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Terry,
If there is current run through the coils continuously and interrupted to make spark, then the DC math would be true for most of the running. For the transients you are right and the inductance would have to be figured in.

marsolgp wrote:As a side note, I have gotten into the habit of switching OFF the secondary ignition once I'm airborne. It makes a huge difference (for me) with voltage I see on my meter and in keeping the battery charged up. I switch it back ON once I'm back in the pattern.

I learned to operate my Aerovee the same way. In addition to charging better, the engine also ran cooler on just the mags. I would typically see a 10 degree drop in CHTs...no change in the way the engine ran or fuel burn rate.

Bryan Cotton wrote:Terry,
If there is current run through the coils continuously and interrupted to make spark, then the DC math would be true for most of the running. For the transients you are right and the inductance would have to be figured in.

Bryan,

The current is not constant because it is being interrupted to make spark, so therefore the inductance does in fact come into play in the calculation.

Terry

SNX1508 wrote:

Bryan Cotton wrote:Terry,
If there is current run through the coils continuously and interrupted to make spark, then the DC math would be true for most of the running. For the transients you are right and the inductance would have to be figured in.

Bryan,

The current is not constant because it is being interrupted to make spark, so therefore the inductance does in fact come into play in the calculation.

Terry

So for the nonmathematicians does that mean when you measure that inductance is it going to mean less than 7 A or more than 7 A and by approximately how much would you expect? Personally if it's less than half an amp that's close enough for our calculations.

The average current will be less on a running engine than when the coils are directly measured.

I haven't directly measured our coils, but similar coils have an inductance of about 14mH and charge to 95% current in about 10 milliseconds.

At 1000 rpm, the coil fires every 30 milliseconds (one of these firings is the wasted spark). The average current through the coil will be about 88% of the steady state DC value.

At 3000 rpm, the average current will be about 67%.

It would be possible to build a system with a single coil and no wasted spark with about half the current draw, but the mechanical complexity would increase.

jerryhain wrote:

SNX1508 wrote:

Bryan Cotton wrote:Terry,
If there is current run through the coils continuously and interrupted to make spark, then the DC math would be true for most of the running. For the transients you are right and the inductance would have to be figured in.

Bryan,

The current is not constant because it is being interrupted to make spark, so therefore the inductance does in fact come into play in the calculation.

Terry

So for the nonmathematicians does that mean when you measure that inductance is it going to mean less than 7 A or more than 7 A and by approximately how much would you expect? Personally if it's less than half an amp that's close enough for our calculations.

Jerry,

I am not intending to do the calculations, I was only pointing out that it was not correct to calculate the current as I=E/R (or by using inductance values from "similar coils").

Terry

I measured the current draw on the electronic ignition on a running Aerovee today, it drew just under 5 amps. It did not seem to vary much with RPM. It was measured using an old Simpson 260 VOM in seried with the ignition switch.

Thanks for sharing an actual measurement while the engine is running with us :-) Out of curiosity, do you remember what the value was without the engine running? I'd expect it to be an amp or so higher.

It's interesting to me that the current didn't change much with RPM. My earlier prediction was for the current at full throttle to be about 75% that of idle. I wonder if the alternator is raising the voltage across the coils enough to approximately compensate at higher rpms.

Just started flying my Xenos. I see a 10 amp difference in charge rate with secondary ignition on. In other words, with primary only I see a 8 amp charge rate, with secondary on,I see -2 amps.... kind of troubling.