injector-duty-cycle discussion simple style

[Seamus]

No disrespect aimed at you at all Bastard, my only problem is this, when I close my eyes, I can picture the pistons pumping up and down, I can see the valves open close op

Lol I thanks for this, I can picture youbreathing from your mouth too.

When you close your eyes hard to think, do you make constipated faces as well?

 

[dsmcurse]

c'mon guy, not LITERALLY

 

[bastarddsm]

I still scratch my head at thought of these chevy v8 guys logging 180%IDC?

Honestly, those guys are idiots.

 

[delta448]

Not that you need my input as everything you needed to know was already correctly posted, but here's a way for you to check it and assure yourselves that steve and bastard are right.

Look at any datalog where you can see raw injector on time in milliseconds, the logger calculated IDC, and RPM.

I'm going to use easy numbers to simplify the math, but you can go check your own datalog and use exact values if you want.

Example: IPW = 5ms
IDC = 5%
RPM = 1200



1200rpm = (divide 1200 by 60 seconds per minute) 20 revs per second
= (divide 1 by 20) 0.05 of a second per revolution
= (multiply by 1000 milliseconds per second) 50 milliseconds per single revolution
= 100 milliseconds per complete cycle (multiply 50 milliseconds by 2 revolutions for whole cycle)

IPW of 5 milliseconds, divide your IPW of 5ms by your 100ms complete cycle and you get IDC of 0.05 or 5%.

 

[Boostdriven]

Whatever dick measuring contest some of you guys want to get in to I'll leave that up to you

First of all I never said that I was right and if you guys read through my posts you will see that several times I use a word "think" as in what I think. If my way of thinking about this whole IDC/IWP thing is wrong then I'm wrong. I gain nothing from being right but if I'm wrong I have an opportunity to learn something.

All I'm asking is for some solid information that shows how the fuel injection system works. Yes I've read through a lot of information and I find contradicting thoughts. A lot of it is just peoples opinion on how the system works and to me that is no different than the opinions expressed on this site, whether its by me or someone else.

Is there a good book a guy could read, maybe a book written by some tech that works at a car manufacture that writes the tune for a car or programs the ECU functions. Maybe some of you have some formulas on how to calculate air density per a given volume and pressure as well as a formula for figuring out how much fuel is needed per a given volume of air to end up with a desired air/fuel ratios. I would like to see that math aspect of things. I never went to school for any of this stuff, never really had the time I guess or major need for that matter I'm not being a smartass, I really would like to understand this better and just saying this is how it is does not explain much, it just states a "theory" no different then my for the next noob that will be reading this.

The thing that I have hard time understanding is how a guy can get more fuel out of the injector when the IDC is already at 100% duty cycle, and the reason why I say that because I've seen it work. I know that IPW is the time frame that the injector stays open and IDC is the calculation of that time frame based on RPM.

This write up shows how to calculate the time frame per each cycle
injector-duty-cycle [ECMTuning - wiki][]=injector&s[]=duty

This is the formula from the site
Revolutions per second = RPM/60
Cycles per second = (RPM/60)/2 or RPM/120
Time available between injector pulses (period) = 120/RPM

So at 7000 rpms you have 17ms to open the injector, deliver the fuel and close the injector.

IDC = PW/Period, or PW/(120/RPM)... which can be simplified to PW*(RPM/120)
Using this formula, if IDC takes 17ms then the duty cycle of that injector would be at 100% This would also mean that its open through out the entire engine cycle and never gets a chance to close, therefore delivering its maximum amount of fuel. What stumps me is the fact that I've seen guys run well over 100% IDC and not run lean and not only the car didn't run lean but that added % over 100 gave the engine just the right amount of fuel to maintain the air/ratios. So where is that over a 100% IDC fuel coming from? Is it just an error in the ECUs calculation or is the ECUs calculation based off of different time frame than the full engine cycle? This is what I don't understand.

Like I said before, I'm not trying to argue or prove my point, I'm just trying to get a better understanding of the process. I speak my thoughts in detail and examples so you guys can get a better understanding of my reasoning. I would like to hear the same from those who have a different understanding.

 

[bastarddsm]

Is there a good book a guy could read, maybe a book written by some tech
that works at a car manufacture that writes the tune for a car or programs the ECU functions.

I'm not aware of any. Most auto tech books are way too dumbed down to get any useful info out of. download a dsmecu dissasembly and start reading it.

 

[Boostdriven]

Not that you need my input as everything you needed to know was already correctly posted, but here's a way for you to check it and assure yourselves that steve and bastard are right.

