Question about injector duty cylce and pulse width

[Jeff S]

Is the formula for calculating duty cycle, if engine RPM and pulsewidth is given, the same on SEFI, bank to bank, and batch fire EFI systems?

The formula I have is:

(1/ (rpm/120)) = 100% duty at a give rpm.
Measured pulse width (in seconds) / 100% duty at a given rpm = duty cycle


THANKS!

 

[JDEstill]

Kinda...

Max time available for the injector to fire in milliseconds = 120,000/rpm

If you have one injector event per ignition event (ie sequential) then the duty cycle = measured pw / max time available

If you two injector events per ignition event (ie batch) then the duty cycle = 2 x measured pw / max time available.

Example:
5500 rpm, max time available for injector to fire = 120,000/5,500 =21.82 ms

Sequential system fires the injectors 18.7 ms. Duty cycle = 18.7/21.82 = 0.857 = 85.7%

Batch system fires the injectors 9.35 ms twice (to get the same amount of fuel as one 18.7 ms squirt with the sequential system), so duty cycle =
2x9.35/21.82 = .857 = 85.7%

John

 

[Jeff S]

JDEstill,

Ok I have DFI Gen 6 "batch fire" and it is showing that at 6000rpm's that the pulse width is 11.2ms. Am I at 56% duty cycle or am I at 112% ???

 

[bruce]

Originally posted by Jeff S
JDEstill,

Ok I have DFI Gen 6 "batch fire" and it is showing that at 6000rpm's that the pulse width is 11.2ms. Am I at 56% duty cycle or am I at 112% ???

There are actually two batch fire strategies.
One (single batch fire) where they fire all the injectors at the same time, and one (double batch fire) where they fire 1/2 of them at a time.

On the single they fore them all on every revolution, and on the double fire only one of the batchs on every revolution.

Now do you have a single or double batch fire, system?.

 

[Craig Smith]

You are at 56%.

Ultimately the answer to the question at hand is that duty cycle is figured as the total time you are injecting divided by the available time to inject. At 6000 RPM you have about 20 mS, so if your total pulsewidth time is 11.2 mS, 11.2/20 = 56%.

Single batch fire strategies fire each injector simultaneously every 360 degrees. Double batch fire (or bank to bank as we call it) fires half the injectors every 180 degrees. The net result in terms of volume of fuel delivered is the same.

 

[bruce]

Originally posted by Craig Smith
Double batch fire (or bank to bank as we call it) fires half the injectors every 180 degrees.

There is a fly in the soup then.
One of the aftermarket ecm's advertised that with their batch fire they were able to use the longer PWs of SEFI. The only way I see that happening is if they fired only 1/2 of them every revolution.

 

[Craig Smith]

Our bank to bank Buick V6 stuff works this way. Half the injectors fire every 360 degrees. This means that even though the injectors are fired in groups, each cylinder still only gets one shot of fuel per combustion cycle.

Because a V6 fires 3 cylinders per rotation and the crank sensor pulses are 120 degrees apart, it isn't possible to fire the injectors in the same fashion that we fire a bank to bank V8.

While our V8 strategy couldn't be applied to a V6, I suppose it is possible for our V6 strategy to be applied to a V8.

Hmm... so many ways to skin a cat.

 

[Jeff S]

Craig Smith,

The car i'm trying to figure out the duty cycle on is a V8 Ford Mustang with a Gen 6 batch fire system on it. You said that the total available time is 20ms, but that is based on the injector firing once per intake valve event(or every 720 crank degrees), correct?

Since I have a batch fire system, which you stated: "Single batch fire strategies fire each injector simultaneously every 360 degrees." (I assume you mean crank degrees), wouldn't the total available time be 10ms? Which would mean i'm at 112% duty cycle.

Thanks for all the replies and please bear with me as i'm trying to understand this better.

 

[Craig Smith]

No matter how many times you fire the injector, there are 20 milliseconds available to inject fuel into a cylinder at 6000 RPM on a V8. This

The Gen 6 fires the injectors twice per combustion cycle. I personally don't know if the pulsewidth you are seeing is the total pulsewidth or if it is the pulsewidth of each of the two shots the injector makes. I would guess it to be the total. Meaning that if you see an 11.2 mS pulsewidth, the injector fires twice for 5.6 milliseconds. This would translate to 56% duty cycle.

 

[540Malibu]

dumb question.....

the lower the duty cycle the better right? at least if your not running injectors that are too big?

 

[redc4]

DFI 6.32 Pulsewidths

Here is the injector pulse width chart used in Accel Gen 6.32 DFI systems. Based on this, you are at 112% duty cycle. Just like Craig said, this is a SDF system which fires the injectors 1 time for every crankshaft revolution, or twice for every combustion cycle. The pulsewidths in the Base Fuel Map screen are for each event, so the total pulsewidth is actually double what is shown.

FWIW, this information was taken from the Accel EMIC manual.

INJECTOR STATIC CHART FOR SDF
RPM PW MATRIX VALUE
3500 17.14 273
3750 16.00 255
4000 15.00 239
4250 14.12 225
4500 13.33 212
4750 12.63 217
5000 12.00 191
5250 11.42 182
5500 10.90 173
5750 10.43 166
6000 10.00 159
6250 9.60 153
6500 9.23 147
6750 8.88 141
7000 8.57 136
7250 8.28 132
7500 8.00 127
7750 7.74 123
8000 7.50 119
NOTE: For SEFI ECU's use same matrix values
to identify static opening but due to the frequency
difference in the firings of the injectors in SDF vs.
SEFI the indicated gross pulse width in the VIEW
screen will be double the matrix value.

 

[Two Lane]

Re: DFI 6.32 Pulsewidths

Originally posted by redc4
Here is the injector pulse width chart used in Accel Gen 6.32 DFI systems. Based on this, you are at 112% duty cycle.

Ouch!

 

[bruce]

Re: dumb question.....

Originally posted by 540Malibu
the lower the duty cycle the better right? at least if your not running injectors that are too big?

The limit is at idle.
At too short of pulse widths, the injectors don't get to fully cycle on and off. Same at the other end at WOT as you get to a 95% Duty cycle injectors really get erratic.

An injector has a min on and min off time.

And just to clarify the 95% that's on a 6 cyl, near 3K rpm.
Depending on what your doing, 85% is alot more reasonable of figure. But again, talking about a 6K engine.

 

[Craig Smith]

Jeff S,

How big are the injectors and how fast is the car? It'll be pretty easy to figure if we are at 56% or at static from this info.

 

[Jeff S]

The car had 30# injectors and made 371rwhp with a stock EEC. I've come to learn that the duty cycle was indeed 112% with the 30's and DFI. The car now has 42's. I'm not sure why the duty cycle was so high originally with the 30's and DFI since when the car had the stock EEC the duty cyle was only ~85% and the A/F as tested on a dyno with wideband was right at 13:1 across the board. I guess this is what happens when people try to tune cars with crappy 3wire 02 sensors!

 

[bruce]

Originally posted by Jeff S
I guess this is what happens when people try to tune cars with crappy 3wire 02 sensors!

Feel free to spread the word!.
I've been trying to tell folks for years about the inaccuracy of using them as tuning aids.

 

[meach13]

our turbo 5.0 made 626hp/636 tq on 42lb injectors with a stock computer