injector-duty-cycle discussion simple style

[dsmcurse]

So I hear the term injector duty cycle thrown out in the forums quite often and I feel that the term is used to loosely and generic and sometimes wonder if they understand this concept completley. Things like " how hard the injectors are working" and terms like "above 80% idc your pissing" so I thought id start a small thread about this subject, maybe my understanding is incorrect and some one in here who chimes in will teach me somethin? Here is my explanation of injector duty cycle explained simple style.

A "four-stroke" engine must complete four differant task's to magically create one full engine revolution completion. 1=intake stroke, 2=compression stroke, 3=power stroke and 4=exhaust stroke. So we have intake, compress, power, exhaust.

On the intake stroke we have piston at the top and engine is just starting out, with intake valve open and exhaust valve closed the piston goes down to inhale its fesh air gulp, as the piston goes down it receives a shot of gas from the injector to create the "air-fuel mixture" at this point in time is the ONLY time the injector fires, is on the intake stroke when piston is going down to inhale its fresh air gulp, this is the ONLY point in time when injector fires.

So now the piston has gone all the way down and has inhaled its fresh air gulp containing air fuel mixture, the intake valve close's and the exhaust valve still remains closed, the piston travels up in a air tight cylinder compressing the air fuel mixture, with both valves still closed and the piston almost reaches the top of the stroke the spark plug receives signal to ignite its lil mini lightning bolt and explode the compressed air fuel mixture gettting squished in the cylinder.

Now that the compressed air fuel mixture has just been ignited by the spark the explosion sends piston down (creating horsepower and sending other pistons up) this is power stroke, the valve up top anre still closed and the explosion forced the piston down with power.

Now the exhaust valve opens and the piston travels up with intake valve closed and exhaust valve open, piston goes up to expell its wasted by product. This is called exhaust stroke.

So we had intake, compress, power, exhaust... and the injector only did its part for a very small portion of this entire process. After the injector received its signal look how much more stuff happened.

So now to the important part, the part that matters, the intake stroke where the injector can do its thing (because if injector fired at any other time during the whole 4-stroke process the engine would go titty-wompass and not function properley) so here's my explanation, here's is what the scientists at Honda or Toyota or where ever you deceided to call. A Mitsubishi engineer would tell you somethin along these lines

On the intake stroke where piston inhales its fresh air gulp the injector only has its normal "window" of operating time from 5* after top dead center to 15* befor bottom dead center. So from piston just bearley starting to head down to piston almost at the bottom of its going down stroke, this is the window of time in which the injector can do its thing.

So... on our laptops when we are data logging this and we see the displayed injector duty cycle, its telling us how long the injector fired for during its normal window of operating time. Ya dig what I'm tryin to say? Example, check this out, ever wonder why its possible to see the lap top data log over %100 IDC? Well, the fuel demand was so great that the injector remained open firing even as the piston went beyond its normal window of operating time, the injector demand was so great it continued to supply fuel x-amount of degrees on the way back up %115IDC, %127 IDC, blah blah blah whatever

So if IDC is at %25 cruising down freeway, then that just means how long it delivered spray on its fresh air gulp, which 25% the fuel squirt was very minimal. So my example of 5*atdc to 15*bbdc isnt EXACTLY EXACT, it just a very close example of how it works and what injector duty cycle really means. Id love to hear some more input fom some of the smarter vetrans on here, beginers just starting out could use this discussion to clarify what IDC really means.

Thanks for reading

 

[bastarddsm]

No, the injector pulse is a squarewave that is half the frequency of engine rotation, and the idc is the percentage of that cycle it is open. The total possible open time is the time it takes to make two revolution or 2/rpm. So for a constant pulse width idc goes up with rpm. Also you can have greater than 100% idc when the commanded pulse width getts bigger than the availible time. The injecter never closes and is considered static at this point. Realistically over 95%idc you can consider it static. The ecu doesn't know idc, it only has a cap at a cercain pulse width, and it is huge. Idc is calculated on the logger. Note, that under wot most engines are spraying on a closed intake valve, and the ecu doesn't care, it has no provisions to restrict spraying to open intake valve. Imho I don't think you want to, you want the time for the fuel to mix and vaporize.

 

[dsmcurse]

Bastard, you always have interesting points and coments. So agreed that it only receives "signal" from 5*atdc to *15bbdc? Give or take?

