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# Injector Timing / Duty Cycle

### Tech Articles

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Hi,

So if the IDC is at 85% then the injector is open for almost the entire 720 deg. cycle of the engine?? (18 min mark of webinar)

Yes that's correct. In that case 85% of 720 degrees or 612 degrees of crank rotation. As you can imagine, this doesn't leave a lot of potential for moving the injection timing dramatically.

Hmmm, I've all ways looked at DC being the % of the pulse width being used to achieve the desired AFR.

So when your at 100% DC the injector is static or open, for the entire pulse which is determined by the frequency.

Thus the setting of Injector end time as your "primary" is critical, as if you haven't delivered the required amount of fuel by then . . Tough

MoTec M1 Help File

Fuel Timing

The injection pulse width will be realised using the specified output settings and timings.

Changes in the Fuel Volume request are immediately applied to the output. Generally, all fuel is delivered with one injection using the specified injection timing. If e.g. due to engine acceleration the Fuel Volume request increases after the first pulse was delivered, a second injection can deliver a Makeup pulse if the timing allows for that. "

If you get to a point where you're operating at 100% IDC then the injector is held wide open for the entire engine cycle.

The cycle time (how long the engine cycle takes) can be calculated by using the formula 120/RPM, so let's say at 6000 rpm the cycle time is 120/6000 = 0.02 seconds or 20 ms. This is the entire window available for injecting fuel. If you're at 85% IDC then the pulse width supplied to the injector is 0.02 x 0.85 = 0.017 or 17 ms. If the IDC is 100% then the pulse width is the same as the cycle time - 0.02 seconds or 20 ms.

So let's say that the EOI timing is set to 0 degrees BTDC (just for a nice simple value). If you're IDC is 85% then you're using 85% of 720 degrees or 720 x 0.85 = 612 degrees. This means the injection event would begin at 612 degrees before TDC on the compression stroke and finish at 0 deg or TDC. If on the other hand you're IDC is 100% then the injection would begin and end at 0 deg TDC - Well actually it would never technically begin or end since the injector is open the entire time.

Is that clear?

It's a different deal if you're talking about a DI engine when the available cycle time is much shorter and dependent to a degree on ignition timing. In a DI engine you only really have the intake stroke plus some portion of the compression stroke available for injection.

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