Look at any datalog where you can see raw injector on time in milliseconds, the logger calculated IDC, and RPM.

I'm going to use easy numbers to simplify the math, but you can go check your own datalog and use exact values if you want.

Example: IPW = 5ms
IDC = 5%
RPM = 1200

1200rpm = (divide 1200 by 60 seconds per minute) 20 revs per second
= (divide 1 by 20) 0.05 of a second per revolution
= (multiply by 1000 milliseconds per second) 50 milliseconds per single revolution
= 100 milliseconds per complete cycle (multiply 50 milliseconds by 2 revolutions for whole cycle)

IPW of 5 milliseconds, divide your IPW of 5ms by your 100ms complete cycle and you get IDC of 0.05 or 5%.

I was a bit distracted when I wrote my last post so I didn't refresh my screen for a while I know this information but that still leaves me with a question I stated above.

 

[delta448]

Sorry if this is short, I don't have much time right now.

Is there a good book a guy could read, maybe a book written by some tech that works at a car manufacture that writes the tune for a car or programs the ECU functions.

http://www.dsmtuners.com/forums/tuning-engine-management/352130-couple-good-tuning-books.html

The thing that I have hard time understanding is how a guy can get more fuel out of the injector when the IDC is already at 100% duty cycle, and the reason why I say that because I've seen it work. I know that IPW is the time frame that the injector stays open and IDC is the calculation of that time frame based on RPM.

The only way to increase the amount of fuel delivered at 100% IDC is to increase the (fuel) pressure differential across the injector. If AFR became richer and fuel pressure was constant, airflow dropped.

What stumps me is the fact that I've seen guys run well over 100% IDC and not run lean and not only the car didn't run lean but that added % over 100 gave the engine just the right amount of fuel to maintain the air/ratios. So where is that over a 100% IDC fuel coming from? Is it just an error in the ECUs calculation or is the ECUs calculation based off of different time frame than the full engine cycle? This is what I don't understand.

IDC is never calculated by the ECU, the ECU does not care about IDC because the stock programming should keep you from ever going over the limit. IDC is an aftermarket value that is important to us as tuners, since increasing max engine rotational speed or increasing airflow dictates what flow rate your injectors will need to be able to flow. The percentage of injection time available that is used up varies by how much fuel you need to introduce to keep things happy. That is why every datalogger worth it's salt has an IDC function pre-programmed to log. If you dig around in the datalogger software, you'll see that the logger only needs RPM and the value of IPW the ECU is trying to command in order to report the IDC. Once the commanded IPW exceeds the cycle time, your logger reports that IDC is now over 100%. The logger does not ever pull an IDC value directly from the ECU data, it pulls RPM and commanded IPW and then it calculates IDC for you based on the already established formula for IDC.

 

[dsmcurse]

Sorry if this is short, I don't have much time right now.


http://www.dsmtuners.com/forums/tuning-engine-management/352130-couple-good-tuning-books.html


The only way to increase the amount of fuel delivered at 100% IDC is to increase the (fuel) pressure differential across the injector. If AFR became richer and fuel pressure was constant, airflow dropped.


IDC is never calculated by the ECU, the ECU does not care about IDC because the stock programming should keep you from ever going over the limit. IDC is an aftermarket value that is important to us as tuners, since increasing max engine rotational speed or increasing airflow dictates what flow rate your injectors will need to be able to flow. The percentage of injection time available that is used up varies by how much fuel you need to introduce to keep things happy. That is why every datalogger worth it's salt has an IDC function pre-programmed to log. If you dig around in the datalogger software, you'll see that the logger only needs RPM and the value of IPW the ECU is trying to command in order to report the IDC. Once the commanded IPW exceeds the cycle time, your logger reports that IDC is now over 100%. The logger does not ever pull an IDC value directly from the ECU data, it pulls RPM and commanded IPW and then it calculates IDC for you based on the already established formula for IDC.

Very good info, well said sir!

 

[94awdcoupe]

The thing that I have hard time understanding is how a guy can get more fuel out of the injector when the IDC is already at 100% duty cycle, and the reason why I say that because I've seen it work. I know that IPW is the time frame that the injector stays open and IDC is the calculation of that time frame based on RPM.



