REFLECTOR: Re: Electronic Ignition-Reflector Digest, Vol 11, Issue 70

[Chuck Harbert]

Good topic to discuss. From what I've read and from my experience, modern
Electronic Ignitions fire the spark plugs at optimum conditions based upon a
matrix (map) of the important inputs (crank angle, manifold pressure, rpm, 
inlet air temp
and (cold) engine temp (water or oil temp). These type of systems provide a 
more powerful
and smoother running engine, plus better fuel efficiency than a fixed timing 
magneto.

Ideally, the cylinder should be fired at a point where you get the most 
power from the fuel. For
example, as the rpms go up the engine should be advanced because the flame 
front has less time
to burn. The density and temperature of the cylinder charge can make a big 
difference in the timing
which EI's can properly deal with rapidly. The old magneto's are reliable, 
but they are limited to a
less than optimal advance, plus they have a much weaker spark at low rpms.

Another advantage of modern EI's with CD (capacitive discharge) ignitions is 
that they will fire leaner
mixtures because of multiple plug firings per firing cycle. All the 
cylinders should be fired at the same timing
(i.e. 20 BTDC (degrees before top dead center)) because the crank is 
geometrically fixed
(180 degrees -4 cyl).

The bigger problem with aircraft engines is the fuel delivery. Carbs are the 
worst, but the current constant flow servos aren't much better. GAMI 
injectors try to even the fuel flow based upon expected cylinder airflow. 
Modern EFI systems provide well atomized fuel at the precise time (not batch 
or gang fire systems) for optimal performance. I thot about installing EFI's 
(injectors) and using a ECU (engine control unit) that would handle both 
fuel and igntion with a carb/servo and mag as backup, but that will need to 
be later. Race engines are built such that each cylinder makes the same 
power using the same amount of fuel and the same timing. I like the 
reliability and efficiency of a conventional mag on one side and an EI on 
the other.

Chuck H

---------

Scott Derrick wrote:
>> lean cylinder where as with a MAG a good portion of it is burning on
>> the way out the tail pipe.  This premature loss of efficiency in the
>> leanest cylinders when the rich ones are lagging behind is what causes
>> the engine to run rough when you try to lean past peak to 50-75 LOP.
>
> Pretty good.
> The biggest problem with LOP operations, even with an engine in perfect
> tune
> is that the swirling airflow inside the cylinder is chaotic.  A rich
> cylinder burns very predictably, while a lean cylinder does not.
> This phenomenon is measured by CoV:  The coefficient of variability
> (Taylor,
> et. al.).
>
> As the mixture gets leaner, and also as the density increases, it becomes
> more difficult to create a reliable spark, and also becomes more difficult
> to establish a viable flame front.
>
> A perfectly tuned engine running excessively LOP will run rough due to
> these
> limitations.  This is where the stratified charge devices Honda plays with
> come into play, or the Saab (I think) method of using two combustion
> chambers; one rich that squirts a flame into the lean main chamber.  There
> are also other ignition schemes to address this.  One is to detect the
> presence of a flame kernel around the spark plug.  If there is no flame,
> then keep sparking the plug.  The power pulse will be late, but it's
> better
> than no power pulse at all.
>
> Continuing to move to a leaner condition leads to progressively worse fuel
> economy.  This is because a failed combustion event dumps that fuel
> overboard, with no extraction of power.  At this point, you may see higher
> EGT's as the mixture is ignited in the exhaust.
>