Scott:

Good points.

Let me remind you of one aspect of the graphs you referenced from Taylor. Relatively large changes in timing at HIGH POWERS have little effect on power, but can have large effects on ICPs. OTOH, small changes in timing at modest power settings can, in fact, have pretty significant effects on HP. It's important to make that distinction. As you pointed out, it ain't all that simple!!!

Walter


On Dec 23, 2005, at 4:23 PM, Sky2high@aol.com wrote:


In a message dated 12/23/2005 10:24:20 A.M. Central Standard Time, lancair@ustek.com writes:

Not enough systems out there to get any trend analysisRobert,

 
Sure there are:
 
10 of thousands of dual mags.  Properties are well known as are the drawbacks.  Mags were developed for airplanes without electrical systems as were buggy whips for 1 or 2 horsepower land vehicles.  Then, in the 60's could you imagine the FAA approving a vacuum-tube computer to manage your engine?  It is time to move on.
 
Thousands of electronic ignitions (hereafter EI).  This includes the new E-mags and P-mags. They have similar timing characteristics I will describe later.  There is a trend - easier starts, improvements in power and high altitude efficiency (especially for NA engines).
 
100's of LASARs (maybe more?) - A unique system using mags as backup thus allowing the user to switch between mags and its' EI function.  Very illuminating.  The major drawback is plug characteristics and gapping is limited to the puny joules developed by the mags.
 
0 PRISMs - Sounds like a modern way of managing the engine, but much like the vaporware of US developed diesel aircraft engines.  I'm too old to wait.
 
Several EFI, EI closed loop systems.
 
Loads of one mag - one EI systems.  What is better, two flame fronts starting at the same time or at possibly widely different times?  Ever notice how the mag check results in a drop in power (as measured by rpm) when on the mag and almost no drop when switched to just the EI?  It works and gives some peace-of-mind but it is not ideal for an engine with two spark plugs.
 
---
 
Some would say timing is everything and it is.  Timing the result of the combustion event to deliver optimal forces to the driving piston over the most effective part of the power stroke is not simple.  The aim of the PRISM system is to actually measure the cylinder pressure generated with respect to the crank angle and adjust those engine operational parameters that are adjustable to make sure the optimal pressure is best utilized. Bad timing can result in not achieving good power results and also in destroying the engine thru pre-ignition.  
 
Taylor talks about minor variation from the optimal ignition timing doesn't have much affect on the power output.  I cannot find much discussion on the timing that leads to ideal engine conditions considering the things that affect it (optimal timing).  Let's say Lycoming, for injected engines, has provided some limiting conditions given the valve operation profile, octane, absolute manifold pressure range, etc. and determined that for certain engines with certain compression ratios the best timing for the spark is 25 DBTDC.  Lo and behold - over a broad range of MAP (NA) achieved at cruise at altitudes below 10,000 MSL, 25 degrees is not a bad compromise, showing that the application of Taylor's research is meaningful in a practical sense.  This timing is pretty close to what you would see in an EI under similar conditions and equipment.
 
What's the list of conditions that affect combustion event timing (timing to reach the best useful pressure in the cylinder)?  Well, it is a long list:
Octane, compression ratio, air composition, temperature, density, spark initiation, spark length, spark intensity, spark reliability, F/A ratio, F/A mix (vaporization), piston speed, etc. Blah, blah, blah....  Besides an EI improving the length, intensity and reliability of the spark, it can adjust the timing, ergo the timing of the combustion event.  Such an EI can utilize easily measurable parameters, MAP and RPM (assuming a controllable pitch prop), to improve the timing to utilize fuel more efficiently in hi RPM, lo MAP environments - i.e. high altitude cruise.  Turbo charging/supercharging just confounds the altitude at which "hi" is accomplished, among other things.  Also, remember that hi compression isn't so high at high altitudes.
 
Piston engines are being developed (not necessarily for aviation) that use variable control valves (no cam), electronic fuel injection (EFI) using non-vented 12 port injectors going directly into the cylinder, EI, multiple plugs, input air temp, etc. - all in a closed loop (exhaust sniffer) system to optimize power and fuel efficiency.  Note, I am too old to wait for that engine, too.
 
The least we can do is take that tiny step for mankind - a giant step for Pilotkind - and use something better than mags for lighting our fires.
 
Let the EI march continue!
 
Scott Krueger AKA Grayhawk
Lancair N92EX IO320 SB 89/96
Aurora, IL (KARR)

Merry Christmas to all!