Fueling & Injection - Is injector duty cycle dependant on camshaft profile?

The Alchemist
02-27-2012, 11:33 AM
I don't have access to HPtuners right now, but isn't injector duty cycle calculated based on how long the injector is open against a certain value?

Isn't the critical value how long the intake valve is open? So wouldn't running a biger camshaft with more duration impact the injector duty cycle?

Ollie8974
02-28-2012, 07:51 AM
Duty cycle is dependent on HP and injector flow rate.
injector flow rate should be calculated at 80% duty cycle

The Alchemist
02-28-2012, 11:07 AM
Yes, I undestand that duty cycle is dependant on flow rate and HP level, but step back for a second and look at the actually equation to calculate injector duty cycle. I don't have it in front of me, but it's dependant on rpm and actual injector pulse width in terms of miliseconds of firing time.

So at a certain rpm, if the injector is firing for 4ms, you would have a different duty cycle than the same 4ms at a different rpm.

My question was doesn't cam profile need to be taken into account for that since it is the duration of the intake valve being open that determines actual duty cycle?

atomic 6
02-28-2012, 05:02 PM
Injector dc is not dependant on camshaft(intake open time)

You could think of it in relation to the time from opening to opening again(no referance to closing), or just RPM.

At high dc the injectors are going to be open when the valve isn't.

The % dc is going to be a percentage of that window, it represents the use of the injector between the point of the valve opening(or any other arbitrary point) and the next instance of that point. Or just the period of the RPM.

This is where X-Tau modeling comes in handy, with it's intake Wall wet ,and intake Wall suck modeling, For a more acurate fuel calculation based on the actual suspended fuel drawn in from the previous cycle of injection into the manifold.

:corn:

Lonnies Performance
02-28-2012, 07:40 PM
Injection is based on desired air/fuel ratio & the calculated airflow.

Cam profile really does not enter the equation, but it would be preferrable to inject fuel only when the intake valve is open for best atomization... this is more injector timing than duty cycle/pulsewidth.

Converesly, duty cycle is dependent on rpm... as RPM increases there are more combustion events & therefore less time to inject fuel for each cycle. you can calculate the combustion cycle time as half the revolutions/second.

6000rpm/60 = 100revs/second=10mlliseconds/revolution
2 rotations *10mS = 20milliseconds per combustion event
At 20mS you are at 100% duty cycle.

At 7000rpm you have only 17mS to feed the cylinder.

bayer-z28
02-28-2012, 09:28 PM
^ Come to think of it, what was the number that is associated with injector timing? Isn't it around 5* or something before the valve opens? Prevents puddling and helps create a bit of a fog in the port for the cylinder to "ingest."... ?? Something like that.. I can't remember it right now. I'd have to look it up.

Valve events DO play a part in injection timing (duh) but the DC has very little to do with the cam profile. IVO events would be the * that you need to pay attention to, IMO.

The Alchemist
02-29-2012, 08:06 AM
So the fact that it's better to have the injector done firing prior to the intake valve closing makes it even more critical to keep injector duty cycles lower.

Sorry for the confusion in the thread. It was one of those things you think about at 2am when you need to go to the bathroom and grab something to drink.

bayer-z28
02-29-2012, 07:33 PM
^ Or a shower thought.. :lol: I think a lot in the shower.

ZL1Killa
02-29-2012, 08:33 PM
At 7000rpm you have only 17mS to feed the cylinder.

damn. thats absolutely crazy

Lonnies Performance
02-29-2012, 10:59 PM
So the fact that it's better to have the injector done firing prior to the intake valve closing makes it even more critical to keep injector duty cycles lower.

Yes & no...

This theory is best for mileage & efficiency, but on a high HP application you will have a hard time getting an injector that can inject enough fuel while the valve is open, that will also be small enough to allow part throttle drivability.

The above 20mS is for 720deg of engine rotation (2 revolutions), even a very healthy cam with 300 deg seat duration, would relegate the injector duty cycle to be under 40%. You also would not want to inject fuel during overlap as this fuel is potentially going right out the exhaust, reducing his value even further. This scenario would make injectors roughly twice the recommended size, having an impact on idle quality, drivability etc.

Keep in mind that at full throttle, there is very little power benefit to sequential (timed to the actual intake) fuel injection, but it will help emissions...

Good theory, but not worth worrying about.

Big Al
03-01-2012, 11:37 AM
Lots of engines fire the injectors every time around with no ill effects. Look at the late 80's Ford's they fire two injectors at the same time every time around.

Or look at the old mechanical FI Corvette, injectors squirt all the time just like most mechanical drag race cars. Hillborn, Enderle or Kinsler all squirt all the time the engine is running.

Al 95 Z28

The Alchemist
03-01-2012, 12:48 PM
Thanks for the reality vs. theory approach Lonnie. Sometimes I get stuck in theory, hence why I ask those in the know what the reality is.