Injector Duty Cycle explained *good stuff*
12-20-2007, 08:56 PM
#1
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Injector Duty Cycle *Come Inside* I ran accross this in a link a board member posted and learned a few things. Based on things I read on the site I KNOW there are plenty of others that are confused as well. It's a bit long but real informative.
Might make a few of us think twice about using stock injectors on even mild Heads/Cam build ups. I did me.
Re'
The duty cycle is an often misunderstood. It's a ratio between the pulse width required for proper AFR and the length of time you have to inject that amount of fuel. It is dictated by the RPM and the Otto cycle. 20ms pulse width at 3000rpm is just fine (50% duty cycle), because the 'window of opportunity' is 40ms. But the same 20ms pulse width at 6000rpm is 100% duty cycle because the 'window of opportunity' is also 20ms. Very often on forums you get people making statements like 'my car runs just fine at 130% duty cycle, so the stock injectors must be underrated.' What they really mean is their injectors don’t have the capacity to provide enough fuel in the time allotted. The computer calculates air mass, figures it needs 30ms worth of fuel and starts spraying, not thinking if it has that much time before the next ignition or not. This is where the ridiculous 'acceptable' duty cycle numbers come from. Computer calculates that it wants 30ms fuel with 20ms to do it in, it's going to show 150% duty cycle. This however doesn't mean the fuel gets there. It physically cannot get there in less time than it's allowed, as the intake valve closes shut. So what happens then?
Not only the ignition will happen without all of the fuel necessary at the time, but the fuel is still spraying during the power stroke, puddling on top of a hot valve. Then the injectors turn off and recharge for another spray cycle half way through another cycle already, when they should be spraying fuel. So while one cycle might be fueled fine, another one is going to be lacking proper fueling. Such sequences of events cause severely unpredictable fueling and widely scattered AFR, fluctuating EGT's, and generally making it unpredictable and difficult to tune. In addition to that, you're stressing the injectors beyond their operational range, overheating them, and possibly causing a pre-ignition.
Without proper injectors, at best your car is going to be running like ****; at worse it will cause a meltdown. It's just not worth risking. Injectors are cheap, I will never understand people who are willing to drop 3k in heads that flow few CFM better than others half the price, but won't even think of spending $300 to make sure that the airflow can be matched with fuel and your expensive parts don’t melt away. Don’t be a cheap dumbass, be fast and reliable instead.
Not only the ignition will happen without all of the fuel necessary at the time, but the fuel is still spraying during the power stroke, puddling on top of a hot valve. Then the injectors turn off and recharge for another spray cycle half way through another cycle already, when they should be spraying fuel. So while one cycle might be fueled fine, another one is going to be lacking proper fueling. Such sequences of events cause severely unpredictable fueling and widely scattered AFR, fluctuating EGT's, and generally making it unpredictable and difficult to tune. In addition to that, you're stressing the injectors beyond their operational range, overheating them, and possibly causing a pre-ignition.
Without proper injectors, at best your car is going to be running like ****; at worse it will cause a meltdown. It's just not worth risking. Injectors are cheap, I will never understand people who are willing to drop 3k in heads that flow few CFM better than others half the price, but won't even think of spending $300 to make sure that the airflow can be matched with fuel and your expensive parts don’t melt away. Don’t be a cheap dumbass, be fast and reliable instead.
Re'
Last edited by RE AND CHERYL; 12-20-2007 at 11:30 PM.
01-01-2008, 09:30 PM
#3
Here is better info:
How does a fuel injector work?
A fuel injector is nothing more than a high-speed valve for gasoline. An engine computer or controller is used to control the fuel injector. Contrary to popular belief, this is NOT done by sending power to the injector. Fuel injectors are normally fed power whenever the ignition key is on. The computer controls the negative, or ground side, of the circuit. When the computer provides the injector with a ground, the circuit is completed and current is allowed to flow through the injector. This energizes an electromagnetic coil inside the injector, which pulls a sealing mechanism (pintle, ball, or disc) away from its seat. This makes it possible for fuel to flow through the injector and into the engine. When the computer removes the electrical ground to the injector, the electromagnetic coil becomes demagnetized and a spring forces the pintle, ball, or disc shut to cut off fuel flow. Even at an engine speed of just 1000 RPM, this is done hundreds of times per minute.
What do the terms “static” and “duty cycle” mean?
