Reasons for dual mags:
- Safety Reasons - continued spark even with electrical failure
That is certainly the main reason for using dual (or even one) mag.
- Maintenance Reasons - better parts availability, mechanics have greater familiarity with magnetos
Very true. Get stuck away from home and a mag repair is still a sure thing.
- No good evidence of improved horsepower with electronic igntion
True again. Klaus, I believe, claims "up to 20% better" something, but under what conditions? Getting real dyno data for either the one mag or no mag setup would not be difficult - it would just cost money. I was tempted, but finally unwilling to pay for the dyno time. Probably should have.
- Possible decrease in TBO with electronic ignition due to unproven spark advance curve
That is a concern, and one can only have faith that the manufacturers were conservative in their advance protocols. As someone else posted, Klaus seems to be unwilling to share this information, but he sells a monitor that reads out advance. Is there anyone out there that can post the data? It would not be difficult to get - just run at different rpms and different MAPs at each rpm - I can sort out the curve from the data. I'm sure the curve is not very complex as I believe the Lightspeed is an analog circuit (not microprocessor-based).
Recently, I have been thinking about using the new E-Mag to replace one or both of my magnetos. E-Mag expects to have their new 6 cylinder model out shortly (I know - don't hold my breath). It has several features that appeal to me:
- An internal generator that will continue to spark even with a complete electrical failure
Certainly a good thing.
- The ability to be set to a fixed advanced curve like a Magneto
You might be able to do that with the Lightspeed, but I'm not sure. But why would you set an EI system to a fixed timing? Might as well run a mag in that case.
- Immunity to high altitude misfire (like a pressurized magneto)
I think this is of no concern with a naturally-aspirated (NA) engine, but is useful in the case of a boosted engine.
- Even if horsepower is increased, does it come at the expense of a decreased TBO? (Advanced timing can increase intra-cylinder pressures)
It most certainly will increase peak cylinder pressure - the two are directly related.
Are you suggesting that the high cylinder pressure due to combustion might:
A) Cause the spark not to form, since the air won't break down and ionize?
This has been discussed before - I believe the mag will have no problem firing into the slightly higher cylinder pressure caused by the other plug firing early. I'd like to know that for sure, though.
B) Or instead are you suggesting that the magneto will not try to make a spark?
I'm no expert, but I think (A) would be unlikely, and (B) doesn't seem feasible since the magneto has no way of knowing what the cylinder pressure is.
Exactly - the mag will always try to make a spark. If the pressure is too high for the mag to spark the coil and wiring will "swallow" the energy created - it won't arc internally unless there is some defect in the system. Where does the energy go? Capacity coupling within the coil causes the voltage to oscillate until the energy is dissipated in the coil resistance. It could, however, arc to an adjacent post in the distributor. The coil will always have an internal defect in the form of a void in the insulation and what happens is that there is a corona discharge (not a full arc, but ionization of the gas in the void) that occurs, the intensity of which is proportional to the voltage. The corona discharge will eventually oxidize or destroy the insulation surrounding the void, increasing its size until eventually a real arc forms and the coil fails. Increasing the average peak voltage it sees will reduce the life of the coil. How much? I don't think there is any hard data.
- more power: in theory you can set up a mag engine to get the same power in one config. In other words, if the main thing you care about is cruise, you could set your spark timing to be just right for your favorite mixture and your favorite rpm.
Standard mag timings do that reasonably well at low altitude. At sea level full throttle the mixture should be rich, richer than that required for best power. This reduces the flame speed and essentially "retards" the spark - the peak cylinder pressure occurs later. At higher altitudes (lower MAP) the mixture can be leaned and at something like the best power mixture or maybe a little leaner (this will be right in the "coffin corner" that George talks about) the flame speed will peak, effectively increasing "advance" and bringing the peak cylinder pressure to an earlier point in the cycle. This compensates for the slower flame speed caused by the lower cylinder pressures at altitude. Up to 7,000 feet or so this works pretty well, so if your flying is mostly from sea level to 10,000 there will probably be no real advantage to be obtained by any of the EI systems. This is also true of a turbocharged engine, which is almost always operating at the same low "altitude."
