Doug,

 

You said:

===My point was that electronic ignition systems measure only 2 of the 3 factors that influence timing.  Mixture can have a significant effect on when Peak Intracylinder Pressure (= effective timing) occurs.  Hence the effective timing at a best power mixture (50 to 100 deg ROP) would be significantly advanced compared to the effective timing at best economy mixture (approx 50 LOP).  This is not accounted for by electronic ignition systems and could lead to excessive spark advance.

===

 

Remember that magneto systems don't measure any factor yet the engine manufacturer has no problem with LOP or ROP operations with some restrictions (see below).  Indeed, there are no timing adjustments for CR while EIs provide fjor such.  As I have said, EIs don't provide much timing change from the base timing (dependent on CR) for power settings over 75% as inferred by a combination of RPM and MAP - a typical way of expressing power.  At lower power settings there is a benefit of advancing the spark to get more work done by the combustion event occurring in the cylinder.  At low power settings it is hard to get bad events to occur, including damaging peak cylinder pressures.

 

Finally, please stop using ROP settings where the EGT is -50F from peak as that is outside the good operating range of my engine (that's info from both Lyc and GAMI, worst peak pressures) - Of course, if you are running a Continental engine, well, uh, you have to find your own info.

 

Scott Krueger

 

PS If you are talking about supercharged/turbocharged engines always running at high power then the benefits of EIs are simply a better, more reliable spark without worrying about esoteric things such as that the magneto is properly pressurized so no sparking occurs in the mag. 

 

 

Scott,

 

I will concede your point about the hotter, longer and more consistent spark, although in most flight regimes (no extreme of F/A mixture) it probably doesn’t make much difference.

 

With respect to your comment:

 

“your argument of moving timing by messing with the mixture is, uh, not good operational practice”

 

I think you misunderstood my point.  I certainly do not advocate “messing with the mixture” – especially at power levels above 70%. 

 

My point was that electronic ignition systems measure only 2 of the 3 factors that influence timing.  Mixture can have a significant effect on when Peak Intracylinder Pressure (= effective timing) occurs.  Hence the effective timing at a best power mixture (50 to 100 deg ROP) would be significantly advanced compared to the effective timing at best economy mixture (approx 50 LOP).  This is not accounted for by electronic ignition systems and could lead to excessive spark advance.

 

D. Brunner

 

 

From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of Sky2high@aol.com
Sent: Tuesday, November 16, 2010 9:25 AM
To: lml@lancaironline.net
Subject: [LML] Re: Electronic Ignition

 

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. 

 

Why mixture? Because air and fuel mixtures burn at different rates depending on how rich or lean they are.  So at a given RPM, MAP and spark advance you can vary your effective timing by changing the mixture. 

 

·         Want to advance your timing? Change your mixture to a 50 deg ROP mixture – which is the fastest burning.

·         Want to retard your timing?  Make your mixture richer or leaner from 50 deg ROP.

 

And since optimal power is achieved at an effective timing of 16 deg ATDC, advancing the spark timing may or MAY NOT increase horsepower depending on the mixture.  But advancing the timing will likely increase the magnitude of peak intracylinder pressure leading to higher intracylinder pressures, higher cylinder head temperatures and greater stress = shorter enging life.

 

For those of you who are using electronic ignition (I am not):

·         Are you sure you are getting more horsepower?  How do you know?

·         If you are getting more horsepower, are you getting it during all modes of engine operation?  Rich of peak, lean of peak, high MAP, low MAP, etc

·         What is happening to your TBO?

 

D. Brunner

 

The biggest advantage of any of the electronic systems (in my opinon) is that they provide a spark advance that is a function of at least manifold pressure, giving an advantage any time the manifold pressure is much lower than maybe 25 inches.  But there is very little combustion taking place in the exhaust system regardless.  The reason the exhaust temperature rises is that more of the combustion occurs after TDC and that means less of the energy is being converted to work.  The down side of advancing the spark is that since more combustion occurs at the highest cylinder pressure, more heat is transferred to the cylinder head and piston.