An injector in an engine turns on and off very quickly to control the amount of fuel delivered. The amount of time an injector is turned on and delivering fuel is known as the duty cycle. This is measured as a percent, so 50% duty cycle indicates that the injector is held open and held closed for an equal amount of time. When the engine needs more fuel, the time that the injector stays on (its duty cycle) increases so that more fuel can flow into the engine. If an injector stays on all the time, it is said to be static (wide open, or 100% duty cycle). INJECTORS SHOULD NOT GO STATIC IN A RUNNING ENGINE! If an injector is static in a running engine (open 100% of the time), that injector is no longer able to control fuel delivery. It is just “along for the ride”. This could be an indication that the injector is too small for the needs of the engine. Injector duty cycle should usually not exceed 80% in a running engine at any time.
What is impedance?
Impedance is the electrical resistance of the electromagnetic coil inside the injector. This is measured in ohms and can be determined with an ohmmeter. Injectors are classified as either high-impedance (also known as “saturated”) or low-impedance (known as “peak and hold”). High-impedance injectors usually range from 11 to 16 ohms of impedance, while low-impedance injectors usually range from 0.7 to 5 ohms of impedance (these impedance numbers are based on what is currently available in the consumer market and are subject to change). Most OEM engine computers are designed to control high-impedance fuel injectors. Low-impedance injectors are generally preferred for racing or ultra-high performance use because they respond more quickly, but aftermarket engine controllers are usually required to control them.
What is an injector’s static flow rate?
Manufacturers rate fuel injectors by the maximum amount of fuel that they can flow in a given amount of time. This measurement is taken with the injector on 100% of the time (100% duty cycle, or wide open) and with the fuel at a given pressure (usually 43.5 psi). For example, a 19 pound per hour (Lb./Hr.) injector flow 19 pounds of fuel in one hour at 100% duty cycle and 43.5 psi of fuel pressure. Injectors in imported vehicles are often rated in cubic centimeters per minute (cc/min) instead of pounds per hour. This is also done at 100% duty cycle.
If injectors should not exceed 80% duty cycle under operating conditions, why do manufacturers rate them at 100% duty cycle?
A test at 100% duty cycle is used to determine the maximum amount of fuel that will flow through an injector in a given time. This test is useful for determining whether an injector’s internal fuel passages were machined properly, but it does NOT check an injector’s ability to cycle on or off. It is usually NOT recommended to run an injector at more than 80% duty cycle under actual driving conditions. This 80% duty cycle operating limit is taken into account to make sure the injector will be large enough to feed the engine under ACTUAL OPERATING CONDITIONS and will not starve the engine for fuel.
How does a fuel injector work?
A fuel injector is nothing more than a high-speed valve for gasoline. An engine computer or controller is used to control the fuel injector. Contrary to popular belief, this is NOT done by sending power to the injector. Fuel injectors are normally fed power whenever the ignition key is on. The computer controls the negative, or ground side, of the circuit. When the computer provides the injector with a ground, the circuit is completed and current is allowed to flow through the injector. This energizes an electromagnetic coil inside the injector, which pulls a sealing mechanism (pintle, ball, or disc) away from its seat. This makes it possible for fuel to flow through the injector and into the engine. When the computer removes the electrical ground to the injector, the electromagnetic coil becomes demagnetized and a spring forces the pintle, ball, or disc shut to cut off fuel flow. Even at an engine speed of just 1000 RPM, this is done hundreds of times per minute.
What do the terms “static” and “duty cycle” mean?
An injector in an engine turns on and off very quickly to control the amount of fuel delivered. The amount of time an injector is turned on and delivering fuel is known as the duty cycle. This is measured as a percent, so 50% duty cycle indicates that the injector is held open and held closed for an equal amount of time. When the engine needs more fuel, the time that the injector stays on (its duty cycle) increases so that more fuel can flow into the engine. If an injector stays on all the time, it is said to be static (wide open, or 100% duty cycle). INJECTORS SHOULD NOT GO STATIC IN A RUNNING ENGINE! If an injector is static in a running engine (open 100% of the time), that injector is no longer able to control fuel delivery. It is just “along for the ride”. This could be an indication that the injector is too small for the needs of the engine. Injector duty cycle should usually not exceed 80% in a running engine at any time.
What is impedance?
Impedance is the electrical resistance of the electromagnetic coil inside the injector. This is measured in ohms and can be determined with an ohmmeter. Injectors are classified as either high-impedance (also known as “saturated”) or low-impedance (known as “peak and hold”). High-impedance injectors usually range from 11 to 16 ohms of impedance, while low-impedance injectors usually range from 0.7 to 5 ohms of impedance (these impedance numbers are based on what is currently available in the consumer market and are subject to change). Most OEM engine computers are designed to control high-impedance fuel injectors. Low-impedance injectors are generally preferred for racing or ultra-high performance use because they respond more quickly, but aftermarket engine controllers are usually required to control them.
