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# Ignition Duty % Calculation.

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Hi. Does anyone know the formula to calculate the ignition duty cycle. (On/off time) My dwell is set at 3ms and the dwell angle is 20 degrees. The ECU also has battery voltage multiplier function as well as MAP dwell multiplier. I have added values to the battery voltage multiplier table and when test driving at 4000 rpm to 5000rpm the duty % goes above 40% to 50%. The ECU duty % table starts to flash orange when above 40% and red when close to 50%. Clearly the dwell time is excessive and I’m almost certain that the values In voltage multiplier table are too high. Thus how does the battery voltage multiplier affect the overall dwell time and how is the duty cycle % calculated? Hope you help.

How many coils do you have? How many cylinders are used by each coil? Since the duty cycle will vary with the engine speed, what is the maximum engine speed?

5 cylinders, 5 individual smart coils. Max engine speed will be 8250rpm.

So with individual coils, each one needs to fire only once per cycle (2 cycles per revolution on a 4-stroke), so the time between firings is:

Rev/sec = RPM / 60 = 8250 / 60 = 137.5

Cycles/sec = Rev/sec / 2 = 137.5 / 2 = 68.75

Sec/Cycle = 1 / Cycles/Sec = 1 / 68.5 = 0.0146 or 14.6 milliseconds.

Coil charge time = 3 ms (correct this as necessary)

Duty Cycle @ 8250 RPM = On Time / Total Time = 3 / 14.6 = 20.5%

I would say the ECU value of duty cycle is incorrect, unless it thinks the coils are being fired as wasted spark (ie, once per rev, not once per cycle). I didn't ask -- is the ECU setup for sequential spark, or could it be mis-configured for wasted spark?

Thanks for the calculations. I will have to check the phase table and let you know if it’s configured for waste spark setup. If it is setup for waste spark how will that effect the duty cycle. Should I disable the voltage battery dwell multiplier function.

Wasted spark will double the duty cycle, since the "total time" will be divided by 2.

Thanks Dave. Your explanation has made it much more understandable. I will double check the spark phase table when i get home tonight.