 

[jakelandry]

No, the injector pulse is a squarewave that is the frequency of engine rotation, and the idc is the percentage of that cycle it is open. The total possible open time is the time it takes to make one revolution or 1/rpm. So for a constant pulse width idc goes up with rpm. Also you can have greater than 100% idc when the commanded pulse width getts bigger than the availible time. The injecter never closes and is considered static at this point. Realistically over 95%idc you can consider it static. The ecu doesn't know idc, it only has a cap at a cercain pulse width, and it is huge. Idc is calculated on the logger. Note, that under wot most engines are spraying on a closed intake valve, and the ecu doesn't care, it has no provisions to restrict spraying to open intake valve. Imho I don't think you want to, you want the time for the fuel to mix and vaporize.

So Theoretically, after reaching 200% your injectors are open 100% of the time? That's fairly interesting if true. If that were the case though wouldn't the afr be lean for one rotation and catch up afterwards due to the fact that lets say on a 123% idc the 23% would be "sitting on the valve" and when it opens it would get the remaining 100% spray?

 

[bastarddsm]

No under wot conditions the the injector can be open almost 2 revolutions. 720degrees.

So Theoretically, after reaching 200% your injectors are open 100% of the time? That's fairly interesting if true. If that were the case though wouldn't the afr be lean for one rotation and catch up afterwards due to the fact that lets say on a 123% idc the 23% would be "sitting on the valve" and when it opens it would get the remaining 100% spray?

I messed up the first post, at 100% idc, the injectors are open for 2 engine revolutions. So 75% would be "sitting on the valve"

 

[jakelandry]

No under wot conditions the the injector can be open almost 2 revolutions. 720degrees.



I messed up the first post, at 100% idc, the injectors are open for 2 engine revolutions. So 75% would be "sitting on the valve"

So technically 25% is actually being spit while the intake valve is open. In order to retain a consistent afr, it continues to spray to compensate for the next intake valve opening? 100% equates to a 720* revolution, which means it would never close because it fires fuel every 2 revolutions correct?

 

[casperfast]

dsmcurse, go take an oscilliscope, connect it to your injector at idle, set the o-scope timing to 1/4 of the engine rpm so you can isolate an individual fire and tell me what you see...... you will see a digital wave with a high peak (first told to open= highest current draw) then a drop below flat followed by a flat line signal for a certain amount of time (pulse width) this all together is the "duty cycle" at a certain range the injectors WILL become static meaning open or as you say "pissing" and no longer efficient. before you try and re-educate people and tell them the answers to all their questions, make sure you know what your talking about.

 

[casperfast]

No, the injector pulse is a squarewave that is half the frequency of engine rotation, and the idc is the percentage of that cycle it is open. The total possible open time is the time it takes to make two revolution or 2/rpm. So for a constant pulse width idc goes up with rpm. Also you can have greater than 100% idc when the commanded pulse width getts bigger than the availible time. The injecter never closes and is considered static at this point. Realistically over 95%idc you can consider it static. The ecu doesn't know idc, it only has a cap at a cercain pulse width, and it is huge. Idc is calculated on the logger. Note, that under wot most engines are spraying on a closed intake valve, and the ecu doesn't care, it has no provisions to restrict spraying to open intake valve. Imho I don't think you want to, you want the time for the fuel to mix and vaporize.

/\/\ exactly what im talking about

 

[dsmcurse]

Closed my explanation with "my understanding" and also openly asked for smart guys opions and version as well, openly said so I can learn something new. So why do you wanna come off like a dick? This is a discussion. I'm glad to learn and keep learning. So why the attitude?

 

[Boostdriven]

If 100% duty cycle was based off of a full engine cycle then that would mean that the injector would open at the top of the intake stroke and remain open throughout the intake, compression, fire and exhaust strokes. If that was the case then due to cam duration roughly 25-35% of the fuel would get used for that engine cycle at 100% injector duty cycle. This is not the case and that's not how duty cycle works.

So let’s break it down in to engine cycle percentages and call full engine cycle a 100% so that intake stroke is 25% compression stroke is 25% fire stroke is 25% and exhaust stoke 25% so in total that would be 100% full engine cycle. Having said all that, the engine is only able to intake air/fuel during the intake stroke which is 25% of the whole engine cycle so during those 25% of the engine cycle is when the injector sprays fuel.