What is an injector’s static flow rate?
Manufacturers rate fuel injectors by the maximum amount of fuel that they can flow in a given amount of time. This measurement is taken with the injector on 100% of the time (100% duty cycle, or wide open) and with the fuel at a given pressure (usually 43.5 psi). For example, a 19 pound per hour (Lb./Hr.) injector flow 19 pounds of fuel in one hour at 100% duty cycle and 43.5 psi of fuel pressure. Injectors in imported vehicles are often rated in cubic centimeters per minute (cc/min) instead of pounds per hour. This is also done at 100% duty cycle.
If injectors should not exceed 80% duty cycle under operating conditions, why do manufacturers rate them at 100% duty cycle?
A test at 100% duty cycle is used to determine the maximum amount of fuel that will flow through an injector in a given time. This test is useful for determining whether an injector’s internal fuel passages were machined properly, but it does NOT check an injector’s ability to cycle on or off. It is usually NOT recommended to run an injector at more than 80% duty cycle under actual driving conditions. This 80% duty cycle operating limit is taken into account to make sure the injector will be large enough to feed the engine under ACTUAL OPERATING CONDITIONS and will not starve the engine for fuel.
01-01-2008, 09:55 PM
#4
This is measured as a percent, so 50% duty cycle indicates that the injector is held open and held closed for an equal amount of time. When the engine needs more fuel, the time that the injector stays on (its duty cycle) increases so that more fuel can flow into the engine. If an injector stays on all the time, it is said to be static (wide open, or 100% duty cycle)
01-02-2008, 09:14 AM
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They don't. DC is a calculation performed by the PCM representing the amount of time the injector needs to be open, relative to the actual available time, to deliver the correct amount of fuel.
At 130% DC, the injector is maxed out (static) and delivering less fuel than the engine needs.
At 130% DC, the injector is maxed out (static) and delivering less fuel than the engine needs.
01-02-2008, 10:44 AM
#6
Good info.
At what moment, say, in an LS type engine, does the beginning of duty cycle begin? Is it the moment at which the piston is sensed to be on its' intake stroke, or, just as spark is applied BTDC for an engine with increasing speed, does the duty cycle begin slightly before the intake stroke?
More precisely, where are those ms of duty cycle applied during the intake stroke?
Maybe I am thinking incorrectly about this and the actual, correct amount of fuel is being dispersed throughout the entire intake cycle?
At what moment, say, in an LS type engine, does the beginning of duty cycle begin? Is it the moment at which the piston is sensed to be on its' intake stroke, or, just as spark is applied BTDC for an engine with increasing speed, does the duty cycle begin slightly before the intake stroke?
More precisely, where are those ms of duty cycle applied during the intake stroke?
Maybe I am thinking incorrectly about this and the actual, correct amount of fuel is being dispersed throughout the entire intake cycle?
01-02-2008, 03:43 PM
#7
They don't. DC is a calculation performed by the PCM representing the amount of time the injector needs to be open, relative to the actual available time, to deliver the correct amount of fuel.
At 130% DC, the injector is maxed out (static) and delivering less fuel than the engine needs.
At 130% DC, the injector is maxed out (static) and delivering less fuel than the engine needs.
Jon
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01-02-2008, 06:29 PM
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I,m confused. My 42#er's saw a duty cycle of 187% on pump gas and ran just fine, AFR's were where they needed to be. The wright-ups appear to be saying that i need twice the amount of time to get the correct amount of fuel, yet my Afr's tell me that I am getting the right amount fuel. Did i miss something. Does this mean when i get bigger injectors i will have to tune the VE table again to get everything back in line? Or maybe my VE table is already off. so the pcm thinks it needs more fuel, artificialy inflatting the duty cycle?
01-02-2008, 08:32 PM
#10
I,m confused. My 42#er's saw a duty cycle of 187% on pump gas and ran just fine, AFR's were where they needed to be. The wright-ups appear to be saying that i need twice the amount of time to get the correct amount of fuel, yet my Afr's tell me that I am getting the right amount fuel. Did i miss something. Does this mean when i get bigger injectors i will have to tune the VE table again to get everything back in line? Or maybe my VE table is already off. so the pcm thinks it needs more fuel, artificialy inflatting the duty cycle?
