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Ignition timing
- Thread starter Bones
- Start date
hgerhardt
Well Known Member
I wondered that too, as I have an IO-360-A3B6D (angle-valve 200HP) in my -6 that's spec'd at 25 deg, when most 8.7:1 angle-valves are spec'd at 20 deg. I heard from a friend who allegedly asked a Lycoming engineer why that was, and he said it was so that Mooney (the original application of my engine) could make the performance numbers it promised. Apparently, the engine made just enough more power at 25 deg to make the numbers. That begs the question of if the engine could tolerate more timing, why weren't ALL of the 8.7:1 engines re-rated at the higher advance? As far as I can tell, Mooney 201 J's didn't have any better cooling than the usual spam can of the day and therefore weren't any more or less tolerant of higher advance. Anybody have any more insight?
Heinrich Gerhardt
RV-6, flying
Heinrich Gerhardt
RV-6, flying
sprucemoose
Well Known Member
I assume you mean more than just looking at the number on the data plate?
Magnetos are kind of like fixed pitch props. They are a compromise, designed to run OK over the whole range of RPM/ MP. Electronic ignition, with variable timing, is like adding a C/S prop to your engine.
Don't you need to update your picture now?
Combustion chamber shape has a lot to do with detonation performance. Fixed timing is a compromise of the different temperatures and pressures that can be expected at airports all over the world to give some degree of detonation margin. For instance, if you are taking off from a sea-level airport where the pressure is 31" and the temperature is -30F, your density altitude is going to be about -6700' and you are going to considerably exceed 100% power at rated rpm if you give the plane full throttle.
Geico266
Well Known Member
While I agree EI may be of some limited benifit and aids in starting, it's not like adding a CS prop to an engine, not even close. The HP gains are usually greatly exagerated by the guys making the EI's. I've taken an IO-540 from EI back to Slicks with very little, to NO difference in performance & fuel burn. Certainly, IMHO not worth price or the risk involved. Granted, I'm pretty much turned off to EI's right now due to recent in flight experiences, the materials used in making the EI's, and other problems I have heard about in talking to other pilots. After market EI on Lycomings is just not worth the risk at this point in time. JMHO.
Not trying to start a war, JHMO.
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David-aviator
Well Known Member
The real question was, "how did they come up with number on the placard."
For sure they don't pull it out of their butt, Larry. There has to be lots of engineering considerations concerning burn rate, compression, detonation, and cooling at high power settings.
There may be something magic about 24 degrees, however. When I was checking myself out with Subaru and had the OBDII scanner hooked up, it too ran at about 24 degrees at take off power. Pull the throttle back and timing advanced as far 42 degrees BTDC.
So far, I don't think anyone who really knows about this stuff has checked in here....
AlexPeterson
Well Known Member
The real question was, "how did they come up with number on the placard."
"They" probably all croaked long ago....
hecilopter
Well Known Member
SWAG guess
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. When you pull the power to less than that, the EI will advance the timing and give a more optimal fuel burn. It would be interesting to see the fuel burns/timing advance of the same engine equipped with Mags and then swapped out for EI when run at 75%. It would make sense that the efficiency is about the same at 75% power, then the EI would be a little more efficient when the power was pulled back. I have Slicks and normally cruise with the throttle to the stop to minimize pumping losses and about 2350 rpm, leaned to 1320 EGT. I routinely see about 201 MPH true (174 knots) at that setting and see about an average of 7.5 - 8 GPH at the pump. I don't think EI would help me much there. JMHO.
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. When you pull the power to less than that, the EI will advance the timing and give a more optimal fuel burn. It would be interesting to see the fuel burns/timing advance of the same engine equipped with Mags and then swapped out for EI when run at 75%. It would make sense that the efficiency is about the same at 75% power, then the EI would be a little more efficient when the power was pulled back. I have Slicks and normally cruise with the throttle to the stop to minimize pumping losses and about 2350 rpm, leaned to 1320 EGT. I routinely see about 201 MPH true (174 knots) at that setting and see about an average of 7.5 - 8 GPH at the pump. I don't think EI would help me much there. JMHO.
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Geico266
Well Known Member
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.
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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...
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
Well Known Member
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.
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.
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
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
Well Known Member
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
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
Glenn Wilkinson
I think you are mistaken. I believe most, if not all, non-turbo Lycs are 25 btdc.
http://www.pilotfriend.com/aero_engines/engine_specs/Lycoming 0 360.htm
hgerhardt
Well Known Member
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
Well, I've got an O-360 in my 172, it's 25 btdc, an O-320 in the RV6, it's 25 btdc, all of the IO-360s in this link are 25 btdc except the turbo motors.
http://www.pilotfriend.com/aero_engines/engine_specs/Lycoming 0 360.htm
I don't know what you're looking at, but 25 btdc is pretty standard for Lycs.