One slight "difficulty" with EI systems, especially those using the capacitor discharge principle (Lightspeed): Their spark is of a very high intensity, but a very short duration. This high intensity spark can speed up the very initial part of combustion by igniting a larger "kernel" than with low-current inductive (mags and Emag) systems. The engine really doesn't care, except that this means the effective spark timing is slightly advanced. The pressure curve will look like it has a few degrees more advance than it really has. In other words, a Lightspeed running at 20 degrees is not the same as a mag running at 20 degrees. Why do we care? The EI sellers might brag about "more power", it will be at the expense of significantly higher peak cylinder pressures. The mag running a couple of degrees more advance would be completely equivalent.
- engine damage. lacking variable timing, all you have is mixture to protect your engine from harm. With fixed timing it's clear what to do (e.g. if you take APS). With some unknown timing curve in an electronic ignition box, I don't know what to do. I asked Klaus what his timing curve was really and he just blew me off. Lacking that information, I wasn't willing to have his box in my airplane.
That would be a reasonable reaction to the lack of information. The advance curve should not be secret and I was reluctant to install a system with an unknown timing. But in the end I did it anyway.
- open loop control systems. As far as I know, all electronic ignition systems that are for sale are open loop. ....meaning they determine timing based on control inputs but don't actually have any sensors in the cylinder to determine what is happening. PRISM's approach is radically different in that it measures the angle of peak pressure and tries to keep it in a very narrow range no matter what you do to the engine controls. As a result, you can run right at peak, ROP, LOP, whatever you want. ...and this has obvious advantages both in terms of power and engine longevity. Unfortunately PRISM isn't for sale at this point.
Exactly. Even with advance that is a function of MAP, it still can't compensate for the different flame speeds caused by mixture. LOP operation causes reduced flame speed, especially at the "lean limit". Being able to advance the spark to compensate would be a big advantage for very lean operation. Of course, very lean operation is very low power operation and unless the engine is boosted, we're probably not going to use the "very low power" regime very much.
- one mag and one electronic. Seems plausible but you lose timing control if you leave the mag on. If I were going to do this I would want to run most of the time with the mag off I think.....and that would mean different timing curves. I don't know if any of the electronic setups deal with that properly or not.
I'm guessing (without knowing the facts) that the mag will spark before the flame gets to the second plug. Therefore, it will be doing some good in terms of increasing the burn rate. You can tell if that is the case by turning the mag off - if there is a power loss the mag was doing something. In either case there is nothing to be gained by turning off the mag. Some years ago investigated splitting the timing curves in dual-plug automotive engines. Compared to firing both at the "optimum" time retarding one reduced the combustion noise and power while increasing fuel consumption. We could come up with no reason to split the timing except to reduce the combustion noise at full power.
conclusion: I decided to stick with 2 mags until I see a) a seriously bullet proof backup power solution. b) a closed loop ignition system or an open loop system that is clearly documented and backed up by test stand analysis. For what I care about I can get most of what I want by cruising LOP and just using a lot of gas in max power configurations. A good electronic system would allow cruising AT PEAK without damaging the engine and that would mean some incremental power.
Even with optimum timing operating at peak EGT will increase the CHTs compared to either richer or leaner (okay, the peaks aren't in exactly the same place, but you get the idea).
I have been thinking a little bit about this issue. I would like to add my 2 cents
and see what others (particularly Adkinson and Braly) have to say about this issue.
Me too.
20 or 25 Degrees BTDC (depending on compression ratio) provides a reasonable compromise for standard flying at 75% (plus or minus a bit). The timing makes a bigger difference at higher altitudes where NA engines are operating at reduced power enforced by lack of MAP. The EI also makes a big difference in starting and high power operations - like take off.
Very true in the case of high altitude (above 10,000 feet) operation. Fixed timing works very well at modest altitudes (75% to 100% power). I disagree about the takeoff power - the same effective timing will produce the same power. A "hotter" spark won't increase power - only timing effects that. Better starting? Probably, as the mixture distribution in the cylinder can be so poor that a hotter spark is more likely to reach a combustible mixture. The same is true in the case of high oil consumption that can foul plugs - when my engine was new and I was doing more ground operations the plugs would foul so much that the mag would almost not be able to run the engine at all, while the Lightspeed system never missed a beat. After the engine quit burning oil there was no difference.
Sorry about the long post. Hopefully, there was some useful information in it.