Ignition timing

[noelf]

Ignition timing number(s) are determined by a number of factors that influence and control the "burn" rate of our hydro-carbon fuel. In the specific case of a/c magneto systems, it is desirable to have as foolproof, reliable, and simplistic system as can be achieved. (and this is not a discussion on any mechanical /vacuum /electronic system that post-dates the 1950's.) And on top of that, we make it redundant with dual magnetos!!!

It takes a specific, finite amount of time from the point at which a spark plug fires and starts the fuel burn process. As the fuel is burned, and the flame front progresses through the air-fuel mixture, pressure builds within the combustion chamber. It would be ideal to have the peak pressure build-up occur just after the piston PASSES top-dead-center. This would then transfer the greatest force to the pistons, rods, crank, and prop. But it does take time for all of the ignition process to take place. At low RPM, there is "more" time for this to happen, at higher RPM, less time. The trick for the engineers is to come up with a single timing number (magneto) that works at all engine speeds, under all conditions, without causing any detrimental effects to engine health.

This number is not an optimal number for all conditions or situations. Air density, turbocharging, supercharging, water injection, air temperature, octane rating, ...they all contribute to the variables of optimum engine timing.

Oh, and the two spark plugs per cylinder?? Yes, they do provide a measure of redundancy, but they also help in establishing a uniform combustion flame front within the relatively large diameter combustion chamber on our 4 banger a/c engines.

Hope this helps...

[AlexPeterson]

Originally Posted by Geico266

I agree in theory, but in practice when I run LOP I burn 10.2 GPH @ 170MPH with dual Slicks. Those were the same numbers I had with the EI. Certainly, nothing like what is claimed by the EI manufacturers. Granted conditions change, temps, baro pressure, all that. Nothing I could not live without, or (for what ever reason) take the chance on another inflight ignition failure.

I had two in-flight failures of Slick magnetos in 700 hours - the shaft inside busted in both cases. The two mags that failed were manufactured several years apart. I did hear that Unison was redesigning that shaft, don't know the status. Granted, these were Lasar mags, but the part that broke was mechanical and is part of the magneto portion. Put Lightspeed EI on after that. Just another viewpoint, I like no moving parts.

[Rivethead]

Originally Posted by noelf

Oh, and the two spark plugs per cylinder?? Yes, they do provide a measure of redundancy, but they also help in establishing a uniform combustion flame front within the relatively large diameter combustion chamber on our 4 banger a/c engines.

Hope this helps...

I couldn't pass this up. My point being that, it's for the reason stated above that I wouldn't mix mags and lightspeed or for that matter any other types of ignition systems on the same engine. Non uniform flame front IMHO is worse for an engine than poor timing.

[Ted Johns]

Originally Posted by Rivethead

I couldn't pass this up. My point being that, it's for the reason stated above that I wouldn't mix mags and lightspeed or for that matter any other types of ignition systems on the same engine. Non uniform flame front IMHO is worse for an engine than poor timing.

I'm curious how you came to this conclusion. Flame front is fractal in nature, no two are alike. Different combustion chambers will have completely unique flame propagation patterns due to shape, amount of squish, and intake port shape, to name a few variables. Additionally, the typical EI user reports better power, better mpg, and improved smoothness of operation. Given this I don't see a reason to be concerned about the uniformity of the combustion flame front.

[robpar]

Here at the University of British Columbia we do engine testing. We set the timing to best torque. This is likely what Lycoming did when developing their ignition timing specs.

Best torque generally occurs when 50% of the heat release occurs at 10 degrees after TDC. This generally is the point of maximum cylinder pressure.
When we change the timing from this point we can keep the power the same by changing fuel flow. The one thing that varies a great deal with timing is emissions. This is something the is coming to the GA community soon. Aircraft piston engines are the only remaining internal combustion engines that have NO emission controls at all. This will change soon likely at the same time that leaded fuel goes away.

The only way to meet the coming emission standards is to adopt the very dependable control systems used in the automotive industry. Fixed timing, leaded fuel and no emission controls are things of the past. I was a mechanic when the emission controls started to appear in the 70's and some of them were very hard on the engine. In the 21st Century we have engines that run perfectly, pollute as little as possible and have a dependability that is truly amazing. The aircraft industry does not have to go through the teething problems that occurred in the auto industry, just adapt the technology to their purpose.

This seems to be what Lycoming is doing with their new engine announcements, they see the writing on the wall.

Bob Parry

[hgerhardt]

Originally Posted by robpar

Here at the University of British Columbia we do engine testing. We set the timing to best torque. This is likely what Lycoming did when developing their ignition timing specs.

Bob Parry

That still doesn't explain why all IO-360-A-series engines EXCEPT -A1B6D and -A3B6D are spec'd at 20 degrees and those two are spec'd at 25 degrees (which have identical cylinders, pistons and compression ratios to the rest of the IO-A models). Those two oddballs were installed in Mooney 201's.

Heinrich Gerhardt
RV-6, flying

[glenn654]

The CAFE Foundation has produced a very good three part research paper comparing EI (various makers) and Mags. Lots of good infomation. What I got from it was what Rooster said, at ~8000 ft and 75% power the engine produces its best power w/ 25 degrees BTDC. EI really shines above 10K ft. where it is more efficient than mags. So if you fly high and fast, EI will work very well for this but if like me, lower and slower then mags will perform well.

Glenn Wilkinson

[elippse]

[quote=hecilopter;247152]Just a guess, but I would expect that 25 BTDC is the optimum setting for max power for a Lyc when at 75% of rated HP at full throttle (ie. at ~8000 ft density altitude). Based on that, when running in the same conditions, I would bet electronic ignition is at about 25 degrees as well.
QUOTE]
25 deg is the setting for sea-level full throttle at rich mixture.The LSE ignition advances timing from the 25 deg static setting based on both MAP and rpm. At 8000' palt you would have well over 5 deg advance just on MAP. Most likely you would have 32 deg to 37 deg advance with a normally-aspirated engine.

[BOBM]

Originally Posted by hgerhardt

That still doesn't explain why all IO-360-A-series engines EXCEPT -A1B6D and -A3B6D are spec'd at 20 degrees and those two are spec'd at 25 degrees (which have identical cylinders, pistons and compression ratios to the rest of the IO-A models).

I think you are mistaken. I believe most, if not all, non-turbo Lycs are 25 btdc.

http://www.pilotfriend.com/aero_engi...%200%20360.htm

[hgerhardt]

Originally Posted by BOBM

I think you are mistaken. I believe most, if not all, non-turbo Lycs are 25 btdc.

Sorry Bob, I'm right. If I could figure out how to post a pdf, I'd post the page out of the Lycoming Operator's Manual here. I emailed it to you at your bobvicky address.

The whole point of my posting this at all is to bring attention to some of the seemingly inane things Lycoming does sometimes. Why would two virtually identical engines have different timing requirements? Why can the same engine tolerate more timing when installed in a different airframe? Airframe cooling differences? Mooney demanded Lycoming to advance the timing so their airplane would go faster? Got a better idea?

Heinrich Gerhardt