01-03-2008, 05:30 PM
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i did the ve tuning up to third gear only, i dont like driving that fast on the street. would that screw things up during a fourth gear pull?
the correction factor was 1.21. when i did the dyno pull the AFR went way lean, 14:1. i have never seen it that lean before, but i had switched over to E-85 the week before, and as stated above i didnt tune it with any fourth gear pulls. The track time was on 91 octane gas on a 95 deg. day. From what i have read E-85 will add 10% more power, i would have liked to dyno before the switch to confirm that but i didn't. the actual power when dynoed was around 530. All that and i suck at drag racing, my 60 ft. was 2.18, tires at 35 psi and the stock 10 bolt that i dont want brake.
The numbers in your sig don't add up at all. The correction factor on those dyno #'s would have to be extreme. The 1/4 mile time is easily doable with 42# injectors on gasoline, but e85 requires a lot more fuel, although still within range. The HP #'s, however, are not even remotely doable on 42's with E85, unless the actual power is MUCH lower uncorrected.
Last edited by vaticano; 01-03-2008 at 05:36 PM.
01-03-2008, 07:08 PM
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Copied from turbmustangs but very relavant to the dicussion here.
Just because the system is broken doesn;t mean everyone else is wrong. Even with your IOT you still have injector closing time which is going to be double in bank and 4x in batch. So if you don't understand the physics of the argument thats fine but lets actually give a true mathmatical example.
Sequential. 20 msec. 18.9 usable 1.1 off time = 20 msec 6000rpm.
Bank to Bank Now we have a pulsewidth of 9.45 for the same amount of fuelinjector pulsewidth but we have to add in that 1.1 to Each event to not be static. Thats 9.45+1.1= 10.55 x 2 = 21.10 to achive the same pulse width as a seuqneutial systems but thats not the whole of it.
Even using the smallest factor avaibale for off time you can see where the issue arrives in a hurry with the blatantly inccorect claim you just made. The mechanical electrical propertie of the injector can be found here on the Precision Turbo website.
Closing time Must be factored into the Actuall Injector PW to determine if you are actually Static.
http://www.precisionturbo.net/fuelsystems.php
Now what you still have to factor in for here is that every time you charge the injector coil. which takes roughly 1.24msec or so on a 96pph peak hold about .8 msec faster then a saturated injector. So we must deduct that from out 20 msec maximum.
20msec - 1.24= 18.76 usable injecting pulsewidth now again subtract the 1.1 nessacary off time and you have 17.66 msec of usable pulsewidth.
now lets work this again in batch but now we have multiples of 2 events per firing cycle.
20msec- 2.48 = 17.52 and again subtract the minimum off time from that usable pulse width and you have 15.32 usable pulse wdith for injecting fuel.
so if we take those two numbers and do this basic peice of math 15.32/17.66 we get this disparity in fuel dilivery the bank to bank at 2 events per cylinder cycle using the same injector delivers 86% of the fuel the sequential system does.
So while you would like to belive you are correct you are in fact wrong. Using a batch system makes it even worse in synchronous fueling mode becuase you have 4 firing events per crank cycle.
So really what people do not understand about injectors is that they have a
charge time "injector not open"
Open time " Injector Open"
Injector Closing Time" injecotr closing"
Off Time "injector Closed"
So injector have 4 basic cycles of operation and inbetween those 2 center cycle we have what called non linear fueling. This is why its so difficult to get consistne idle AFR with large injectors running very short pulsewidths. This is also why precision in the fueling algorythm and injector control circut is so very important.
http://www.lindertech.com/newsletters/nwk7~03.pdf
this is some info on how to diagnose bad injectors but from this you can see that injectors have 4 operational states and they all must be accounted for in the fueling euqations.
1
mS = _______________________________________
rev/min x .5/rev x 1min/60sec x 1sec/1000mS
thats a formula to figure out total avaiable pulsewdith for injecto on time. Subtract your minimum off time of 1.1 from your maximum on time and you have your non staitc Pulsewdith.
Sequential. 20 msec. 18.9 usable 1.1 off time = 20 msec 6000rpm.
Bank to Bank Now we have a pulsewidth of 9.45 for the same amount of fuelinjector pulsewidth but we have to add in that 1.1 to Each event to not be static. Thats 9.45+1.1= 10.55 x 2 = 21.10 to achive the same pulse width as a seuqneutial systems but thats not the whole of it.
Even using the smallest factor avaibale for off time you can see where the issue arrives in a hurry with the blatantly inccorect claim you just made. The mechanical electrical propertie of the injector can be found here on the Precision Turbo website.