The standard timing was 25* btdc and the change was to 20*btdc. The SI that deals with the change said it was done primarily to reduce CHT's in cruise and to increase detonation margin during cold weather operation at high power. It's SI 1325A if you are interested.
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
Good Luck,
Mahlon
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
I checked the Lycoming website & no SI 1325A is listed. Can you provide a link?
On our EI systems which are user programmable, we recommend testing for best power or speed at a variety of manifold pressures and rpm. All engines are quite different and all EI manufacturers including those for non-certified engines should be doing a lot of cell/ dyno/ flight testing to determine safe timing curves for their products.
Timing would certainly vary with CR changes even on the same basic engine type so if an EI maker doesn't ask questions like what is your CR, be a little suspicious that timing may not be optimized for your engine.
As Paul stated, chamber design has a lot to do with the required advance to optimize where peak cylinder pressure occurs. For peak power, we see something like a shallow pent roof, 4 valve Toyota 4AG Formula Atlantic engine with 12.8 CR require only about 22 degrees of total timing, a Toyota 2TG with 10 to 1 CR, high domed pistons and Hemi chamber, 40 degrees, a twin plug Porsche 3.2 turbo around 28 degrees and various Continental and Lycoming engines at 26-30 degrees.
This is for max SL power. When you reduce manifold pressures either through throttling or higher altitudes, increased advance may show benefits on certain engines. Only through lots of testing on a specific engine combination can this best timing values over the whole operational range be found.
Turbocharged engines require a lot more work in this regard especially when operated on relatively low octane fuel. We sometimes have to reduce timing around torque peak rpm where the highest cylinder pressure occurs to avoid detonation, then we can re-advance a bit as cylinder pressure falls off due to lower volumetric efficiency.
This is another area where most have blind faith that the supplier did the job right or has at least been very conservative with advance.
Timing would certainly vary with CR changes even on the same basic engine type so if an EI maker doesn't ask questions like what is your CR, be a little suspicious that timing may not be optimized for your engine.
As Paul stated, chamber design has a lot to do with the required advance to optimize where peak cylinder pressure occurs. For peak power, we see something like a shallow pent roof, 4 valve Toyota 4AG Formula Atlantic engine with 12.8 CR require only about 22 degrees of total timing, a Toyota 2TG with 10 to 1 CR, high domed pistons and Hemi chamber, 40 degrees, a twin plug Porsche 3.2 turbo around 28 degrees and various Continental and Lycoming engines at 26-30 degrees.
This is for max SL power. When you reduce manifold pressures either through throttling or higher altitudes, increased advance may show benefits on certain engines. Only through lots of testing on a specific engine combination can this best timing values over the whole operational range be found.
Turbocharged engines require a lot more work in this regard especially when operated on relatively low octane fuel. We sometimes have to reduce timing around torque peak rpm where the highest cylinder pressure occurs to avoid detonation, then we can re-advance a bit as cylinder pressure falls off due to lower volumetric efficiency.
This is another area where most have blind faith that the supplier did the job right or has at least been very conservative with advance.
Ignition Timing
That engineer was pulling an answer out his behind.
There is a Lycoming service instruction circa 1977 that calls for changing the timing, due to problems at extreme cold temperatures, and every angle valve 200 hp engine since has been built by Lycoming at 20 degrees, and now they claim it actually makes equal or better power at that setting.
The 25 degrees was in use when Mooney selected the IO360A1A for the E model in 1963, before the A3B6 or A1B6 were on the drawing boards.
Also, the A1B6(and A1B6D) was first used on the C177RG in 1971, then adopted by Mooney in 1976, then converted to A3B6D by changing prop orientation for slightly smoother operation. In other words, the A3B6D wasn't even the original application for the engine, so the engineer was making up an answer with no basis in fact. If you haven't guessed, I have an A1A engine, which runs fine on 25 degrees, per data plate, with Bendix mags.
Kelly
A&P/IA
That engineer was pulling an answer out his behind.
There is a Lycoming service instruction circa 1977 that calls for changing the timing, due to problems at extreme cold temperatures, and every angle valve 200 hp engine since has been built by Lycoming at 20 degrees, and now they claim it actually makes equal or better power at that setting.
The 25 degrees was in use when Mooney selected the IO360A1A for the E model in 1963, before the A3B6 or A1B6 were on the drawing boards.
Also, the A1B6(and A1B6D) was first used on the C177RG in 1971, then adopted by Mooney in 1976, then converted to A3B6D by changing prop orientation for slightly smoother operation. In other words, the A3B6D wasn't even the original application for the engine, so the engineer was making up an answer with no basis in fact. If you haven't guessed, I have an A1A engine, which runs fine on 25 degrees, per data plate, with Bendix mags.
Kelly
A&P/IA