IDC = PW/Period, or PW/(120/RPM)... which can be simplified to PW*(RPM/120)
Using this formula, if IDC takes 17ms then the duty cycle of that injector would be at 100% This would also mean that its open through out the entire engine cycle and never gets a chance to close, therefore delivering its maximum amount of fuel. What stumps me is the fact that I've seen guys run well over 100% IDC and not run lean and not only the car didn't run lean but that added % over 100 gave the engine just the right amount of fuel to maintain the air/ratios. So where is that over a 100% IDC fuel coming from? Is it just an error in the ECUs calculation or is the ECUs calculation based off of different time frame than the full engine cycle? This is what I don't understand.

Like I said before, I'm not trying to argue or prove my point, I'm just trying to get a better understanding of the process. I speak my thoughts in detail and examples so you guys can get a better understanding of my reasoning. I would like to hear the same from those who have a different understanding.

I have seen people claim as well that tey get more fuel at 108% than they did at 100%. I think it is because dead times are a large part of the equation. In your example of 17ms, 3ms is for on and off times. so 14ms is left for actual opening. So when you go over 100% the actual opening actually gets bigger like 15ms . The calculated on time is bleeding deeper into the dead times alloted.

I personally dont allow my duty to go over 85%. If it does I shop for larger injectors.

 

[Boostdriven]

So I did some more research on this whole IDC and I have to say that my assumption that IDC is based off of 180* of crank rotation was wrong. Few weeks ago I sat down and did some serious math calculations to put this whole thing in perspective.

To find out how much fuel you need to generate desired a/f ratios you need to take the weight of air in a given volume and divided it by the weight of fuel witch will give you the a/f ratio. Also if you know the given air volume (engine displacement) and you know what a/f ratio you want to see then you can take a/f ratio and divide it by the weight of the air per given volume, that will give you the total weight of the fuel you will need to generate those a/f ratios. If you know the injector size then you can figure out how long that injector needs to be open to spray the right amount of fuel to produce those a/f ratios.

I wish that some of you more knowledgeable guys out there would have take the time to post up some formulas on how to figure this whole thing out, it would have made it easier for me as well as other people who would have or are reading this to have a better understanding of how things work. In some ways I'm glad that this thread was started because it gave me a good reason to do my own research from which I have learned new things.

 

[tkelly27]

You can prove to yourself that the car can run when the fuel injectors spray 180 degrees out of phase. That is to say, the car will run no matter which stroke the fuel injector opens on. Simply swap injector plugs on 1 & 4, and 2 & 3. I believe this can also be done by putting the CAS in 180 degrees out.

You can also think of Chevy central throttle injection with 4 injectors for 8 cylinders, or batch fire injectors which fire a whole bank at a time, regardless of the position of the intake valve.

The effect is only noticed at idle, some folks get a little misfire. It is supposed to have an effect on fuel economy too, but I've never tested it long term.

Here is a fuel map for an AEM EMS for you to consider.

http://www.innovatemotorsports.com/resources/images/EFI_FuelMapping.jpg

The values in the cells are milliseconds. With an AEM you directly edit injector pulse width. At 8000rpm and top load you are getting

1min * 60s * 1000ms = 7.5ms per revolution
8000r * 1min * 1s

The intake stroke occurs for half of a revolution so you only have a 3.75ms window to open during the intake valve. There simply isn't time to have a 9.6ms pulse during this window, yet that is what is required for the AEM system to inject enough fuel.

You can see that as the RPM goes up, the denominator in the above formula gets bigger. A bigger denominator means even less time for the injector to be open before the next fueling event.

I hope this makes sense to you, that as RPMs go up no matter how big your injector is the time required for fueling can not fit inside the window of time offered when the intake valve is open.

Duty cycle, to my understanding, is

Time open
time between triggering events

The time between events is twice the time of a single crankshaft rotation.

2 * 7.5ms = 15ms

You could see that if you had injectors that were half as big, and needed twice as much time to fuel

2 * 9.5 = 19ms

The number 19 divided by the number 15 would make

19 = 1.266
15

1.266 is equivilent to 127%. What this means is that the amount of time you need to have the injector open to get the right amount of fuel is longer than the amount of time between fueling events. That's what a duty cycle of more than 100% means.

The ECU only knows miliseconds, like the AEM fueling table, and RPM. True, it does need to know the time between RPM events to know RPM, but from what I understand it dumps that info ever time it calculates a new RPM. The logging software post processes this information to give duty cycle.

You are not doing yourself any favors having duty cycles over 80%, so to worry about the how and why of 120% DC's is foolish. If you are pushing your engine hard enough to have 120%DC's it's not going to be pleasant when the injector gets stuck open or stuck shut. Every DSM ECU has the ability to run very large injectors, especially with a voltage box, P&H drivers, or even staged injection if you are at that point.