Closing time Must be factored into the Actuall Injector PW to determine if you are actually Static.
http://www.precisionturbo.net/fuelsystems.php
Now what you still have to factor in for here is that every time you charge the injector coil. which takes roughly 1.24msec or so on a 96pph peak hold about .8 msec faster then a saturated injector. So we must deduct that from out 20 msec maximum.
20msec - 1.24= 18.76 usable injecting pulsewidth now again subtract the 1.1 nessacary off time and you have 17.66 msec of usable pulsewidth.
now lets work this again in batch but now we have multiples of 2 events per firing cycle.
20msec- 2.48 = 17.52 and again subtract the minimum off time from that usable pulse width and you have 15.32 usable pulse wdith for injecting fuel.
so if we take those two numbers and do this basic peice of math 15.32/17.66 we get this disparity in fuel dilivery the bank to bank at 2 events per cylinder cycle using the same injector delivers 86% of the fuel the sequential system does.
So while you would like to belive you are correct you are in fact wrong. Using a batch system makes it even worse in synchronous fueling mode becuase you have 4 firing events per crank cycle.
So really what people do not understand about injectors is that they have a
charge time "injector not open"
Open time " Injector Open"
Injector Closing Time" injecotr closing"
Off Time "injector Closed"
So injector have 4 basic cycles of operation and inbetween those 2 center cycle we have what called non linear fueling. This is why its so difficult to get consistne idle AFR with large injectors running very short pulsewidths. This is also why precision in the fueling algorythm and injector control circut is so very important.
http://www.lindertech.com/newsletters/nwk7~03.pdf
this is some info on how to diagnose bad injectors but from this you can see that injectors have 4 operational states and they all must be accounted for in the fueling euqations.
1
mS = _______________________________________
rev/min x .5/rev x 1min/60sec x 1sec/1000mS
thats a formula to figure out total avaiable pulsewdith for injecto on time. Subtract your minimum off time of 1.1 from your maximum on time and you have your non staitc Pulsewdith.
01-03-2008, 11:33 PM
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I thought I under stood this when I posted it, but now I'm not sure. I know older batch fire injection systems would inject fuel when the valve was closed or open depending on the cylinder and the luck of the draw. I thought sequential fire systems only injected fuel when the valves were opened, that was the whole advantage.
Re'
Re'
01-04-2008, 02:45 AM
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I thought I under stood this when I posted it, but now I'm not sure. I know older batch fire injection systems would inject fuel when the valve was closed or open depending on the cylinder and the luck of the draw. I thought sequential fire systems only injected fuel when the valves were opened, that was the whole advantage.
Re'
Re'
So thanks Re, I may not be able to replicate the math yet, but I have a much better understanding of injector duty cycle. I was thinking it was akin to electrical compononet or relay duty cycle on time/off time, but there's much more to it. It's the on time needed to provided fuel/the window of time avialable minus the overhead of time for cycle and charge...roughly speaking at nealry 1:00am
And this is very relevent for me as I eye my injecetors and will soon be looking at thier duty cycle.
01-04-2008, 10:38 AM
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Comparing an Angle or Degree driven system vs a system based on time would be your problem in comprehending the data. You have 720 degree of crank rotation in a sequential 360 in a bank and 90 in a synchronous batch.
The real trick is knowing how much time you have per degree of roatation. You can open an injector with the valve closed. the engine does not care. If we only opened the injector when the valve was open a stock ls1 would need like 3x the injector it comes with from the factory. the injection window would be very short. The basic premise is that you put the needed quantity of fuel in the port and when the valve opens it mixes with the air. Also the injector will be spraying while the valve is open.
The real trick is knowing how much time you have per degree of roatation. You can open an injector with the valve closed. the engine does not care. If we only opened the injector when the valve was open a stock ls1 would need like 3x the injector it comes with from the factory. the injection window would be very short. The basic premise is that you put the needed quantity of fuel in the port and when the valve opens it mixes with the air. Also the injector will be spraying while the valve is open.
I thought I under stood this when I posted it, but now I'm not sure. I know older batch fire injection systems would inject fuel when the valve was closed or open depending on the cylinder and the luck of the draw. I thought sequential fire systems only injected fuel when the valves were opened, that was the whole advantage.
Re'
Re'
01-04-2008, 06:05 PM
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It does... thats the point to give a larger amount of time for fuel to enter the cylinder... hence why cams with larger duration generally make more power...
01-04-2008, 06:43 PM
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Yes, for an air charge, but that's interesting that accordingly a longer duration cam can then make more power with a smaller injector since it has more time to inject fuel. Somewhat counter inutative.
01-05-2008, 10:58 AM
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