How can injector DC exceed 100%?

[Redline Motorsports]

How come the pid for injector duty cycle read more the 100% duty cycle?

Howard

[mr.prick]

Math :hihi:
{GM.IBPW1}*{SAE.RPM}/1200

high IBPW*high/1200 = high duty cycle.

There is no set max limit to be displayed for that PID like some other PIDs.

Example:
Long Term Fuel Trim Bank 2 {SAE.LONGFT2}

Units: %
Range: -100 to 99.22%

[Redline Motorsports]

So this would mean that "if" the PCM commanded a injector duty cycle higher the 20 ms over 6000 rpms it would be a DC of over 100%.

I guess the PCM doesn't really care about the physics but will keep commanding regardless.

HT

[mr.prick]

I don't think injectors are physically limited at 100%.

[Redline Motorsports]

I don't think injectors are physically limited at 100%.

If an injector is open 100% of a 100 precent window of time....I would think we are "static" and is not an ideal situation. The whole point of tuning is to have control of fueling. At 100% DC I think we have lost control. The injector drivers in the PCM also don't like 100% DC either.

[joecar]

When injector DC goes above 100% it is an visual indication that the fuelmass is not able to be met with the current IFR/VE/MAF/CFOL tables.

The IDC is calculated over 2 crank revolutions (4 strokes, 1 complete cycle, intake valve opens/closes once)... the time taken to spin 2 complete revolutions represents 100%.

The PCM does this:
- calculates the airmass,
- looks up the AFR,
- calculates the fuelmass required to meet that AFR,
- looks up the IFR,
- calculates the IBPW required to deliver that fuelmass...

if the IBPW exceeds the time taken to for 2 revolutions, then:
- one or more of those calculations/lookups were wrong,
- motor's fuel requirements exceed the physical abilities of the hardware.

(ok, we already knew the above... but for for the benefit of anyone else)

The scantool just displays the calculated DC, if it shows above 100% then it is a visual indication (set your scantool dash gauge to show red).

[mr.prick]

If an injector is open 100% of a 100 precent window of time....I would think we are "static" and is not an ideal situation. The whole point of tuning is to have control of fueling. At 100% DC I think we have lost control. The injector drivers in the PCM also don't like 100% DC either.

Could this damage the drivers?

[joecar]

The transistors in the driver circuit would get hot which would degrade them (I don't know for certain... but I do know that a continuous current thru a Darlington transistor makes it get very hot)... they probably won't fail instantly, but they would suffer long term damage... $0.02.

The coils in the injectors would get hot which would degrade them also (this is a known)... long term damage.

And the motor will be leaner than commanded, combustion temps will go up, hight temp damage may occur, knock may occur, fueling protection modes will have no effect (they won't be able to add fuel).

[mr.prick]

The transistors in the driver circuit would get hot which would degrade them (I don't know for certain... but I do know that a continuous current thru a Darlinton transistor makes it get very hot)... they probably won't fail instantant, but they would suffer long term damage... $0.02.

The coils in the injectors would get hot which would degrade them also (this is a known)... long term damage.

And the motor will be leaner than commanded, combustion temps will go up, hight temp damage may occur, knock may occur, fueling protection modes will have no effect (they won't be able to add fuel).

Good to know.