My understanding of 100% injector duty cycle is that it begins somewhere at the top of the intake stroke and ends somewhere at the bottom of intake stroke. In reality the engine does not have a sensor that tells the ECU when the piston is at the bottom of the intake stroke, it’s all calculated off of time and time is effected by the engine RPMs. The faster the engine spins the less time the injector has to spray the right amount of fuel to keep air/fuel ratios happy, so at higher RPMs you will get higher injector duty cycle and when you add boost (more air) to the equation then the injector duty cycle has to be increased even more to maintain the right air/fuel rations. All this has to happen during the intake stroke of the engine rotation which is only 25% of the whole engine cycle.

Ok now let’s talk about the intake stroke. As the intake valve opens and the piston starts to travel down it starts to take air in and that's when the injector starts to spray fuel. Now based on the engine speed (RPMs) and the volume of air read by the MAF sensor or MAP sensor the ECU makes a calculation of how long it has to keep the injector open to give the engine enough fuel to maintain a desired air/fuel rations. Here is the catch; the final intake stroke is determined by how long the intake valve remains open allowing the engine to intake air and fuel that's being sprayed by the injector, this is calculated on every car that comes from the factory. Once the intake valve is closed then any fuel being sprayed in to the engine will just puddle up on top of the closed valves and do no good for that compression stroke. It is only on the next intake stroke is when that fuel will be taken in to the cylinder but it will not be atomized as well as sprayed fuel because it comes from a puddled fuel. I think this is why the theory of (don't run more than 85% injector duty cycle) was born, but this theory is not always proven to be true. I've seen people run their injectors to 115% duty cycle and never lean out and make really good power, and the reason for that is the cam duration. Any cam that has a long duration meaning it keep the valve open well past the bottom dead center (BDC) allows the air/fuel to enter the engine after the piston has reached the bottom and starts to travel back up on a compression stroke. As the piston travels down on the intake stroke it picks up speed till the crank is at 90* of rotation then it starts to slow down and as it reaches the bottom and it comes to a complete stop right before it starts to come back up. At high RPMs it takes more than just 25% of engine cycle (intake stroke) to equalize the pressure between the cylinder and intake manifold so that’s why we buy long duration cams to allow the engine to take as much air as it can. As the piston reaches BDC and starts to travel up the momentum of entering air in to the cylinder still remains and because the intake valve is still open it allows the momentum of air to equalize the pressure in the cylinder giving it all the air it can take before it goes on the compression stroke. This is the time where the injector can exceed its pre programmed 100% duty cycle(time frame) and continue to spray fuel because the cam still keep the intake valve open allowing air and sprayed fuel to enter the cylinder, therefore you end up with injector duty cycle that’s greater then 100%

If I’m not making sense somewhere I’ll be glad to explain things in better detail, it’s hard to gather your thoughts when you’re with family at thanksgiving dinner and you have kids running around the house

 

[bastarddsm]

No that is wrong. 100% idc means it's open for 2 revolutons, all 4 cycles. The injectors spray on closed valves. Do the math, youll see it's almost impossible for us to get all the fuel in, in 180* of crank rotation.

 

[Boostdriven]

No that is wrong. 100% idc means it's open for 2 revolutons, all 4 cycles. The injectors spray on closed valves. Do the math, youll see it's almost impossible for us to get all the fuel in, in 180* of crank rotation.

To me that don't make sense. How could ECU read over 100% duty cycle if the injector is already open and the ECU has not giving it a commend to close. The ECU would not be able to calculated duty cycle over 100% if it never tells the injector to close, it would just reach 100% duty cycle meaning injector is open at all times and that's it.

If 720* of crank rotation is 100% duty cycle of the injector then as the engine goes past 720* of rotation meaning starting a new cycle and that injector that is still open and has not been given a commend from ECU to close then the ECU would just keep the negative signal to the injector live meaning 100% (on) 100% duty cycle.

Here is where I think this theory is wrong. In order to measure distance or duration of time you have to have a beginning and an end. If the calculation of duty cycle starts when the ECU sends the negative signal to the injector and ends when the ECU cuts the signal then that would mean the injector has a point in the engine cycle (720* in this case) when its closed. If the ECU never cuts a signal to the injector then the duty cycle could not be measured because there is no end of the duty cycle time.

So let's take for example, if it takes an engine 1 second to make 720* of rotation and 1 second of open injector would also be 100% duty cycle then how could the ECU calculate 115% duty cycle if it never cuts the signal to the injector? The injector is already open so I don't see a way the ECU can predict where that injector should have closed to determine 115% duty cycle. At 115% or 1.15 seconds of duty cycle that injector should technically close at 108* of engine rotation of a new cycle (720*x.15%=108*) so at 115% injector duty cycle the engine would have made 828* of rotation, (720*+108*=828*) that just don't make any sense.

