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# Calculating Injector Pulse Width Example

### Tech Articles

Discussion and questions related to the course EFI Tuning Fundamentals

In the example you show

120/6000 = .02 = 20 milliseconds

20 milliseconds * 0.462 duty cycle = 9.24 milliseconds

Now i know this is for a gasoline engine but wouldn't a diesel be the same?

For example

120/3000 = .04 = 40 milliseconds

40 milliseconds * 0.2156 duty cycle = 8.264 milliseconds

but my tuning software shows the numbers in microseconds with the stock ECM file.

Any help would be greatly appreciated as i am trying to make an excel file to plot all the info into graphical form.

Engine is 408 Cubic Inches

pressure is 14.33

Temp is 70F or 529.67

AFR = 14.6

(408 * (3000/2)) / 1728 = Airflow 354.17

2.7 * (14.33/529.67) = Air Density 0.073

354.17 * 0.073 = Mass Air Flow 25.854 lb/min

25.854 / 6 cylinders = 4.309 lb/min

4.309 / 14.6 = Fuel Mass 0.295 lb/min

0.295 / 2.0 = Duty Cycle .1475 or 14.75%

40ms cycle time * .1475 = 5.9 ms

Does all the math look correct?

Anyone know?

First of all I'll mention that right now our courses are solely focussed on gasoline engine tuning, not diesel. There are very large fundamental differences between the two engine designs so most of our course material is not applicable. That being said in this particular case a diesel 4 stroke engine will always require 720 degrees of rotation for a complete engine cycle so yes, the 120/rpm calculation will still give you the cycle time.

Your calculations all look ok as far as the mass air flow and fuel flow go but remember that this assumes 100% VE which is unlikely. When it comes to injector duty cycle or pulse width though a diesel engine uses direct injection where the available time to inject fuel is much shorter than the full engine cycle time. Generally you'll inject the fuel somewhere close to TDC on the compression stroke and continue some way into the power stroke.

We usually reply within 12hrs (often sooner)

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