  It seems to be upside down.  In reference I refer you to my 11/2/2010 email entitled "NA Injected Engine Performance LOP".  In any event, for any given proper fuel/air ratio (mixture), there is a narrow timing range that produces efficient cylinder pressures at the correct crank angle range.  The Lycoming engine manual (for my 320) displays the acceptable range for the EGT for best power as 100F to 180F ROP, and best economy as 0F to 80F LOP.  Running 50F ROP is very bad for cylinder pressures as stated by GAMI - thus your argument of moving timing by messing with the mixture is, uh, not good operational practice.

 

Secondly, you imply that the only function of an electronic ignition is to advance the timing.  Au contraire mon ami!  An electronic ignition provides for a hotter, longer, more consistent spark over a greater plug gap that better insures the reliable and timely ignition of the combustible mixture even though the F/A may be at some extreme.  This results in greater efficiency/HP depending on the way one looks at it.  See the pamphlet on the benefits of the LASAR electronic ignition that shows speed increases for the same fuel burn or reduced fuel burn at the same speed of a magneto driven engine.

 

The fixed timing of a magneto (i.e. 25 DBTDC) is a compromise.  Generally speaking, EIs advance the base spark timing minimally until the power, as measured by RPM and MAP, is reduced below 75%.  Bad events, such as pre-ignition, detonation and incorrectly timed cyl pressure, are less likely in this regime (that is why the GAMI lean test is done at less than 75% power).  Furthermore, EGTs (representing F/A ratios) are still maintained in the engine manufacturer's recommended ranges and the timing is a better match for the combustion event pressures.

 

Note that during my best power cruises at middle altitudes (2500 RPM, MAP > 22", EGT > 120F ROP), the EI timing is about 25 DBTDC plus or minus a degree. 

 

Yes, more HP.  Better speed for the same fuel burn holding other conditions equal.

Yes, in all the modes you mentioned.

 

I haven't reached TBO yet, I go too fast to accumulate that much time.

 

Scott Krueger

IO 320 Dual Lightspeed Plasma III EI

 

In a message dated 11/15/2010 8:21:27 P.M. Central Standard Time, douglasbrunner@earthlink.net writes:

At the risk of starting a “theological” controversy, I would like to raise some reservations about electronic ignition systems (other than reliability and loss of electrical power)

 

Peak intracylinder pressure (the point in the combustion cycle at which pressure in the cylinder is highest = effective timing) is related to 3 things; spark timing, rpm and MIXTURE.