 

[bastarddsm]

No, the ecu calculates a pulse width. It doesn't consider how long it has before the next cycle begins. It just never gets a signal to turn off the injector.

 

[Boostdriven]

No, the ecu calculates a pulse width. It doesn't consider how long it has before the next cycle begins. It just never gets a signal to turn off the injector.

Yes I agree and that's why I think the ECU could not measure duty cycle over 100% because the new cycle would start after 720* of crank rotation. How can the ECU calculate 115% duty cycle or pulse width when it never sees the ending point of that pulse width? 100% would just mean the injector is always open and when the crank passes 720* of rotation and the new cycle begins then there is no need to open the injector because its already open. So now think about it, you say "the ECU calculates a pulse width" and I say, how can ECU calculate that pulse width if it never ends meaning injector never closes?

That is why I say that 100% injector duty cycle takes less time then what it takes the engine to make 720* (all 4 cycles) it has to be somewhere around 180*+/- of crank rotation on the intake stroke which is exactly where it would be most efficient.

My thoughts on this are solely based on common sense and understanding of engine mechanics. I'm not arguing that I'm right, its just so far I have not seen anything that makes more sense to me and makes me think otherwise

 

[jakelandry]

Even though noone has said anything but theory, I'm enjoying this thread so far.

 

[bastarddsm]

The ecu calculates the pulse width based on airflow injector size and fuel map. It has no limit to what the pulse width can be. If the pulse width is longer than a cycle, it just never turns it off.

The ecu reports engine RPM and commanded pulse width to the logger. The logger goes idc = (pulsewidth/(2/rpm))

 

[steve]

IDC is just a ratio of the injector pulse width to the engine cycle time at the current RPM.

IDC's > 100% just means that the ECU wants to keep the injector open for longer than the current RPM allow. You can't actually get > 100% due to the laws of physics.

 

[Seamus]

Dsm curse,

You may have closed your post with "discuss" but its pretty obvious you thought you were preaching the word on this post. There is nothing worse than an Internet jockey who does nothing but read posts all day and try to pass off misinformation like he knows what he is talking about. Why don't you spend a few more years learning your shit and less time posting your "accomplishments" and bad info online. This is foundational stuff for fuel mapping and setting up a fuel system with enough capacity. So it's a good thing you let the people you constantly nursing to build your car.

Btw I know you are going to reply asking me what I've ever done. So I'll make it easy for you. I was tuning old school standalone a what required manual pulse width input and calculating my own duty cycle when your pride and joy was still rolling down the assembly line.

 

[boost97gst]

Yes, get an O scope(like the gentleman above said) and see whats really going on. Thats the best way to have tangible, in your face data, in front of you. I was troubleshooting an injector problem when i got my Lincoln and the scope helped a ton. then again, most people dont have oscilliscopes in their trunk like me .
Neat thread though. Seen it before but always like to comb through these for entertainment.

 

[LandSpeed-DSM]

Get a couple of the usual suspects together and you can pretty easily predict how it's going to turn out..

 

[boost97gst]

^^^^Hahahaha, wheres urban? Just kidding urban....

 

[black97spyder]

Seamus, these guys are merely putting out there their interpreted understanding of how IDC works. They are not trying to be better than anyone else and both dsmcurse and boostdriven said they "were open to learning and if someone else's understanding is better than chime in" so that we can put useful info on this site. They are simply trying to help everyone who is not so mechanically Inclined, to learn more of the principle behind injector duty cycle..... so no need to be a dick and try to get into a "pissing competition", as most people on here are just on the site to learn things or help others with their problems.

 

[Boostdriven]

IDC is just a ratio of the injector pulse width to the engine cycle time at the current RPM.

IDC's > 100% just means that the ECU wants to keep the injector open for longer than the current RPM allow. You can't actually get > 100% due to the laws of physics.

Very well put and I agree with your statement but that's just a general explanation of how it works. It does not explain how much of the RPM duration is considered to be 100% duty cycle for the ECU, is it the time it takes an engine to make 180* 360* or is it the full cycle of 720*

Here are few links that kind of explain why IDC has to happen before the intake valve closes or roughly with in 180* of crank rotation.

