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I am trying to calculate the amount of power that a Honda CRF450 dirtbike's ignition system (ignition coil + spark plug) will consume. I'm not very familiar with ignition systems and the currents they involve - what is the easiest way to calculate this?

So far, I've found that the ignition coil's resistance is 0.1 - 0.3 \$\Omega\$, so the power (while charging the coil) is $$ P_{active} = \frac{V^2}{R} = \frac{(12~V)^2}{0.1 \text{ to } 0.3~\Omega} = 480~\text{to}~1440~W $$ Then, if I pick an engine speed of 5000 RPM as an example, the time between sparks is $$ T = \frac{2\text{ revs/spark}}{(5000~\text{RPM}) / (60~\text{s}/\text{min})} = 24~\text{ms} $$ If I estimate the dwell time as 2 ms, then the average power consumption is $$ P_{avg} = \frac{2~\text{ms}}{24~\text{ms}} (480~W \text{ to } 1440~W) = 40~\text{to}~120~W $$ This is a huge range, and the higher end of the range sounds too large. Is there a more precise way to estimate this average power?

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  • \$\begingroup\$ such a range should not pose a problem, consider the energy output of the engine over the same speed range. \$\endgroup\$ – Jasen Nov 8 '15 at 1:03
  • \$\begingroup\$ @Jasen: I've picked one specific engine speed (5000 RPM), where I know my engine's power output. \$\endgroup\$ – Greg d'Eon Nov 8 '15 at 1:13
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Your power estimates are way too high like you say.You should measure or google or get the inductance of the coil .Once you have L then in your formula it goes .Its now an LR circuit where the current builds slowly starting from zero and limiting out given dwell time at your DCR value .With sensible parameters the coil finishing current wont get that high so your average coil current which is your current consumption on inductive ignition system will be much lower than your estimate .Realy your estimate assumes that L is zero or can be neglected which is not true here .

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  • \$\begingroup\$ Hmm. I guess I'm approaching this in the wrong way, then. In my "dwell time" link, is the % charge in the coil basically just the current? \$\endgroup\$ – Greg d'Eon Nov 8 '15 at 1:16
  • \$\begingroup\$ Hmmm Greg .... % charge is a mechanics term which means Spark energy remember that energy in a inductor is one half L times current squared. \$\endgroup\$ – Autistic Nov 8 '15 at 2:15

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