Injector Duty Cycle explained *good stuff* - LS1TECH

Injector Dutycycle explained [Archive] - IGOTASUBARU.COM FORUM

I don't know how smart those guys are and how much they know but to me that makes sense.

Now explain to me how a carbureted engine works when there is no fuel being injected under pressure but its only being drawn in through the jet in the carburetor ONLY when the intake valve is open and the piston is taking air in.

 

[dsmcurse]

Dsm curse,

You may have closed your post with "discuss" but its pretty obvious you thought you were preaching the word on this post. There is nothing worse than an Internet jockey who does nothing but read posts all day and try to pass off misinformation like he knows what he is talking about. Why don't you spend a few more years learning your shit and less time posting your "accomplishments" and bad info online. This is foundational stuff for fuel mapping and setting up a fuel system with enough capacity. So it's a good thing you let the people you constantly nursing to build your car.

Btw I know you are going to reply asking me what I've ever done. So I'll make it easy for you. I was tuning old school standalone a what required manual pulse width input and calculating my own duty cycle when your pride and joy was still rolling down the assembly line.

Okay dude, since this is a DISCUSSION, why dont you explain this in simple terms please? This is aimed towards better understanding, and Ill openly admit, I DONT UNDERSTAND 720* spraying on closed valves? I dont get get it? An earlier attempt to explain things in my own words left me "stupid" right? I have plenty of books and magazines I could open up but a big scientific explanation isnt what im looking for here, I want simple understanding, draw me a picture with a crayon please. No body on here likes to get qoted then called out by a smartassy reply and thats what I did to you in a differant thread and now you wanna be "internet tuff guy"

In a differant thread homeboy claims to sell "exotic" cars for a living with the ocasional high-end boats... Sure you dont sweep the floors at a high end dealership? Claims like "200 thousand dollar boats are life rafts where im from" c'mon dude, get real. Im so sure. Judging by your pictures id say your car aint $hit. And if you were slanging Lambos and Ferraris for a living I highly doubt that you'd even waste your time in this forum as this is for 20 year-old relic cars that have no business being owned by a rich "big shot" Wouldnt you and your pretend Lambo be better off at www.Lambosareforassholes.com?

Im by no means trying to start a d!ck measuring contest, an open discussion of IDC aimed towards a better understanding is what the point is. Just like in this other thread where EVERYONE is picking on dudes chassis work I sorta try to stick up for his attempt and get hated on fr not joining the band wagon and flaming the dude. Ive done enough of that "picking-on" and am pround to say I have grown out of that. You pop into the thread to not offer any advice or constructive critisim but rather set out strickly to hurt the guys feelings. C'mon bigshot, high end car dealer like you gotta be a bully to a younger guy who is trying? What ever makes you feel good about yourself huh? Just like in your profile and dude leaves you comments like "wheres my flippin used turbo i bought from you? Im pist" Your a flake...

So mr. exotic, why dont you explain how the duty cycle on your Ferrari injectors work? Because I for one have already offered my explanation as I see it in my head. My above post wasnt read out of a magazine or watched on a youtube video, that was my brain offering an explanation as I see it working. I would LOVE to hear your brain spit out an explanation in YOUR OWN WORDS

My explanation wasnt by no means set in stone or labled as THE LAW. Im open to learing, as i honestly admit, spaying on a closed valve seems alienated to me??? I think your trying to be a bigger "player" than what you really are . I dont know any exotic car salesman that would drive a VW or a 25year old DSM that aint even modded.

Next your gonna fire back with "oh you lil young punk ( hey im 30 ) I was running custom obd0 tunes when you were still in diapers" Well, with all those years of experience you should have a magazine feature by now, care to share the link?

 

[Boostdriven]

Get a couple of the usual suspects together and you can pretty easily predict how it's going to turn out..

So why don't you enlighten all of us on how the whole injector duty cycle works but please don't tell me the same old shit that everyone else has said. So far none of you here have posted any solid evidence that shows that my way of thinking is wrong. Only thing I've seen so far is a simple explanation that says IDC is a calculation of IPW which we all know.

I'm not saying that I'm right but I want to be proven wrong, and if it turns out that I'm wrong then I will admit that I'm wrong. I want to see some facts that show that car manufacturers have designed injectors to spray on the closed valves and that roughly 25% IDC is actually spraying fuel when the valves are open and the other 55% (80-25=55) is sprayed when valves are close.